View source for Boron

{{about|the chemical element}}
{{pp-semi-indef|small=yes}}
{{Use dmy dates|date=October 2021}}
{{Infobox boron}}
'''Boron''' is a [[chemical element]]; it has [[Chemical symbol|symbol]]&nbsp;'''B''' and [[atomic number]]&nbsp;5. In its crystalline form it is a brittle, dark, lustrous [[metalloid]]; in its [[Amorphous solid|amorphous]] form it is a brown powder. As the lightest element of the [[boron group]] it has three [[valence electron]]s for forming [[covalent bond]]s, resulting in many compounds such as [[boric acid]], the mineral [[borax|sodium borate]], and the ultra-hard crystals of [[boron carbide]] and [[boron nitride]].

Boron is synthesized entirely by [[cosmic ray spallation]] and [[supernova]]e and not by [[stellar nucleosynthesis]], so it is a low-abundance element in the [[Solar System]] and in the [[Crust (geology)|Earth's crust]].<ref>{{cite web|url = http://van.physics.illinois.edu/qa/listing.php?id=17594|title = Q & A: Where does the element Boron come from?|website = physics.illinois.edu|access-date = 2011-12-04|archive-url = https://web.archive.org/web/20120529072641/http://van.physics.illinois.edu/qa/listing.php?id=17594|archive-date = 29 May 2012|url-status = dead}}</ref> It constitutes about 0.001 percent by weight of Earth's crust.<ref>{{cite web|url=https://www.britannica.com/science/boron-chemical-element|title=Boron|website=Britannica encyclopedia|access-date=4 August 2020|archive-date=4 August 2020|archive-url=https://web.archive.org/web/20200804181151/https://www.britannica.com/science/boron-chemical-element|url-status=live}}</ref> It is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the [[borate mineral]]s. These are mined industrially as [[evaporite]]s, such as [[borax]] and [[kernite]]. The largest known deposits are in [[Turkey]], the largest producer of boron minerals.

Elemental boron is a [[metalloid]] that is found in small amounts in [[meteoroid]]s, but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, the very pure element is produced with difficulty because of contamination by carbon or other elements that resist removal.<ref>{{cite book |last1=Hobbs |first1=Dale Z. |last2=Campbell |first2=Thomas T. |last3=Block |first3=F. E. |title=Methods Used in Preparing Boron |date=1964 |publisher=U.S. Department of the Interior, Bureau of Mines |page=14 |url=https://books.google.com/books?id=37NtbclQPRgC&pg=PA14 |language=en}}</ref> Several [[allotropes of boron|allotropes]] exist: [[Amorphous solid|amorphous]] boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the [[Mohs scale of mineral hardness|Mohs scale]]), and a poor [[electrical conductor]] at room temperature. The primary use of the element itself is as [[Boron fiber|boron filaments]] with applications similar to [[carbon fibers]] in some high-strength materials.

Boron is primarily used in chemical compounds. About half of all production consumed globally is an additive in [[fiberglass]] for insulation and structural materials. The next leading use is in [[polymer]]s and [[ceramic]]s in high-strength, lightweight structural and [[Refractory|heat-resistant]] materials. [[Borosilicate glass]] is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. As [[sodium perborate]], it is used as a [[bleach]]. A small amount is used as a [[dopant]] in [[semiconductor]]s, and [[reagent]] intermediates in the [[Organic synthesis|synthesis of organic fine chemicals]]. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which ([[#Enriched boron (boron-10)|boron-10]]) has a number of uses as a neutron-capturing agent.

The intersection of boron with biology is very small.  Consensus on it as essential for mammalian life is lacking. [[Borate]]s have low toxicity in mammals (similar to [[Salt|table salt]]) but are more toxic to [[arthropod]]s and are occasionally used as [[insecticide]]s. Boron-containing organic antibiotics are known. Although only traces are required, it is an essential [[plant nutrition|plant nutrient]].

==History==
The word ''boron'' was coined from ''[[borax]]'', the mineral from which it was isolated, by analogy with ''carbon'', which boron resembles chemically.
[[File:Sassolite.jpg|thumb|left|[[Sassolite]]]]

Borax in its mineral form (then known as tincal) first saw use as a glaze, beginning in [[China]] circa 300 AD. Some crude borax traveled westward, and was apparently mentioned by the alchemist [[Jabir ibn Hayyan]] around 700 AD. [[Marco Polo]] brought some glazes back to Italy in the 13th century. [[Georgius Agricola]], in around 1600, reported the use of borax as a flux in [[metallurgy]]. In 1777, [[boric acid]] was recognized in the hot springs ([[soffioni]]) near [[Florence]], Italy, at which point it became known as ''sal sedativum'', with ostensible medical benefits. The mineral was named [[sassolite]], after [[Sasso Pisano]] in Italy. Sasso was the main source of [[Europe]]an borax from 1827 to 1872, when [[United States|American]] sources replaced it.<ref name="borates">{{Cite book|pages=102; 385–386|title=Borates: handbook of deposits, processing, properties, and use| author = Garrett, Donald E.| publisher =Academic Press| date= 1998| isbn=978-0-12-276060-0}}</ref><ref name="boron">{{cite web| access-date = 2009-05-05| url = http://mysite.du.edu/~jcalvert/phys/boron.htm| title = Boron| publisher = University of Denver| author = Calvert, J. B.| archive-date = 24 September 2018| archive-url = https://web.archive.org/web/20180924201219/http://mysite.du.edu/~jcalvert/phys/boron.htm| url-status = live}}</ref> Boron compounds were relatively rarely used until the late 1800s when [[Francis Marion Smith]]'s [[Pacific Coast Borax Company]] first popularized and produced them in volume at low cost.<ref>Hildebrand, G. H. (1982) "Borax Pioneer: Francis Marion Smith." San Diego: Howell-North Books. p. 267 {{ISBN|0-8310-7148-6}}</ref>

Boron was not recognized as an element until it was isolated by Sir [[Humphry Davy]]<ref name="Davy" /> and by [[Joseph Louis Gay-Lussac]] and [[Louis Jacques Thénard]].<ref name="Lussac" /> In 1808 Davy observed that electric current sent through a solution of borates produced a brown precipitate on one of the electrodes. In his subsequent experiments, he used potassium to [[Reducing agent|reduce]] boric acid instead of [[electrolysis]]. He produced enough boron to confirm a new element and named it ''boracium''.<ref name="Davy" /> Gay-Lussac and Thénard used iron to reduce boric acid at high temperatures. By oxidizing boron with air, they showed that boric acid is its oxidation product.<ref name="Lussac" /><ref name="weeks">{{Cite book|last = Weeks|first = Mary Elvira|author-link = Mary Elvira Weeks|date = 1933|title = The Discovery of the Elements|publisher = Journal of Chemical Education|location = Easton, PA|chapter = XII. Other Elements Isolated with the Aid of Potassium and Sodium: Beryllium, Boron, Silicon and Aluminum|page = 156|chapter-url = https://books.google.com/books?id=SJIk9BPdNWcC&pg=PA156|isbn = 978-0-7661-3872-8|access-date = 5 January 2016|archive-date = 20 September 2014|archive-url = https://web.archive.org/web/20140920165959/http://books.google.com/books?id=SJIk9BPdNWcC&pg=PA156|url-status = live}}</ref> [[Jöns Jacob Berzelius]] identified it as an element in 1824.<ref>Berzelius produced boron by reducing a borofluoride salt; specifically, by heating potassium borofluoride with potassium metal. See: Berzelius, J. (1824) [https://books.google.com/books?id=pJlPAAAAYAAJ&pg=PA46 "Undersökning af flusspatssyran och dess märkvärdigaste föreningar"] {{Webarchive|url=https://web.archive.org/web/20160613221714/https://books.google.com/books?id=pJlPAAAAYAAJ&pg=PA46 |date=13 June 2016 }} (Part 2) (Investigation of hydrofluoric acid and of its most noteworthy compounds), ''Kongliga Vetenskaps-Academiens Handlingar'' (Proceedings of the Royal Science Academy), vol. 12, pp. 46–98; see especially pp. 88ff. Reprinted in German as: Berzelius, J. J. (1824) [http://gallica.bnf.fr/ark:/12148/bpt6k150878/f123.image.r=Annalen%20der%20Physic.langEN "Untersuchungen über die Flußspathsäure und deren merkwürdigste Verbindungen"], Poggendorff's ''Annalen der Physik und Chemie'', vol. 78, pages 113–150.</ref> Pure boron was arguably first produced by the American chemist Ezekiel Weintraub in 1909.<ref>{{cite journal|author = Weintraub, Ezekiel|date = 1910|title = Preparation and properties of pure boron|journal = Transactions of the American Electrochemical Society|volume = 16|pages = 165–184|url = https://books.google.com/books?id=e5USAAAAYAAJ&pg=PA165|access-date = 5 January 2016|archive-date = 9 May 2016|archive-url = https://web.archive.org/web/20160509105556/https://books.google.com/books?id=e5USAAAAYAAJ&pg=PA165|url-status = live}}</ref><ref name="Laubengayer" /><ref>{{Cite journal| author = Borchert, W. | author2 = Dietz, W. | author3 = Koelker, H.| title = Crystal Growth of Beta–Rhombohedrical Boron| journal= Zeitschrift für Angewandte Physik|date=1970|page=277| volume=29|osti=4098583}}</ref>

==Preparation of elemental boron in the laboratory==
The earliest routes to elemental boron involved the reduction of [[boric oxide]] with metals such as [[magnesium]] or [[aluminium]]. However, the product is almost always contaminated with [[boride]]s of those metals.{{citation needed|date=June 2021}} Pure boron can be prepared by reducing volatile boron halides with [[hydrogen]] at high temperatures. Ultrapure boron for use in the semiconductor industry is produced by the decomposition of [[diborane]] at high temperatures and then further purified by the [[zone melting]] or [[Czochralski process]]es.<ref name="berger">{{Cite book| title = Semiconductor materials| author = Berger, L. I.| publisher = CRC Press| date = 1996| isbn = 978-0-8493-8912-2| pages = [https://archive.org/details/semiconductormat0000berg/page/37 37–43]| url = https://archive.org/details/semiconductormat0000berg/page/37}}</ref>

The production of boron compounds does not involve the formation of elemental boron, but exploits the convenient availability of borates.

==Characteristics==

===Allotropes===
{{Main|Allotropes of boron}}
[[File:Bor 1.jpg|upright=0.5|thumb|right|Boron chunks]]
Boron is similar to [[carbon]] in its capability to form stable [[covalent bond|covalently bonded]] molecular networks. Even nominally disordered ([[Amorphous solid|amorphous]]) boron contains regular boron [[icosahedra]] which are bonded randomly to each other without [[long-range order]].<ref>{{Cite journal| title = A neutron diffraction study of amorphous boron|author = Delaplane, R.G.|journal = Journal of Non-Crystalline Solids|volume = 104| date =1988| pages = 249–252| doi = 10.1016/0022-3093(88)90395-X| last2 = Dahlborg| first2 = U.| last3 = Graneli| first3 = B.| last4 = Fischer| first4 = P.| last5 = Lundstrom| first5 = T.| issue = 2–3|bibcode = 1988JNCS..104..249D}}</ref><ref>{{Cite journal| title = A neutron diffraction study of amorphous boron using a pulsed source| author = R.G. Delaplane| journal = Journal of Non-Crystalline Solids|volume = 106| issue = 1–3| date =1988| pages = 66–69| doi = 10.1016/0022-3093(88)90229-3| last2 = Dahlborg| first2 = U.| last3 = Howells| first3 = W.| last4 = Lundstrom| first4 = T.|bibcode = 1988JNCS..106...66D}}</ref> [[Crystalline]] boron is a very hard, black material with a melting point of above 2000&nbsp;°C. It forms four major [[allotrope]]s: α-rhombohedral <ref>[https://log-web.de/chemie/Start.htm?name=B_aRh&lang=en visualisisation of the crystal structure]</ref> and β-rhombohedral <ref>[https://log-web.de/chemie/Start.htm?name=B_bRh2&lang=en visualisisation of the crystal structure]</ref> (α-R and β-R), γ-orthorhombic <ref>[https://log-web.de/chemie/Start.htm?name=B_24&lang=en visualisisation of the crystal structure]</ref> (γ) and β-tetragonal <ref>[https://log-web.de/chemie/Start.htm?name=B_tetra2&lang=en visualisisation of the crystal structure]</ref> (β-T). All four phases are stable at [[Standard conditions for temperature and pressure|ambient conditions]], and β-rhombohedral is the most common and stable. An α-tetragonal phase also exists (α-T), but is very difficult to produce without significant contamination. Most of the phases are based on B<sub>12</sub> icosahedra, but the γ phase can be described as a [[Sodium chloride|rocksalt]]-type arrangement of the icosahedra and B<sub>2</sub> atomic pairs.<ref name="oganov">{{Cite journal| author = Oganov, A.R.| author2 = Chen J.| author3 = Gatti C.| author4 = Ma Y.-M.| author5 = Yu T.| author6 = Liu Z.| author7 = Glass C.W.| author8 = Ma Y.-Z.| author9 = Kurakevych O.O.| author10 = Solozhenko V.L.| title = Ionic high-pressure form of elemental boron| doi = 10.1038/nature07736| journal = Nature| volume = 457| date = 2009| pages = 863–867| url = http://mysbfiles.stonybrook.edu/~aoganov/files/Boron-Nature-2009.pdf| pmid = 19182772| issue = 7231| bibcode = 2009Natur.457..863O| arxiv = 0911.3192| s2cid = 4412568| access-date = 9 May 2009| archive-date = 28 July 2018| archive-url = https://web.archive.org/web/20180728071425/https://mysbfiles.stonybrook.edu/~aoganov/files/Boron-Nature-2009.pdf| url-status = live}}</ref> It can be produced by compressing other boron phases to 12–20&nbsp;GPa and heating to 1500–1800&nbsp;°C; it remains stable after releasing the temperature and pressure. The β-T phase is produced at similar pressures, but higher temperatures of 1800–2200&nbsp;°C. The α-T and β-T phases might coexist at ambient conditions, with the β-T phase being the more stable.<ref name="oganov" /><ref>{{Cite journal|title = Thermodynamic stability of boron: The role of defects and zero point motion|author = van Setten M.J.|author2 = Uijttewaal M.A.|author3 = de Wijs G.A.|author4 = de Groot R.A.|journal = J. Am. Chem. Soc.|volume = 129|date = 2007|pages = 2458–2465|doi = 10.1021/ja0631246|pmid = 17295480|issue = 9| s2cid=961904 |url = https://pure.rug.nl/ws/files/2796591/2007JAmChemSocvSetten.pdf|access-date = 14 July 2019|archive-date = 15 April 2021|archive-url = https://web.archive.org/web/20210415015024/https://pure.rug.nl/ws/files/2796591/2007JAmChemSocvSetten.pdf|url-status = dead}}</ref><ref name="widom">{{Cite journal| title = Symmetry-broken crystal structure of elemental boron at low temperature|author = Widom M.|author2 = Mihalkovic M.|journal = Phys. Rev. B|volume = 77|date = 2008|page= 064113| doi = 10.1103/PhysRevB.77.064113| issue = 6|bibcode = 2008PhRvB..77f4113W|arxiv = 0712.0530 |s2cid = 27321818}}</ref> Compressing boron above 160&nbsp;GPa produces a boron phase with an as yet unknown structure, and this phase is a [[superconductor]] at temperatures below 6–12 K.<ref>{{Cite journal| author = Eremets, M. I.|title = Superconductivity in Boron| doi=10.1126/science.1062286 |journal = Science |volume = 293 |date = 2001 | pmid = 11452118| last2 = Struzhkin| first2 = V. V.| last3 = Mao| first3 = H.| last4 = Hemley| first4 = R. J.| issue = 5528|bibcode = 2001Sci...293..272E| pages = 272–4|s2cid = 23001035}}</ref><ref>{{Cite journal |display-authors=etal |author= Zarechnaya E. Y. |title = Superhard semiconducting optically transparent high pressure phase of boron |journal = Physical Review Letters |volume = 102 |date = 2009 |issue= 18 | pages =185501–185501–4|doi= 10.1103/PhysRevLett.102.185501 |pmid= 19518885 |bibcode= 2009PhRvL.102r5501Z }} structure determination</ref>[[Borospherene]] ([[fullerene]]-like B<sub>40</sub> molecules) and [[borophene]] (proposed [[graphene]]-like structure) were described in 2014.

{| class="wikitable" style="margin:auto; text-align:center;"
|-
!Boron phase
!α-R
!β-R
!γ
!β-T
|-
!Symmetry
|[[Rhombohedral]]
|[[Rhombohedral]]
|[[Orthorhombic]]
|[[Tetragonal]]
|-
!Atoms/unit cell<ref name="oganov" />
|12
|~105
|28
|
|-
!Density (g/cm<sup>3</sup>)<ref name="wentorf">{{Cite journal|first = R. H. Jr|last = Wentorf|title = Boron: Another Form|journal = Science|volume = 147 |date=1 January 1965|doi = 10.1126/science.147.3653.49|pmid = 17799779|issue = 3653 |pages = 49–50|bibcode = 1965Sci...147...49W|s2cid = 20539654}}</ref><ref>{{Cite journal| author = Hoard, J. L.| author2 = Sullenger, D. B.| author3 = Kennard, C. H. L.| author4 = Hughes, R. E.|title = The structure analysis of β-rhombohedral boron| journal = J. Solid State Chem. |volume = 1 |pages =268–277 |date =1970 |doi = 10.1016/0022-4596(70)90022-8| issue = 2|bibcode = 1970JSSCh...1..268H}}</ref><ref>{{Cite journal| title = Electron Deformation Density in Rhombohedral a-Boron| author = Will, G.| author2 = Kiefer, B.| journal = Zeitschrift für Anorganische und Allgemeine Chemie| volume = 627 |page = 2100| date = 2001| doi=10.1002/1521-3749(200109)627:9<2100::AID-ZAAC2100>3.0.CO;2-G| issue = 9}}</ref><ref name="talley">{{Cite journal| author = Talley, C. P.| author2 = LaPlaca, S.| author3 = Post, B.|title = A new polymorph of boron| journal= Acta Crystallogr. |volume =13| pages = 271–272| date = 1960| doi =10.1107/S0365110X60000613| issue = 3| bibcode = 1960AcCry..13..271T| doi-access = }}</ref>
|2.46
|2.35
|2.52
|2.36
|-
![[Vickers hardness]] (GPa)<ref>{{Cite journal|first1 =V. L.|last1 = Solozhenko|title = On the hardness of a new boron phase, orthorhombic γ-B<sub>28</sub>|journal = Journal of Superhard Materials|volume = 30|date = 2008|pages = 428–429|doi =10.3103/S1063457608060117|last2 =Kurakevych|first2 =O. O.|last3 =Oganov|first3 =A. R.|issue =6|arxiv = 1101.2959|s2cid = 15066841}}</ref><ref name="prl" />
|42
|45
|50–58
|
|-
![[Bulk modulus]] (GPa)<ref name="prl">
{{cite journal|last1=Zarechnaya|first1=E. Yu.|last2=Dubrovinsky|first2=L.|last3=Dubrovinskaia|first3=N.|last4=Filinchuk|first4=Y.|last5=Chernyshov|first5=D.|last6=Dmitriev|first6=V.|last7=Miyajima|first7=N.|last8=El Goresy|first8=A.|last9=Braun|first9=H. F.|last10=van Smaalen|first10=S.|last11=Kantor|first11=I.|last12=Kantor|first12=A.|last13=Prakapenka|first13=V.|last14=Hanfland|first14=M.|last15=Mikhaylushkin|first15=A. S.|last16=Abrikosov|first16=I. A.|last17=Simak|first17=S. I.|title=Superhard Semiconducting Optically Transparent High Pressure Phase of Boron|date=2009|journal=Phys. Rev. Lett.|volume=102|issue=18|page=185501|doi=10.1103/PhysRevLett.102.185501|pmid=19518885|bibcode=2009PhRvL.102r5501Z|display-authors=8}}</ref><ref>{{Cite journal|author =Nelmes, R. J. |title= Neutron- and x-ray-diffraction measurements of the bulk modulus of boron| journal = Phys. Rev. B| volume =47 |pages =7668–7673| date =1993| doi =10.1103/PhysRevB.47.7668|pmid= 10004773|last2 =Loveday|first2 =J. S.|last3 =Allan|first3 =D. R.|last4 =Hull|first4 =S.|last5 =Hamel|first5 =G.|last6 =Grima|first6 =P.|last7 =Hull|first7 =S.|issue =13|bibcode = 1993PhRvB..47.7668N}}</ref>
|185
|224
|227
|
|-
![[Bandgap]] (eV)<ref name="prl" /><ref>{{Cite book| title = Landolt-Bornstein, New Series| editor= Madelung, O.|publisher = Springer-Verlag|place= Berlin| date =1983 |volume =17e}}</ref>
|2
|1.6
|2.1
|
|}

===Chemistry of the element===
{{Category see also|Boron compounds}}
Elemental boron is rare and poorly studied because the pure material is extremely difficult to prepare. Most studies of "boron" involve samples that contain small amounts of carbon. The chemical behavior of boron [[diagonal relationship|resembles]] that of [[silicon]] more than [[aluminium]]. Crystalline boron is chemically inert and resistant to attack by boiling [[hydrofluoric acid|hydrofluoric]] or [[hydrochloric acid]]. When finely divided, it is attacked slowly by hot concentrated [[hydrogen peroxide]], hot concentrated [[nitric acid]], hot [[sulfuric acid]] or hot mixture of sulfuric and [[chromic acid]]s.<ref name="Laubengayer">{{Cite journal|title = Boron. I. Preparation and Properties of Pure Crystalline Boron| doi =10.1021/ja01250a036|date =1943|first1 =A. W.|last1 = Laubengayer|journal =Journal of the American Chemical Society|volume =65|pages =1924–1931|last2 = Hurd|first2 = D. T.|last3 = Newkirk|first3 = A. E.|last4 = Hoard|first4 = J. L.|issue = 10}}</ref>

The rate of oxidation of boron depends on the crystallinity, particle size, purity and temperature. Boron does not react with air at room temperature, but at higher temperatures it burns to form [[boron trioxide]]:<ref name="HollemanAF">{{cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 814–864|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first1 = Arnold F.|last1 = Holleman|last2=Wiberg|first2=Egon|last3=Wiberg|first3=Nils|chapter = Bor| language = de}}</ref>
:4 B + 3 O<sub>2</sub> → 2 B<sub>2</sub>O<sub>3</sub>
[[File:Tetraborate-xtal-3D-balls.png|thumb|right|Ball-and-stick model of tetraborate anion, [B<sub>4</sub>O<sub>5</sub>(OH)<sub>4</sub>]<sup>2−</sup>, as it occurs in crystalline borax, Na<sub>2</sub>[B<sub>4</sub>O<sub>5</sub>(OH)<sub>4</sub>]·8H<sub>2</sub>O. Boron atoms are pink, with bridging oxygens in red, and four hydroxyl hydrogens in white. Note two borons are trigonally bonded sp<sup>2</sup> with no formal charge, while the other two borons are tetrahedrally bonded sp<sup>3</sup>, each carrying a formal charge of −1. The oxidation state of all borons is III. This mixture of boron coordination numbers and formal charges is characteristic of natural boron minerals.]]

Boron undergoes halogenation to give trihalides; for example,
:2 B + 3 Br<sub>2</sub> → 2 BBr<sub>3</sub>
The trichloride in practice is usually made from the oxide.<ref name="HollemanAF" />

====Atomic structure====
Boron is the lightest element having an [[electron]] in a [[atomic orbital|p-orbital]] in its ground state. Unlike most other [[p-block|p-elements]], it rarely obeys the [[octet rule]] and usually places only six electrons<ref name="Key">{{Cite web |url=https://opentextbc.ca/introductorychemistry/chapter/violations-of-the-octet-rule-2/ |title=Violations of the Octet Rule |last=Key |first=Jessie A. |date=2014-09-14 |website=Introductory Chemistry |access-date=2019-08-14 |archive-date=17 May 2019 |archive-url=https://web.archive.org/web/20190517090709/https://opentextbc.ca/introductorychemistry/chapter/violations-of-the-octet-rule-2/ |url-status=live }}</ref> (in three [[molecular orbital]]s) onto its [[valence shell]]. Boron is the prototype for the [[boron group]] (the [[group (periodic table)|IUPAC group]]&nbsp;13), although the other members of this group are metals and more typical p-elements (only aluminium to some extent shares boron's aversion to the octet rule).

Boron also has much lower [[electronegativity]] than subsequent [[period 2 element]]s.  For the latter, lithium salts are common e.g. [[lithium fluoride]], [[lithium hydroxide]], [[lithium amide]], and [[Organolithium|methyllithium]], but lithium boryllides are extraordinarily rare.<ref name="yasutomo">{{cite journal |author=Segawa Yasutomo, Yamashita Makoto, Nozaki Kyoko |year=2006 |title=Boryllithium: Isolation, Characterization, and Reactivity as a Boryl Anion |journal=[[Science (journal)|Science]] |volume=314 |issue=5796 |pages=113–115 |bibcode=2006Sci...314..113S |doi=10.1126/science.1131914 |pmid=17023656 |s2cid=21040230}}</ref><ref name="bethany">Bethany Halford ''Boron Attacks Electropositive element pressed into action as nucleophilic boryllithium''  ''[[Chemical & Engineering News]]'' 2006; Volume 84(41): 11 [http://pubs.acs.org/cen/news/84/i41/8441notw8.html Link]</ref>    Strong [[Base (chemistry)|bases]] do not [[Deprotonation|deprotonate]] a borohydride R<sub>2</sub>BH to the boryl anion R<sub>2</sub>B<sup>−</sup>, instead forming the octet-complete adduct R<sub>2</sub>HB-base.<ref name="dennis">''Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine''. Dennis G. Hall {{ISBN|3-527-30991-8}}</ref>

====Chemical compounds====
{{Main|boron compounds}}
[[File:Boron-trifluoride-pi-bonding-2D.png|upright=0.5|thumb|left|[[Boron trifluoride|Boron (III) trifluoride]] structure, showing "empty" boron p orbital in pi-type [[coordinate covalent bond]]s]]
In the most familiar compounds, boron has the formal oxidation state III. These include oxides, sulfides, nitrides, and halides.<ref name="HollemanAF" />

The trihalides adopt a planar trigonal structure. These compounds are [[Lewis acid]]s in that they readily form [[adduct]]s with electron-pair donors, which are called [[Lewis base]]s. For example, fluoride (F<sup>−</sup>) and [[boron trifluoride]] (BF<sub>3</sub>) combined to give the [[tetrafluoroborate]] anion, BF<sub>4</sub><sup>−</sup>. Boron trifluoride is used in the petrochemical industry as a catalyst. The halides react with water to form [[boric acid]].<ref name="HollemanAF" />

It is found in nature on Earth almost entirely as various oxides of B(III), often associated with other elements. More than one hundred [[borate mineral]]s contain boron in oxidation state +3. These minerals resemble silicates in some respect, although it is often found not only in a tetrahedral coordination with oxygen, but also in a trigonal planar configuration. Unlike silicates, boron minerals never contain it with coordination number greater than four. A typical motif is exemplified by the tetraborate anions of the common mineral [[borax]], shown at left. The formal negative charge of the tetrahedral borate center is balanced by metal cations in the minerals, such as the sodium (Na<sup>+</sup>) in borax.<ref name="HollemanAF" /> The tourmaline group of borate-silicates is also a very important boron-bearing mineral group, and a number of borosilicates are also known to exist naturally.<ref>{{Cite web|url=https://www.mindat.org/|title=Mindat.org - Mines, Minerals and More|website=mindat.org|access-date=4 August 2019|archive-date=22 April 2011|archive-url=https://web.archive.org/web/20110422205859/http://www.mindat.org/|url-status=live}}</ref>

=====Boranes=====
{{Main|Boranes}}
[[File:Deltahedral-borane-cluster-array-numbered-3D-bs-17.png|thumb|[[Ball-and-stick model]]s showing the structures of the boron skeletons of [[Boranes|borane]] [[Atom cluster|clusters]]. The structures can be rationalised by [[polyhedral skeletal electron pair theory]].<ref>{{cite journal|title=The significance and impact of Wade's rules |first=Alan J. |last=Welch |journal=Chem. Commun. |date=2013|volume=49 |issue=35 |pages=3615–3616 |doi=10.1039/C3CC00069A|pmid=23535980 }}</ref>]]

Boranes are chemical compounds of boron and hydrogen, with the generic formula of B<sub>x</sub>H<sub>y</sub>. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The parent member BH<sub>3</sub> is called borane, but it is known only in the gaseous state, and dimerises to form diborane, B<sub>2</sub>H<sub>6</sub>. The larger boranes all consist of boron clusters that are polyhedral, some of which exist as isomers. For example, isomers of B<sub>20</sub>H<sub>26</sub> are based on the fusion of two 10-atom clusters.

The most important boranes are diborane B<sub>2</sub>H<sub>6</sub> and two of its pyrolysis products, pentaborane B<sub>5</sub>H<sub>9</sub> and decaborane B<sub>10</sub>H<sub>14</sub>. A large number of anionic boron hydrides are known, e.g. [B<sub>12</sub>H<sub>12</sub>]<sup>2−</sup>.

The formal [[oxidation number]] in boranes is positive, and is based on the assumption that hydrogen is counted as −1 as in active metal hydrides. The mean oxidation number for the borons is then simply the ratio of hydrogen to boron in the molecule. For example, in diborane B<sub>2</sub>H<sub>6</sub>, the boron oxidation state is +3, but in decaborane B<sub>10</sub>H<sub>14</sub>, it is <sup>7</sup>/<sub>5</sub> or +1.4. In these compounds the oxidation state of boron is often not a whole number.

=====Boron nitrides=====
{{Main|Boron nitride}}

The [[boron nitride]]s are notable for the variety of structures that they adopt. They exhibit structures analogous to various [[allotropes of carbon]], including graphite, diamond, and nanotubes. In the diamond-like structure, called cubic boron nitride (tradename [[Borazon]]), boron atoms exist in the tetrahedral structure of carbon atoms in diamond, but one in every four B-N bonds can be viewed as a [[coordinate covalent bond]], wherein two electrons are donated by the nitrogen atom which acts as the [[Lewis base]] to a bond to the [[Lewis acid]]ic boron(III) centre. Cubic boron nitride, among other applications, is used as an abrasive, as it has a hardness comparable with diamond (the two substances are able to produce scratches on each other). In the BN compound analogue of graphite, hexagonal boron nitride (h-BN), the positively charged boron and negatively charged nitrogen atoms in each plane lie adjacent to the oppositely charged atom in the next plane. Consequently, graphite and h-BN have very different properties, although both are lubricants, as these planes slip past each other easily. However, h-BN is a relatively poor electrical and thermal conductor in the planar directions.<ref name="dkg">{{cite journal| title = Hexagonal Boron Nitride (hBN) – Applications from Metallurgy to Cosmetics| url = http://www.esk.com/uploads/tx_userjspresseveroeff/PR_0712_CFI_12-2007_Hexagonales-BN_e_01.pdf| author = Engler, M.| journal = Cfi/Ber. DKG| volume = 84| date = 2007| page = D25| issn = 0173-9913| access-date = 8 January 2012| archive-date = 13 June 2013| archive-url = https://web.archive.org/web/20130613174727/http://www.esk.com/uploads/tx_userjspresseveroeff/PR_0712_CFI_12-2007_Hexagonales-BN_e_01.pdf| url-status = live}}</ref><ref name="b1">{{cite book| author = Greim, Jochen| author2 = Schwetz, Karl A.| title = Ullmann's Encyclopedia of Industrial Chemistry| name-list-style = amp |publisher = Wiley-VCH: Weinheim |date = 2005 |doi = 10.1002/14356007.a04_295.pub2| chapter = Boron Carbide, Boron Nitride, and Metal Borides| isbn = 978-3527306732}}</ref>

=====Organoboron chemistry=====
{{Main|Organoboron chemistry}}
A large number of organoboron compounds are known and many are useful in [[organic synthesis]]. Many are produced from [[hydroboration–oxidation reaction|hydroboration]], which employs [[diborane]], B<sub>2</sub>H<sub>6</sub>, a simple [[borane]] chemical, or [[carboboration]]. Organoboron(III) compounds are usually tetrahedral or trigonal planar, for example, [[tetraphenylborate]], [B(C<sub>6</sub>H<sub>5</sub>)<sub>4</sub>]<sup>−</sup> vs. [[triphenylborane]], B(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>. However, multiple boron atoms reacting with each other have a tendency to form novel dodecahedral (12-sided) and icosahedral (20-sided) structures composed completely of boron atoms, or with varying numbers of carbon heteroatoms.

Organoboron chemicals have been employed in uses as diverse as [[boron carbide]] (see below), a complex very hard ceramic composed of boron-carbon cluster anions and cations, to [[carborane]]s, carbon-boron [[cluster chemistry]] compounds that can be halogenated to form reactive structures including [[carborane acid]], a [[superacid]]. As one example, carboranes form useful molecular moieties that add considerable amounts of boron to other biochemicals in order to synthesize boron-containing compounds for [[boron neutron capture therapy]] for cancer.

=====Compounds of B(I) and B(II)=====
As anticipated by its [[boron hydride|hydride cluster]]s, boron forms a variety of stable compounds with formal oxidation state less than three. [[Diboron tetrafluoride|B<sub>2</sub>F<sub>4</sub>]] and B<sub>4</sub>Cl<sub>4</sub> are well characterized.<ref>{{Greenwood&Earnshaw2nd}}</ref>
[[File:Magnesium-diboride-3D-balls.png|thumb|Ball-and-stick model of superconductor magnesium diboride. Boron atoms lie in hexagonal aromatic graphite-like layers, with a charge of −1 on each boron atom. Magnesium(II) ions lie between layers]]
Binary metal-boron compounds, the metal borides, contain boron in negative oxidation states. Illustrative is [[magnesium diboride]] (MgB<sub>2</sub>). Each boron atom has a formal −1 charge and magnesium is assigned a formal charge of +2. In this material, the boron centers are trigonal planar with an extra double bond for each boron, forming sheets akin to the carbon in [[graphite]]. However, unlike hexagonal boron nitride, which lacks electrons in the plane of the covalent atoms, the delocalized electrons in magnesium diboride allow it to conduct electricity similar to isoelectronic graphite. In 2001, this material was found to be a high-temperature [[superconductor]].<ref>{{cite journal |title = The Preparation and Structure of Magnesium Boride, MgB<sub>2</sub>|author = Jones, Morton E.|author2 = Marsh, Richard E.|name-list-style = amp |journal = Journal of the American Chemical Society|volume = 76 |issue = 5| pages = 1434–1436| date = 1954|doi = 10.1021/ja01634a089}}</ref><ref>{{cite journal|doi = 10.1063/1.1570770|url = http://cmp.physics.iastate.edu/canfield/pub/pt0303.pdf|title = Magnesium Diboride: Better Late than Never|date = 2003|last1 = Canfield|first1 = Paul C.|author-link2 = George Crabtree|last2 = Crabtree|first2 = George W.|journal = Physics Today|volume = 56|issue = 3|pages = 34–40|bibcode = 2003PhT....56c..34C|access-date = 8 January 2012|archive-date = 26 February 2012|archive-url = https://web.archive.org/web/20120226233637/http://cmp.physics.iastate.edu/canfield/pub/pt0303.pdf|url-status = live}}</ref> It is a superconductor under active development. A project at [[CERN]] to make MgB<sub>2</sub> cables has resulted in superconducting test cables able to carry 20,000 amperes for extremely high current distribution applications, such as the contemplated high luminosity version of the [[Large Hadron Collider]].<ref>{{Cite web|url=https://cds.cern.ch/journal/CERNBulletin/2014/16/News+Articles/1693853|title=Category "News+Articles" not found - CERN Document Server|website=cds.cern.ch|access-date=9 October 2020|archive-date=20 February 2022|archive-url=https://web.archive.org/web/20220220075257/https://cds.cern.ch/journal/CERNBulletin/2014/16/News+Articles/1693853|url-status=live}}</ref>

Certain other metal borides find specialized applications as hard materials for cutting tools.<ref>{{cite book|chapter-url = https://books.google.com/books?id=PvU-qbQJq7IC&pg=PA638|chapter = Titanium Diboride|pages = 638–639|title = Materials handbook: A concise desktop reference|isbn = 978-1-84628-668-1|author = Cardarelli, François|date = 2008| publisher=Springer |access-date = 5 January 2016|archive-date = 8 January 2017|archive-url = https://web.archive.org/web/20170108051112/https://books.google.com/books?id=PvU-qbQJq7IC&pg=PA638|url-status = live}}</ref> Often the boron in borides has fractional oxidation states, such as −1/3 in [[calcium hexaboride]] (CaB<sub>6</sub>).

From the structural perspective, the most distinctive chemical compounds of boron are the hydrides. Included in this series are the cluster compounds [[dodecaborate]] ({{chem|B|12|H|12|2-}}), [[decaborane]] (B<sub>10</sub>H<sub>14</sub>), and the [[carborane]]s such as C<sub>2</sub>B<sub>10</sub>H<sub>12</sub>. Characteristically such compounds contain boron with coordination numbers greater than four.<ref name="HollemanAF" />

===Isotopes===
{{Main|Isotopes of boron}}
Boron has two naturally occurring and stable [[isotope]]s, <sup>11</sup>B (80.1%) and <sup>10</sup>B (19.9%). The mass difference results in a wide range of δ<sup>11</sup>B values, which are defined as a fractional difference between the <sup>11</sup>B and <sup>10</sup>B and traditionally expressed in parts per thousand, in natural waters ranging from −16 to +59. There are 13 known isotopes of boron; the shortest-lived isotope is <sup>7</sup>B which decays through [[proton emission]] and [[alpha decay]] with a [[half-life]] of 3.5×10<sup>−22</sup> s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)<sub>3</sub> and [[tetrahydroxyborate|[B(OH)<sub>4</sub>]<sup>−</sup>]]. Boron isotopes are also fractionated during mineral crystallization, during H<sub>2</sub>O phase changes in [[hydrothermal]] systems, and during [[hydrothermal alteration]] of [[rock (geology)|rock]]. The latter effect results in preferential removal of the [<sup>10</sup>B(OH)<sub>4</sub>]<sup>−</sup> [[ion]] onto clays. It results in solutions enriched in <sup>11</sup>B(OH)<sub>3</sub> and therefore may be responsible for the large <sup>11</sup>B enrichment in seawater relative to both [[ocean]]ic crust and [[continent]]al crust; this difference may act as an [[isotopic signature]].<ref>{{Cite journal| first = S.|last = Barth| title = Boron isotopic analysis of natural fresh and saline waters by negative thermal ionization mass spectrometry| journal = Chemical Geology| volume = 143|date = 1997|pages = 255–261|doi = 10.1016/S0009-2541(97)00107-1| issue = 3–4|bibcode = 1997ChGeo.143..255B}}</ref>

The exotic <sup>17</sup>B exhibits a [[nuclear halo]], i.e. its radius is appreciably larger than that predicted by the [[semi-empirical mass formula|liquid drop model]].<ref>{{Cite journal| title = Two-body and three-body halo nuclei| first = Z.|last = Liu|journal = Science China Physics, Mechanics & Astronomy| volume = 46| date= 2003| page = 441|doi = 10.1360/03yw0027| issue = 4| doi-broken-date = 23 January 2024|bibcode = 2003ScChG..46..441L| s2cid = 121922481}}</ref>

The <sup>10</sup>B isotope is useful for capturing [[thermal neutron]]s (see [[neutron cross section#Typical cross sections]]). The [[nuclear power|nuclear industry]] enriches natural boron to nearly pure <sup>10</sup>B. The less-valuable by-product, depleted boron, is nearly pure <sup>11</sup>B.

====Commercial isotope enrichment====
Because of its high neutron cross-section, boron-10 is often used to control fission in nuclear reactors as a neutron-capturing substance.<ref>{{cite web|url=http://bibliothek.fzk.de/zb/berichte/FZKA6746.pdf |title=Results of the B4C Control Rod Test QUENCH-07 |website=Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft |date=2004 |author=Steinbrück, Martin |url-status=dead |archive-url=https://web.archive.org/web/20110719031833/http://bibliothek.fzk.de/zb/berichte/FZKA6746.pdf |archive-date=19 July 2011 }}</ref> Several industrial-scale enrichment processes have been developed; however, only the fractionated vacuum distillation of the [[dimethyl ether]] adduct of [[boron trifluoride]] (DME-BF<sub>3</sub>) and column chromatography of borates are being used.<ref>{{cite web|url = http://library.igcar.gov.in/html/Contents/IGCNewsletter/nl48/A2.htm|archive-url = https://web.archive.org/web/20081208114410/http://library.igcar.gov.in/html/Contents/IGCNewsletter/nl48/A2.htm|archive-date = 8 December 2008|title = Commissioning of Boron Enrichment Plant|publisher = Indira Gandhi Centre for Atomic Research|access-date = 2008-09-21}}</ref><ref>{{cite journal|doi = 10.1080/01496398608056140|title = Chromatographic Enrichment of 10B by Using Weak-Base Anion-Exchange Resin|date = 1986|last1 = Aida|first1 = Masao|last2 = Fujii|first2 = Yasuhiko|last3 = Okamoto|first3 = Makoto|journal = Separation Science and Technology|volume = 21|issue = 6|pages = 643–654}} showing an enrichment from 18% to above 94%.</ref>

====Enriched boron (boron-10)====
[[File:Neutroncrosssectionboron.png|right|upright=1.5|thumb|Neutron cross section of boron (top curve is for <sup>10</sup>B and bottom curve for <sup>11</sup>B)]]

Enriched boron or <sup>10</sup>B is used in both radiation shielding and is the primary nuclide used in [[neutron capture therapy of cancer]]. In the latter ("boron neutron capture therapy" or BNCT), a compound containing <sup>10</sup>B is incorporated into a pharmaceutical which is selectively taken up by a malignant tumor and tissues near it. The patient is then treated with a beam of low energy neutrons at a relatively low neutron radiation dose. The neutrons, however, trigger energetic and short-range secondary [[alpha particle]] and lithium-7 heavy ion radiation that are products of the boron-neutron [[nuclear reaction]], and this ion radiation additionally bombards the tumor, especially from inside the tumor cells.<ref>{{Cite journal|title =A Critical Assessment of Boron Neutron Capture Therapy: An Overview|journal = Journal of Neuro-Oncology|volume = 62|issue = 1|date = 2003|doi = 10.1023/A:1023262817500|pages = 1–5|first = Rolf F.|last = Barth|pmid = 12749698|s2cid = 31441665}}</ref><ref>{{Cite journal|journal = Radiation Research|pages = 1–18|volume =151| issue =1|date = 1999|title =The Radiation Biology of Boron Neutron Capture Therapy|first1 = Jeffrey A.|last1 = Coderre|doi = 10.2307/3579742|pmid = 9973079|last2 = Morris|first2 = G. M.|jstor = 3579742|bibcode = 1999RadR..151....1C}}</ref><ref>{{Cite journal|title = Boron Neutron Capture Therapy of Cancer|first =Rolf F.|last =Barth|journal = Cancer Research|volume = 50|pages = 1061–1070|date=1990|pmid = 2404588|issue = 4|author2 = S|author3 = F}}</ref><ref>{{cite web |url=http://www.pharmainfo.net/reviews/boron-neutron-capture-therapy-overview |title=Boron Neutron Capture Therapy – An Overview |publisher=Pharmainfo.net |date=22 August 2006 |access-date=2011-11-07 |archive-url=https://web.archive.org/web/20110723014243/http://www.pharmainfo.net/reviews/boron-neutron-capture-therapy-overview |archive-date=23 July 2011 |url-status=dead }}</ref>

In nuclear reactors, <sup>10</sup>B is used for reactivity control and in [[scram|emergency shutdown systems]]. It can serve either function in the form of [[borosilicate]] [[control rods]] or as [[boric acid]]. In [[pressurized water reactor]]s, <sup>10</sup>B [[boric acid]] is added to the reactor coolant after the plant is shut down for refueling. When the plant is started up again, the boric acid is slowly filtered out over many months as [[fissile]] material is used up and the fuel becomes less reactive.<ref name="reactor">{{Cite book|last1 = Duderstadt|first1 = James J.|last2= Hamilton|first2=Louis J.| title = Nuclear Reactor Analysis|url = https://archive.org/details/nuclearreactoran00dude|url-access = limited|publisher = Wiley-Interscience|date = 1976|isbn = 978-0-471-22363-4|page= [https://archive.org/details/nuclearreactoran00dude/page/n267 245]}}</ref>

In future crewed interplanetary spacecraft, <sup>10</sup>B has a theoretical role as structural material (as boron fibers or [[boron nitride nanotube|BN nanotube]] material) which would also serve a special role in the radiation shield. One of the difficulties in dealing with [[cosmic rays]], which are mostly high energy protons, is that some secondary radiation from interaction of cosmic rays and spacecraft materials is high energy [[spallation]] neutrons. Such neutrons can be moderated by materials high in light elements, such as [[polyethylene]], but the moderated neutrons continue to be a radiation hazard unless actively absorbed in the shielding. Among light elements that absorb thermal neutrons, <sup>6</sup>Li and <sup>10</sup>B appear as potential spacecraft structural materials which serve both for mechanical reinforcement and radiation protection.<ref>{{cite journal|author= Yu, J. | title = Isotopically Enriched 10BN Nanotubes| journal=Advanced Materials| volume =18|date =2006| url =http://www.rsphysse.anu.edu.au/nanotube/pdf/B10Nnanotubes.pdf | archive-url =https://web.archive.org/web/20080803192520/http://www.rsphysse.anu.edu.au/nanotube/pdf/B10Nnanotubes.pdf | archive-date =3 August 2008 |doi =10.1002/adma.200600231|pages= 2157–2160|last2= Chen|first2= Y.|last3= Elliman|first3= R. G.|last4= Petravic|first4= M.|issue= 16| bibcode = 2006AdM....18.2157Y| s2cid = 135710601}}</ref>

====Depleted boron (boron-11)====

===== Radiation-hardened semiconductors =====
[[Cosmic radiation]] will produce secondary neutrons if it hits spacecraft structures. Those neutrons will be captured in <sup>10</sup>B, if it is present in the spacecraft's [[semiconductor]]s, producing a [[gamma ray]], an [[alpha particle]], and a [[lithium]] ion. Those resultant decay products may then irradiate nearby semiconductor "chip" structures, causing data loss (bit flipping, or [[single event upset]]). In [[radiation-hardened]] semiconductor designs, one countermeasure is to use ''depleted boron'', which is greatly enriched in <sup>11</sup>B and contains almost no <sup>10</sup>B. This is useful because <sup>11</sup>B is largely immune to radiation damage. Depleted boron is a byproduct of the [[nuclear power|nuclear industry]] (see above).<ref name="reactor" />

===== Proton-boron fusion =====
{{main | Proton-boron fusion }}
<sup>11</sup>B is also a candidate as a fuel for [[aneutronic fusion]]. When struck by a proton with energy of about 500&nbsp;k[[electronvolt|eV]], it produces three alpha particles and 8.7&nbsp;MeV of energy. Most other fusion reactions involving hydrogen and helium produce penetrating neutron radiation, which weakens reactor structures and induces long-term radioactivity, thereby endangering operating personnel. The [[alpha particles]] from <sup>11</sup>B fusion can be turned directly into electric power, and all radiation stops as soon as the reactor is turned off.<ref>{{Cite journal|first = W. M.|last = Nevins|title = A Review of Confinement Requirements for Advanced Fuels|journal = Journal of Fusion Energy|volume = 17|issue = 1|date = 1998|doi = 10.1023/A:1022513215080|pages = 25–32|bibcode = 1998JFuE...17...25N |s2cid = 118229833}}</ref>

====NMR spectroscopy====
Both <sup>10</sup>B and <sup>11</sup>B possess [[nuclear spin]]. The nuclear spin of <sup>10</sup>B is 3 and that of <sup>11</sup>B is {{sfrac|3|2}}. These isotopes are, therefore, of use in [[nuclear magnetic resonance]] spectroscopy; and spectrometers specially adapted to detecting the boron-11 nuclei are available commercially. The <sup>10</sup>B and <sup>11</sup>B nuclei also cause splitting in the [[resonances]] of attached nuclei.<ref>{{cite web| title = Boron NMR| url = http://rmn.iqfr.csic.es/guide/eNMR/chem/B.html| access-date= 2009-05-05|publisher = BRUKER Biospin| archive-url = https://web.archive.org/web/20090502140944/http://rmn.iqfr.csic.es/guide/eNMR/chem/B.html <!--Added by H3llBot-->| archive-date = 2 May 2009}}</ref>

===Occurrence===
{{main|Borate minerals}}
{{Category see also|Borate minerals}}
[[File:ulexita br.jpg|thumb|left|A fragment of ulexite]]
[[File:Borax crystals.jpg|thumb|right|Borax crystals]]
Boron is rare in the Universe and solar system due to trace formation in the [[Big Bang]] and in stars. It is formed in minor amounts in [[cosmic ray spallation]] [[nucleosynthesis]] and may be found uncombined in [[cosmic dust]] and [[meteoroid]] materials.

In the high oxygen environment of Earth, boron is always found fully oxidized to borate. Boron does not appear on Earth in elemental form. Extremely small traces of elemental boron were detected in Lunar regolith.<ref>Mokhov, A.V., Kartashov, P.M., Gornostaeva, T.A., Asadulin, A.A., Bogatikov, O.A., 2013: Complex nanospherulites of zinc oxide and native amorphous boron in the Lunar regolith from Mare Crisium. Doklady Earth Sciences 448(1) 61-63</ref><ref>Mindat, http://www.mindat.org/min-43412.html {{Webarchive|url=https://web.archive.org/web/20160306043016/http://www.mindat.org/min-43412.html |date=6 March 2016 }}</ref>

Although boron is a relatively rare element in the Earth's crust, representing only 0.001% of the crust mass, it can be highly concentrated by the action of water, in which many borates are soluble.
It is found naturally combined in compounds such as [[borax]] and [[boric acid]] (sometimes found in [[volcano|volcanic]] spring waters). About a hundred [[borate minerals]] are known.

On 5 September 2017, scientists reported that the [[Curiosity (rover)|''Curiosity'' rover]] detected boron, an essential ingredient for [[life]] on [[Earth]], on the planet [[Mars]]. Such a finding, along with previous discoveries that water may have been present on ancient Mars, further supports the possible early habitability of [[Gale (crater)|Gale Crater]] on Mars.<ref name="GPL-20170905">{{cite journal |author=Gasda, Patrick J. |display-authors=et al |title=In situ detection of boron by ChemCam on Mars |date=5 September 2017 |journal=[[Geophysical Research Letters]] |volume=44 |issue=17 |pages=8739–8748 |doi=10.1002/2017GL074480 |bibcode=2017GeoRL..44.8739G |url=https://curis.ku.dk/ws/files/185473087/Gasda_et_al_2017_Geophysical_Research_Letters.pdf |doi-access=free |access-date=28 August 2019 |archive-date=28 August 2019 |archive-url=https://web.archive.org/web/20190828182218/https://curis.ku.dk/ws/files/185473087/Gasda_et_al_2017_Geophysical_Research_Letters.pdf |url-status=live }}</ref><ref name="GZ-20170906">{{cite news |last=Paoletta |first=Rae |title=Curiosity Has Discovered Something That Raises More Questions About Life on Mars |url=https://gizmodo.com/curiosity-has-discovered-something-that-raises-more-que-1800879035 |date=6 September 2017 |work=[[Gizmodo]] |access-date=6 September 2017 |archive-date=4 August 2019 |archive-url=https://web.archive.org/web/20190804164517/https://gizmodo.com/curiosity-has-discovered-something-that-raises-more-que-1800879035 |url-status=live }}</ref>

==Production==
Economically important sources of boron are the minerals [[colemanite]], rasorite ([[kernite]]), [[ulexite]] and [[tincal]]. Together these constitute 90% of mined boron-containing ore. The largest global borax deposits known, many still untapped, are in Central and Western [[Turkey]], including the provinces of [[Eskişehir]], [[Kütahya]] and [[Balıkesir]].<ref>{{Cite journal|last = Kistler|first = R. B.|date = 1994|url = http://kisi.deu.edu.tr/cahit.helvaci/Boron.pdf|title = Boron and Borates|journal = Industrial Minerals and Rocks|edition = 6th|pages = 171–186|access-date = 20 September 2008|archive-date = 4 June 2016|archive-url = https://web.archive.org/web/20160604063540/http://kisi.deu.edu.tr/cahit.helvaci/Boron.pdf|url-status = dead}}</ref><ref>{{Cite journal|journal = Mineral Processing and Extractive Metallurgy Review|volume = 9|issue = 1–4|date = 1992|pages = 245–254|doi = 10.1080/08827509208952709|title = Mining and Processing of Borates in Turkey|author = Zbayolu, G.|author2 = Poslu, K.|bibcode = 1992MPEMR...9..245O}}</ref><ref>{{Cite journal|title = Boron Minerals in Turkey, Their Application Areas and Importance for the Country's Economy|first1 = Y.|last1 = Kar|journal = Minerals & Energy&nbsp;– Raw Materials Report|date = 2006|volume = 20|issue = 3–4|pages = 2–10|doi = 10.1080/14041040500504293|last2 = Şen|first2 = Nejdet|last3 = Demİrbaş|first3 = Ayhan}}</ref> Global proven boron mineral mining reserves exceed one billion metric tonnes, against a yearly production of about four million tonnes.<ref>[http://content.yudu.com/Library/A1vi5r/AsianCeramicsFeb12/resources/62.htm Global reserves chart] {{Webarchive|url=https://web.archive.org/web/20141031121512/http://content.yudu.com/Library/A1vi5r/AsianCeramicsFeb12/resources/62.htm |date=31 October 2014 }}. Retrieved 14 August 2014.</ref>

[[Turkey]] and the United States are the largest producers of boron products. Turkey produces about half of the global yearly demand, through [[Eti Mine Works]] ({{lang-tr|Eti Maden İşletmeleri}}) a [[Turkey|Turkish]] [[state-owned company|state-owned]] [[mining industry|mining]] and [[chemical industry|chemicals]] company focusing on boron products. It holds a [[government monopoly]] on the mining of [[borate minerals]] in Turkey, which possesses 72% of the world's known deposits.<ref>{{cite news|author=Şebnem Önder|author2=Ayşe Eda Biçer|author3=Işıl Selen Denemeç|name-list-style=amp|title=Are certain minerals still under state monopoly?|url=http://www.cakmak.av.tr/articles/Mining_Metals/Are%20Certain%20Minerals%20Still%20Under%20State%20Monopoly.pdf|access-date=21 December 2013|newspaper=Mining Turkey|date=September 2013|archive-date=3 March 2016|archive-url=https://web.archive.org/web/20160303170324/http://www.cakmak.av.tr/articles/Mining_Metals/Are%20Certain%20Minerals%20Still%20Under%20State%20Monopoly.pdf|url-status=dead}}</ref> In 2012, it held a 47% [[market share|share]] of production of global borate minerals, ahead of its main competitor, [[Rio Tinto Group]].<ref>{{cite web | url=http://www.easpd.eu/sites/default/files/sites/default/files/EVENTS/Conference2013/presentation_ayfer_atabey.pdf | title=Turkey as the global leader in boron export and production | publisher=European Association of Service Providers for Persons with Disabilities Annual Conference 2013 | access-date=18 December 2013 | archive-date=3 March 2016 | archive-url=https://web.archive.org/web/20160303170457/http://www.easpd.eu/sites/default/files/sites/default/files/EVENTS/Conference2013/presentation_ayfer_atabey.pdf | url-status=dead }}</ref>

Almost a quarter (23%) of global boron production comes from the single [[Rio Tinto Borax Mine]] (also known as the U.S. Borax Boron Mine) {{coord|35|2|34.447|N|117|40|45.412|W|type:landmark_globe:earth_region:US-CA|name=Rio Tinto Borax Mine|display=inline}} near [[Boron, California]].<ref>{{cite web |url=http://ludb.clui.org/ex/i/CA4982/ |title=U.S. Borax Boron Mine |website=The Center for Land Use Interpretation, Ludb.clui.org |access-date=2013-04-26 |archive-url=https://web.archive.org/web/20120211220543/http://ludb.clui.org/ex/i/CA4982 |archive-date=11 February 2012 |url-status=dead }}</ref><ref>{{cite web |url=http://www.riotinto.com/ourproducts/218_our_companies_4438.asp |title=Boras |publisher=Rio Tinto |date=10 April 2012 |access-date=2013-04-26 |url-status=dead |archive-url=https://archive.today/20120918084003/http://www.riotinto.com/ourproducts/218_our_companies_4438.asp |archive-date=18 September 2012 }}</ref>

===Market trend===
The average cost of crystalline elemental boron is US$5/g.<ref>{{cite web|url = http://www.rareearth.org/boron_properties.htm|publisher = Los Alamos National Laboratory|title = Boron Properties|access-date = 2008-09-18|archive-date = 26 September 2018|archive-url = https://web.archive.org/web/20180926224305/http://www.rareearth.org/boron_properties.htm|url-status = dead}}</ref> Elemental boron is chiefly used in making boron fibers, where it is deposited by [[chemical vapor deposition]] on a [[tungsten]] core (see below). Boron fibers are used in lightweight composite applications, such as high strength tapes. This use is a very small fraction of total boron use. Boron is introduced into semiconductors as boron compounds, by ion implantation.

Estimated global consumption of boron (almost entirely as boron compounds) was about 4&nbsp;million tonnes of B<sub>2</sub>O<sub>3</sub> in 2012. As compounds such as borax and kernite its cost was US$377/tonne in 2019.<ref>{{cite web |title=BORON |url=https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-boron.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-boron.pdf |archive-date=2022-10-09 |url-status=live |publisher=USGS |access-date=27 July 2022}}</ref> Boron mining and refining capacities are considered to be adequate to meet expected levels of growth through the next decade.

The form in which boron is consumed has changed in recent years. The use of ores like [[colemanite]] has declined following concerns over [[arsenic]] content. Consumers have moved toward the use of refined borates and boric acid that have a lower pollutant content.

Increasing demand for boric acid has led a number of producers to invest in additional capacity. Turkey's state-owned [[Eti Mine Works]] opened a new boric acid plant with the production capacity of 100,000 tonnes per year at [[Emet]] in 2003. [[Rio Tinto Group]] increased the capacity of its boron plant from 260,000 tonnes per year in 2003 to 310,000 tonnes per year by May 2005, with plans to grow this to 366,000 tonnes per year in 2006. Chinese boron producers have been unable to meet rapidly growing demand for high quality borates. This has led to imports of sodium tetraborate ([[borax]]) growing by a hundredfold between 2000 and 2005 and boric acid imports increasing by 28% per year over the same period.<ref name="roskill">{{Cite book| title = The Economics of Boron| edition = 11th| date = 2006| isbn = 978-0-86214-516-3| publisher = Roskill Information Services, Ltd.}}</ref><ref>{{cite web| url = http://www.ceramicindustry.com/Articles/Cover_Story/4b0b7a6ed1cb8010VgnVCM100000f932a8c0____| access-date = 2009-05-05| title = Raw and Manufactured Materials 2006 Overview| archive-url = https://web.archive.org/web/20110708133327/http://www.ceramicindustry.com/Articles/Cover_Story/4b0b7a6ed1cb8010VgnVCM100000f932a8c0____| archive-date = 8 July 2011| url-status = dead}}</ref>

The rise in global demand has been driven by high growth rates in [[glass fiber]], [[fiberglass]] and [[borosilicate]] glassware production. A rapid increase in the manufacture of reinforcement-grade boron-containing fiberglass in Asia, has offset the development of boron-free reinforcement-grade fiberglass in Europe and the US. The recent rises in energy prices may lead to greater use of insulation-grade fiberglass, with consequent growth in the boron consumption. Roskill Consulting Group forecasts that world demand for boron will grow by 3.4% per year to reach 21 million tonnes by 2010. The highest growth in demand is expected to be in Asia where demand could rise by an average 5.7% per year.<ref name="roskill" /><ref>{{cite web|url = http://www.roskill.com/reports/boron|archive-url = https://web.archive.org/web/20031004160834/http://www.roskill.com/reports/boron|url-status = dead|archive-date = 2003-10-04|title = Roskill reports: boron|publisher = Roskill|access-date = 2009-05-05}}</ref>

==Applications==
{{see also|Boron deficiency (plant disorder)|Boron deficiency (medicine)}}
Nearly all boron ore extracted from the Earth is destined for refinement into [[boric acid]] and [[sodium tetraborate|sodium tetraborate pentahydrate]]. In the United States, 70% of the boron is used for the production of glass and ceramics.<ref>{{cite web| url = http://minerals.usgs.gov/minerals/pubs/commodity/boron/| title = Boron: Statistics and Information| access-date = 2009-05-05| publisher = USGS| archive-date = 16 September 2008| archive-url = https://web.archive.org/web/20080916114142/http://minerals.usgs.gov/minerals/pubs/commodity/boron/| url-status = live}}</ref><ref name="CRC">{{Cite book| author = Hammond, C. R.| title = The Elements, in Handbook of Chemistry and Physics| edition = 81st| publisher = CRC press| isbn = 978-0-8493-0485-9| date = 2004| url = https://archive.org/details/crchandbookofche81lide}}</ref>
The major global industrial-scale use of boron compounds (about 46% of end-use) is in production of [[glass fiber]] for boron-containing insulating and structural [[fiberglass]]es, especially in Asia. Boron is added to the glass as borax pentahydrate or boron oxide, to influence the strength or fluxing qualities of the glass fibers.<ref>[http://www.etimineusa.com/en/applications-fiberglass-and-specialty-glass] {{Webarchive|url=https://web.archive.org/web/20141006081049/http://www.etimineusa.com/en/applications-fiberglass-and-specialty-glass|date=6 October 2014}} Discussion of various types of boron addition to glass fibers in fiberglass. Retrieved 14 August 2014.</ref> Another 10% of global boron production is for [[borosilicate glass]] as used in high strength glassware. About 15% of global boron is used in boron ceramics, including super-hard materials discussed below. Agriculture consumes 11% of global boron production, and bleaches and detergents about 6%.<ref>[http://www.webstreetangels.com/MiningPresentations/0-940-Orhan-YILMAZ.pdf Global end use of boron in 2011] {{Webarchive|url=https://web.archive.org/web/20160422002657/http://www.webstreetangels.com/MiningPresentations/0-940-Orhan-YILMAZ.pdf |date=22 April 2016 }}. Retrieved 14 August 2014</ref>

===Elemental boron fiber===
[[Boron fiber]]s (boron filaments) are high-strength, lightweight materials that are used chiefly for advanced [[aerospace]] structures as a component of [[composite material]]s, as well as limited production consumer and sporting goods such as [[golf club (equipment)|golf clubs]] and [[fishing rod]]s.<ref>{{cite web|title = Selected Mechanical and Physical Properties of Boron Filaments|date = 1966|first = H. W.|last = Herring|publisher = NASA|access-date = 2008-09-20|url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660005941_1966005941.pdf|archive-date = 22 February 2014|archive-url = https://web.archive.org/web/20140222135127/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660005941_1966005941.pdf|url-status = live}}</ref><ref>{{Cite journal|title = Fracture behaviour of boron filaments|first = G. K.|last = Layden| journal = Journal of Materials Science|volume = 8|issue = 11|date = 1973|pages = 1581–1589| doi = 10.1007/BF00754893|bibcode=1973JMatS...8.1581L|s2cid = 136959123}}</ref> The fibers can be produced by [[chemical vapor deposition]] of boron on a [[tungsten]] filament.<ref name="USGS-YB-B-2008">{{cite web|publisher = [[United States Geological Survey]]|url = http://minerals.usgs.gov/minerals/pubs/commodity/boron/myb1-2006-boron.pdf|first = Dennis S.|last = Kostick|date = 2006|title = Mineral Yearbook: Boron|access-date = 2008-09-20|archive-date = 20 September 2008|archive-url = https://web.archive.org/web/20080920072528/http://minerals.usgs.gov/minerals/pubs/commodity/boron/myb1-2006-boron.pdf|url-status = live}}</ref><ref>{{Cite journal|title = Inorganic Fibers—A Literature Review|first = Theodore F.|last = Cooke|journal = Journal of the American Ceramic Society|volume = 74|issue = 12|pages = 2959–2978|doi = 10.1111/j.1151-2916.1991.tb04289.x|date = 1991}}</ref>

Boron fibers and sub-millimeter sized crystalline boron springs are produced by [[laser]]-assisted [[Laser chemical vapor deposition|chemical vapor deposition]]. Translation of the focused laser beam allows production of even complex helical structures. Such structures show good mechanical properties ([[elastic modulus]] 450&nbsp;GPa, fracture strain 3.7%, fracture stress 17&nbsp;GPa) and can be applied as reinforcement of ceramics or in [[microelectromechanical systems|micromechanical systems]].<ref>{{Cite journal| title = Microfabrication of three-dimensional boron structures by laser chemical processing| journal = Journal of Applied Physics|volume = 72| pages = 5956–5963|date =1992| doi = 10.1063/1.351904|first1 = S.|last1 = Johansson| last2 = Schweitz| first2 = Jan-Åke| last3 = Westberg| first3 = Helena| last4 = Boman| first4 = Mats| issue = 12|bibcode = 1992JAP....72.5956J}}</ref>

=== Boronated fiberglass ===
{{Main|Fiberglass}}
Fiberglass is a [[fiber reinforced plastic|fiber reinforced polymer]] made of [[plastic]] reinforced by [[glass fiber]]s, commonly woven into a mat. The glass fibers used in the material are made of various types of glass depending upon the fiberglass use. These glasses all contain silica or silicate, with varying amounts of oxides of calcium, magnesium, and sometimes boron. The boron is present as borosilicate, borax, or boron oxide, and is added to increase the strength of the glass, or as a fluxing agent to decrease the melting temperature of [[silica]], which is too high to be easily worked in its pure form to make glass fibers.

The highly boronated glasses used in fiberglass are E-glass (named for "Electrical" use, but now the most common fiberglass for general use). E-glass is alumino-borosilicate glass with less than 1% w/w alkali oxides, mainly used for glass-reinforced plastics. Other common high-boron glasses include C-glass, an alkali-lime glass with high boron oxide content, used for glass staple fibers and insulation, and D-glass, a [[borosilicate glass]], named for its low dielectric constant.<ref name="ullmann1">
{{Cite book
|author = E. Fitzer
|chapter = Fibers, 5. Synthetic Inorganic
|title = Ullmann's Encyclopedia of Industrial Chemistry
|display-authors=etal
|doi=10.1002/14356007.a11_001 |year = 2000
|isbn = 978-3527306732
}}</ref>

Not all fiberglasses contain boron, but on a global scale, most of the fiberglass used does contain it. Because of the ubiquitous use of fiberglass in construction and insulation, boron-containing fiberglasses consume half the global production of boron, and are the single largest commercial boron market.

===Borosilicate glass===
{{Main|Borosilicate glass}}
[[File:Schott Duran glassware.jpg|thumb|upright|Borosilicate glassware. Displayed are two beakers and a test tube.]]
[[Borosilicate glass]], which is typically 12–15% B<sub>2</sub>O<sub>3</sub>, 80% SiO<sub>2</sub>, and 2% Al<sub>2</sub>O<sub>3</sub>, has a low [[coefficient of thermal expansion]], giving it a good resistance to [[thermal shock]]. [[Schott AG]]'s "Duran" and [[Owens-Corning]]'s trademarked [[Pyrex]] are two major brand names for this glass, used both in [[laboratory glassware]] and in consumer [[cookware and bakeware]], chiefly for this resistance.<ref>{{Cite book| title = Schott guide to glass| url = https://archive.org/details/schottguidetogla00pfae| url-access = limited| first = H. G.|last = Pfaender| edition = 2nd| publisher = Springer| date = 1996| isbn = 978-0-412-62060-7| page = [https://archive.org/details/schottguidetogla00pfae/page/n131 122]}}</ref>

===Boron carbide ceramic===
{{main|Boron carbide}}
[[File:Borfig11a.png|thumb|left|upright=0.7|Unit cell of B<sub>4</sub>C. The green sphere and [[icosahedron|icosahedra]] consist of boron atoms, and black spheres are carbon atoms.<ref name="zhangyb28.5c4">{{cite journal|author=Zhang F X|author2=Xu F F|author3=Mori T|author4= Liu Q L|author5= Sato A|author6=Tanaka T|name-list-style=amp|date=2001|title=Crystal structure of new rare-earth boron-rich solids: REB28.5C4|journal=J. Alloys Compd.|volume=329|issue=1–2|pages=168–172|doi=10.1016/S0925-8388(01)01581-X}}</ref>]]
Several boron compounds are known for their extreme hardness and toughness.
[[Boron carbide]] is a ceramic material which is obtained by decomposing B<sub>2</sub>O<sub>3</sub> with carbon in an electric furnace:
:2 B<sub>2</sub>O<sub>3</sub> + 7 C → B<sub>4</sub>C + 6 CO

Boron carbide's structure is only approximately B<sub>4</sub>C, and it shows a clear depletion of carbon from this suggested stoichiometric ratio. This is due to its very complex structure. The substance can be seen with [[empirical formula]] B<sub>12</sub>C<sub>3</sub> (i.e., with B<sub>12</sub> dodecahedra being a motif), but with less carbon, as the suggested C<sub>3</sub> units are replaced with C-B-C chains, and some smaller (B<sub>6</sub>) octahedra are present as well (see the boron carbide article for structural analysis). The repeating polymer plus semi-crystalline structure of boron carbide gives it great structural strength per weight. It is used in [[vehicle armour|tank armor]], [[bulletproof vest]]s, and numerous other structural applications.

Boron carbide's ability to absorb neutrons without forming long-lived [[radionuclide]]s (especially when doped with extra boron-10) makes the material attractive as an [[Neutron poison#Burnable poison|absorbent for neutron radiation arising in nuclear power plants]].<ref>''[https://books.google.com/books?id=czTi4G6-Hq8C&dq=Carborundum+B4C+nuclear&pg=PA311 Fabrication and Evaluation of Urania-Alumina Fuel Elements and Boron Carbide Burnable Poison Elements] {{Webarchive|url=https://web.archive.org/web/20200727125515/https://books.google.com/books?id=czTi4G6-Hq8C&pg=PA311&lpg=PA311&dq=Carborundum+B4C+nuclear&source=bl&ots=Hc8OPQTMsR&sig=ACfU3U3Qe5IZtR99SlCfRuKKTpro3mIebw&hl=en&sa=X&ved=2ahUKEwiexfnYtPziAhUjxVkKHT6KCb4Q6AEwGHoECDAQAQ#v=onepage&q=Carborundum%20B4C%20nuclear&f=false |date=27 July 2020 }}'', Wisnyi, L. G. and Taylor, K.M., in "ASTM Special Technical Publication No. 276: Materials in Nuclear Applications", Committee E-10 Staff, [[American Society for Testing Materials]], 1959</ref> Nuclear applications of boron carbide include shielding, control rods and shut-down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.<ref>{{Cite book| first = Alan W.|last = Weimer| title = Carbide, Nitride and Boride Materials Synthesis and Processing| isbn = 978-0-412-54060-8| date = 1997| publisher = Chapman & Hall (London, New York)}}</ref>

===High-hardness and abrasive compounds===
{{Main|Superhard materials}}

{| class="wikitable" style="margin:30px; text-align:center; float:right;"
|+ Mechanical properties of BCN solids<ref>{{Cite journal| title = Ultimate Metastable Solubility of Boron in Diamond: Synthesis of Superhard Diamondlike BC5| first1 = V. L.| last1 = Solozhenko| journal = Phys. Rev. Lett.| volume = 102| page = 015506| date = 2009| doi = 10.1103/PhysRevLett.102.015506| last2 = Kurakevych| first2 = Oleksandr O.| last3 = Le Godec| first3 = Yann| last4 = Mezouar| first4 = Mohamed| last5 = Mezouar| first5 = Mohamed| pmid = 19257210| bibcode = 2009PhRvL.102a5506S| issue = 1| url = http://bib-pubdb1.desy.de/record/87949/files/GetPDFServlet.pdf| access-date = 23 October 2017| archive-date = 21 September 2017| archive-url = https://web.archive.org/web/20170921211544/http://bib-pubdb1.desy.de/record/87949/files/GetPDFServlet.pdf| url-status = live}}</ref> and ReB<sub>2</sub><ref name="qin">{{cite journal|last1=Qin|first1=Jiaqian|last2=He|first2=Duanwei|last3=Wang|first3=Jianghua|last4=Fang|first4=Leiming|last5=Lei|first5=Li|last6=Li|first6=Yongjun|last7=Hu|first7=Juan|last8=Kou|first8=Zili|last9=Bi|first9=Yan|title=Is Rhenium Diboride a Superhard Material?|date=2008|journal=Advanced Materials|volume=20|issue=24|pages=4780–4783|doi=10.1002/adma.200801471|bibcode=2008AdM....20.4780Q |s2cid=98327405}}</ref>
!Material
!Diamond
!cubic-BC<sub>2</sub>N
!cubic-BC<sub>5</sub>
!cubic-BN
!B<sub>4</sub>C
!ReB<sub>2</sub>
|-
![[Vickers hardness]] (GPa)
|115
|76
|71
|62
|38
|22
|-
![[Fracture toughness]] (MPa m<sup>1⁄2</sup>)
|5.3
|4.5
|9.5
|6.8
|3.5
|
|}
Boron carbide and cubic boron nitride powders are widely used as abrasives. [[Boron nitride]] is a material isoelectronic to [[carbon]]. Similar to carbon, it has both hexagonal (soft graphite-like h-BN) and cubic (hard, diamond-like c-BN) forms. h-BN is used as a high temperature component and lubricant. c-BN, also known under commercial name [[borazon]],<ref>{{Cite journal| first = R. H.|last = Wentorf|title = Cubic form of boron nitride|journal = J. Chem. Phys. |volume = 26|date = 1957| page = 956| doi = 10.1063/1.1745964| issue = 4|bibcode = 1957JChPh..26..956W}}</ref> is a superior abrasive. Its hardness is only slightly smaller than, but its chemical stability is superior, to that of diamond. [[Heterodiamond]] (also called BCN) is another diamond-like boron compound.

===Metallurgy===
{{main|Boron steel|Boriding}}
Boron is added to [[boron steel]]s at the level of a few parts per million to increase hardenability. Higher percentages are added to steels used in the [[nuclear industry]] due to boron's neutron absorption ability.

Boron can also increase the surface hardness of steels and alloys through [[boriding]]. Additionally metal [[boride]]s are used for coating tools through [[chemical vapor deposition]] or [[physical vapor deposition]]. Implantation of boron ions into metals and alloys, through [[ion implantation]] or [[ion beam deposition]], results in a spectacular increase in surface resistance and microhardness. Laser alloying has also been successfully used for the same purpose. These borides are an alternative to diamond coated tools, and their (treated) surfaces have similar properties to those of the bulk boride.<ref>{{Cite book| title = Materials Science of Carbides, Nitrides and Borides| url = https://archive.org/details/materialsscience00andr| url-access = limited| author = Gogotsi, Y. G.| author2 = Andrievski, R.A.| name-list-style = amp| publisher = Springer| date = 1999| isbn = 978-0-7923-5707-0| pages = [https://archive.org/details/materialsscience00andr/page/n279 270]}}</ref>

For example, [[rhenium diboride]] can be produced at ambient pressures, but is rather expensive because of rhenium. The hardness of ReB<sub>2</sub> exhibits considerable [[anisotropy]] because of its hexagonal layered structure. Its value is comparable to that of [[tungsten carbide]], [[silicon carbide]], [[titanium diboride]] or [[zirconium diboride]].<ref name="qin" />
Similarly, AlMgB<sub>14</sub> + TiB<sub>2</sub> composites possess high hardness and wear resistance and are used in either bulk form or as coatings for components exposed to high temperatures and wear loads.<ref name="tib1">{{Cite journal|doi = 10.1016/j.stam.2007.06.009|title = Preparation of titanium diboride TiB<sub>2</sub> by spark plasma sintering at slow heating rate|date = 2007|last1 = Schmidt|first1 = Jürgen|journal = Science and Technology of Advanced Materials|volume = 8|pages = 376–382|last2 = Boehling|first2 = Marian|last3 = Burkhardt|first3 = Ulrich|last4 = Grin|first4 = Yuri|issue = 5|bibcode = 2007STAdM...8..376S|doi-access = free}}</ref>

===Detergent formulations and bleaching agents===
[[Borax]] is used in various household laundry and cleaning products,<ref>{{CPID|id=10|name=Sodium borate decahydrate (borax)}}</ref> including the "[[20 Mule Team Borax]]" laundry booster and "[[Boraxo]]" powdered hand soap. It is also present in some [[tooth bleaching]] formulas.<ref name="CRC" />

[[Sodium perborate]] serves as a source of [[active oxygen]] in many [[detergent]]s, [[laundry detergent]]s, [[cleaning product]]s, and laundry [[bleach (chemical)|bleaches]]. However, despite its name, "Borateem" laundry bleach no longer contains any boron compounds, using [[sodium percarbonate]] instead as a bleaching agent.<ref>{{cite journal|doi = 10.1351/pac197439040547|title = Industrial applications of boron compounds|date = 1974|last1 = Thompson|first1 = R.|journal = Pure and Applied Chemistry|volume = 39|issue = 4|page = 547|doi-access = free}}</ref>

===Insecticides===
Boric acid is used as an insecticide, notably against ants, fleas, and cockroaches.<ref name="Klotz 1994 1534–1536">{{Cite journal|title = Oral toxicity of boric acid and other boron compounds to immature cat fleas (Siphonaptera: Pulicidae)|first1 = J. H.|last1 = Klotz|journal = J. Econ. Entomol.|volume = 87|issue = 6|pages = 1534–1536|date = 1994|pmid = 7836612
|last2 = Moss
|first2 = J. I.
|last3 = Zhao
|first3 = R.
|last4 = Davis Jr.
|first4 = L. R.
|last5 = Patterson
|first5 = R. S.|doi = 10.1093/jee/87.6.1534}}</ref>

===Semiconductors===
Boron is a useful [[dopant]] for such semiconductors as [[silicon]], [[germanium]], and [[silicon carbide]]. Having one fewer valence electron than the host atom, it donates a [[electron hole|hole]] resulting in [[p-type semiconductor|p-type]] conductivity. Traditional method of introducing boron into semiconductors is via its [[atomic diffusion]] at high temperatures. This process uses either solid (B<sub>2</sub>O<sub>3</sub>), liquid (BBr<sub>3</sub>), or gaseous boron sources (B<sub>2</sub>H<sub>6</sub> or BF<sub>3</sub>). However, after the 1970s, it was mostly replaced by [[ion implantation]], which relies mostly on BF<sub>3</sub> as a boron source.<ref>{{Cite book|pages = [https://archive.org/details/fundamentalssemi00mayg/page/n66 51]–54|title = Fundamentals of semiconductor manufacturing and process control|url = https://archive.org/details/fundamentalssemi00mayg|url-access = limited|last1 = May|first1 = Gary S.|last2= Spanos|first2=Costas J.|publisher = John Wiley and Sons|date = 2006|isbn=978-0-471-78406-7}}</ref> Boron trichloride gas is also an important chemical in semiconductor industry, however, not for doping but rather for [[plasma etching]] of metals and their oxides.<ref>{{Cite book|pages = 39–60|title = Semiconductor industry: wafer fab exhaust management|first = J. Michael|last = Sherer|publisher = CRC Press|date = 2005|isbn = 978-1-57444-720-0}}</ref> [[Triethylborane]] is also injected into [[chemical vapor deposition|vapor deposition]] reactors as a boron source.<ref>{{Cite journal |last1=Jeong |first1=Hokyeong |last2=Kim |first2=Dong Yeong |last3=Kim |first3=Jaewon |last4=Moon |first4=Seokho |last5=Han |first5=Nam |last6=Lee |first6=Seung Hee |last7=Okello |first7=Odongo Francis Ngome |last8=Song |first8=Kyung |last9=Choi |first9=Si-Young |last10=Kim |first10=Jong Kyu |date=2019-04-05 |title=Wafer-scale and selective-area growth of high-quality hexagonal boron nitride on Ni(111) by metal-organic chemical vapor deposition |journal=Scientific Reports |language=en |volume=9 |issue=1 |pages=5736 |doi=10.1038/s41598-019-42236-4 |pmid=30952939 |pmc=6450880 |bibcode=2019NatSR...9.5736J |issn=2045-2322}}</ref> Examples are the plasma deposition of boron-containing hard carbon films, silicon nitride–boron nitride films, and for [[doping (semiconductor)|doping]] of [[diamond]] film with boron.<ref>{{Cite book |author=Zschech, Ehrenfried |url=https://archive.org/details/materialsforinfo0000euro/page/44 |title=Materials for information technology: devices, interconnects and packaging |author2=Whelan, Caroline |author3=Mikolajick, Thomas |date=2005 |publisher=Birkhäuser |isbn=978-1-85233-941-8 |page=44 |url-access=registration |name-list-style=amp}}</ref>

===Magnets===
Boron is a component of [[neodymium magnet]]s (Nd<sub>2</sub>Fe<sub>14</sub>B), which are among the strongest type of permanent magnet. These magnets are found in a variety of electromechanical and electronic devices, such as [[magnetic resonance imaging]] (MRI) medical imaging systems, in compact and relatively small motors and [[actuator]]s. As examples, computer HDDs (hard disk drives), CD (compact disk) and DVD (digital versatile disk) players rely on neodymium magnet motors to deliver intense rotary power in a remarkably compact package. In mobile phones 'Neo' magnets provide the magnetic field which allows tiny speakers to deliver appreciable audio power.<ref>{{Cite book|page=45|title=Permanent magnet materials and their application| first = Peter|last = Campbell| publisher =Cambridge University Press| date= 1996| isbn=978-0-521-56688-9}}</ref>

===Shielding and neutron absorber in nuclear reactors===
Boron shielding is used as a control for [[nuclear reactor]]s, taking advantage of its high [[Neutron cross section|cross-section]] for neutron capture.<ref>{{cite book|url = https://books.google.com/books?id=4GzRaq0rIEwC&pg=PA660|pages = 660–661|title = Physics for Radiation Protection: A Handbook|isbn = 978-3-527-61880-4|author = Martin, James E|date = 2008| publisher=John Wiley & Sons |access-date = 5 January 2016|archive-date = 3 June 2016|archive-url = https://web.archive.org/web/20160603212753/https://books.google.com/books?id=4GzRaq0rIEwC&pg=PA660|url-status = live}}</ref>

In [[pressurized water reactor]]s a variable concentration of boronic acid in the cooling water is used as a [[Neutron poison#Soluble poison|neutron poison]] to compensate the variable reactivity of the fuel. When new rods are inserted the concentration of boronic acid is maximal, and is reduced during the lifetime.<ref name="PastinaIsabey1999">{{cite journal| last1=Pastina|first1=B.|last2=Isabey|first2=J. |last3=Hickel|first3=B.|title=The influence of water chemistry on the radiolysis of the primary coolant water in pressurized water reactors |journal=Journal of Nuclear Materials|volume=264|issue=3|year=1999|pages=309–318 |issn=0022-3115|doi=10.1016/S0022-3115(98)00494-2 |bibcode=1999JNuM..264..309P}}</ref>

===Other nonmedical uses===
[[File:Apollo 15 launch.ogv|thumb|right|Launch of ''Apollo 15'' Saturn V rocket, using triethylborane ignitor]]
* Because of its distinctive green flame, amorphous boron is used in [[Flare|pyrotechnic flares]].<ref>{{Cite book|title = Pyrotechnic Chemistry|author = Kosanke, B. J.|publisher = Journal of Pyrotechnics|page = 419|date = 2004|isbn = 978-1-889526-15-7|display-authors=etal}}</ref>
* In the 1950s, there were several studies of the use of [[boranes]] as energy-increasing "[[Zip fuel]]" additives for jet fuel.
* [[Starch]] and [[casein]]-based adhesives contain sodium tetraborate decahydrate (Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>·10 H<sub>2</sub>O)
* Some anti-corrosion systems contain borax.<ref>{{cite web| url = http://chemicalland21.com/industrialchem/inorganic/BORAX%20DECAHYDRATE.htm| title = Borax Decahydrate| access-date = 2009-05-05| archive-date = 20 April 2009| archive-url = https://web.archive.org/web/20090420125306/http://chemicalland21.com/industrialchem/inorganic/BORAX%20DECAHYDRATE.htm| url-status = live}}</ref>
* Sodium borates are used as a [[flux (metallurgy)|flux]] for soldering silver and gold and with [[ammonium chloride]] for welding ferrous metals.<ref>{{Cite book| page = 56| title = The Science and Practice of Welding: Welding science and technology| author = Davies, A. C.|publisher = Cambridge University Press| date = 1992| isbn = 978-0-521-43565-9}}</ref> They are also fire retarding additives to plastics and rubber articles.<ref>{{Cite book| page = [https://archive.org/details/fireretardantmat00horr_765/page/n67 55]| title = Fire Retardant Materials| url = https://archive.org/details/fireretardantmat00horr_765| url-access = limited|author = Horrocks, A.R.|author2 = Price, D.|name-list-style = amp|date = 2001| publisher = Woodhead Publishing Ltd.| isbn = 978-1-85573-419-7}}</ref>
* [[Boric acid]] (also known as orthoboric acid) H<sub>3</sub>BO<sub>3</sub> is used in the production of textile fiberglass and [[flat panel display]]s<ref name="CRC" /><ref>{{Cite journal| title = Information technology and polymers. Flat panel display| author = Ide, F.| journal = Engineering Materials| volume = 51| page = 84| date = 2003| url = http://sciencelinks.jp/j-east/article/200311/000020031103A0287941.php| access-date = 28 May 2009| archive-url = https://web.archive.org/web/20120313115907/http://sciencelinks.jp/j-east/article/200311/000020031103A0287941.php| archive-date = 13 March 2012| url-status = dead}}</ref> and in many [[PVAc]]- and [[PVOH]]-based adhesives.
* [[Triethylborane]] is a substance which ignites the [[JP-7]] fuel of the [[Pratt & Whitney J58]] [[turbojet]]/[[ramjet]] engines powering the [[Lockheed Corporation|Lockheed]] [[SR-71 Blackbird]].<ref>{{cite web|url=http://www.marchfield.org/sr71a.htm |title=Lockheed SR-71 Blackbird |access-date=2009-05-05 |publisher=March Field Air Museum |url-status=dead |archive-url=https://web.archive.org/web/20000304181849/http://www.marchfield.org/sr71a.htm |archive-date=4 March 2000 }}</ref> It was also used to ignite the [[F-1 (rocket engine)|F-1 Engines]] on the [[Saturn V]] Rocket utilized by [[NASA]]'s [[Apollo program|Apollo]] and [[Skylab]] programs from 1967 until 1973. Today [[SpaceX]] uses it to ignite the engines on their [[Falcon 9]] rocket.<ref name="sfn20100602">[http://www.spaceflightnow.com/falcon9/001/status.html Mission Status Center, June 2, 2010, 1905 GMT] {{Webarchive|url=https://web.archive.org/web/20100530232910/http://www.spaceflightnow.com/tracking/index.html|date=30 May 2010}}, ''[[SpaceflightNow]]'', accessed 2010-06-02, Quotation: "The flanges will link the rocket with ground storage tanks containing liquid oxygen, kerosene fuel, helium, gaseous nitrogen and the first stage ignitor source called triethylaluminum-triethylborane, better known as TEA-TEB."</ref> Triethylborane is suitable for this because of its [[pyrophoric]] properties, especially the fact that it burns with a very high temperature.<ref>{{Cite book| page = 86| title = The Saturn V F-1 Engine: Powering Apollo into History| author = Young, A.| publisher = Springer| date = 2008| isbn = 978-0-387-09629-2}}</ref> Triethylborane is an industrial [[radical initiator|initiator]] in [[radical (chemistry)|radical]] reactions, where it is effective even at low temperatures.
* Borates are used as environmentally benign [[wood preservative]]s.<ref>{{Cite journal
| last1 = Carr | first1 = J. M.
| last2 = Duggan | first2 = P. J.
| last3 = Humphrey | first3 = D. G.
| last4 = Platts | first4 = J. A.
| last5 = Tyndall | first5 = E. M.
| title = Wood Protection Properties of Quaternary Ammonium Arylspiroborate Esters Derived from Naphthalene 2,3-Diol, 2,2'-Biphenol and 3-Hydroxy-2-naphthoic Acid
| doi = 10.1071/CH10132
| journal = Australian Journal of Chemistry
| volume = 63
| issue = 10
| pages = 1423
| year = 2010
| doi-access = free
}}</ref>

===Pharmaceutical and biological applications===
Boron plays a role in pharmaceutical and biological applications as it is found in various bacteria-produced antibiotics, such as [[boromycin]]s, [[aplasmomycin]]s, [[borophycin]]s, and [[tartrolon]]s. These antibiotics have shown inhibitory effects on certain bacteria, fungi, and protozoa growth. Boron is also being studied for its potential medicinal applications, including its incorporation into biologically active molecules for therapies like boron neutron capture therapy for brain tumors. Some boron-containing biomolecules may act as signaling molecules interacting with cell surfaces, suggesting a role in cellular communication.<ref name="pmid17991498">{{cite journal |vauthors=Rezanka T, Sigler K |title=Biologically active compounds of semi-metals |journal=Phytochemistry |volume=69 |issue=3 |pages=585–606 |date=February 2008 |pmid=17991498 |doi=10.1016/j.phytochem.2007.09.018 |bibcode=2008PChem..69..585R |url=}}</ref>

'''[[Boric acid]]''' has antiseptic, antifungal, and antiviral properties and, for these reasons, is applied as a water clarifier in swimming pool water treatment.<ref name="acid">{{cite web| url = http://chemicalland21.com/industrialchem/inorganic/BORIC%20ACID.htm| title = Boric acid| publisher = chemicalland21.com| access-date = 28 May 2009| archive-date = 3 June 2009| archive-url = https://web.archive.org/web/20090603032815/http://chemicalland21.com/industrialchem/inorganic/BORIC%20ACID.htm| url-status = live}}</ref> Mild solutions of boric acid have been used as eye antiseptics.

'''[[Bortezomib]]''' (marketed as '''Velcade''' and '''Cytomib'''). Boron appears as an active element in the organic pharmaceutical bortezomib, a new class of drug called the proteasome inhibitor, for treating myeloma and one form of lymphoma (it is currently in experimental trials against other types of lymphoma). The boron atom in bortezomib binds the catalytic site of the [[proteasome|26S proteasome]]<ref name="pmid17268529">{{cite journal |author=Bonvini P |author2= Zorzi E |author3=Basso G |author4=Rosolen A |title=Bortezomib-mediated 26S proteasome inhibition causes cell-cycle arrest and induces apoptosis in CD-30<sup>+</sup> anaplastic large cell lymphoma |journal=Leukemia |volume=21 |issue=4 |pages=838–42 |date=2007 |pmid=17268529 |doi=10.1038/sj.leu.2404528|s2cid= 23570446 |doi-access= }}</ref> with high affinity and specificity.
* A number of potential boronated pharmaceuticals using [[boron-10]], have been prepared for use in [[boron neutron capture therapy]] (BNCT).<ref>{{cite web |url=http://www.pharmainfo.net/reviews/boron-neutron-capture-therapy-overview |title=Overview of neutron capture therapy pharmaceuticals |publisher=Pharmainfo.net |date=22 August 2006 |access-date=2013-04-26 |archive-url=https://web.archive.org/web/20110723014243/http://www.pharmainfo.net/reviews/boron-neutron-capture-therapy-overview |archive-date=23 July 2011 |url-status=dead }}</ref>
* Some boron compounds show promise in treating [[arthritis]], though none have as yet been generally approved for the purpose.<ref>{{Cite journal|title = Boron and Arthritis: The Results of a Double-blind Pilot Study|first1 = Richard L.|last1 = Travers|journal = Journal of Nutritional Medicine|volume = 1|issue = 2|pages = 127–132|date = 1990|doi = 10.3109/13590849009003147|last2 = Rennie|first2 = George|last3 = Newnham|first3 = Rex}}</ref>

'''[[Tavaborole]]''' (marketed as '''Kerydin''') is an [[Aminoacyl tRNA synthetase]] inhibitor which is used to treat toenail fungus. It gained FDA approval in July 2014.<ref name="Thompson">{{cite web|url=http://www.ashp.org/menu/News/PharmacyNews/NewsArticle.aspx?Id=4077|title=FDA Approves Boron-based Drug to Treat Toenail Fungal Infections|last=Thompson|first=Cheryl|date=8 July 2014|publisher=ashp|access-date=7 October 2015|archive-date=8 December 2015|archive-url=https://web.archive.org/web/20151208160134/http://www.ashp.org/menu/News/PharmacyNews/NewsArticle.aspx?Id=4077|url-status=live}}</ref>

Dioxaborolane chemistry enables radioactive [[fluoride]] ([[Fluorine-18|<sup>18</sup>F]]) labeling of [[Antibody|antibodies]] or [[red blood cell]]s, which allows for [[positron emission tomography]] (PET) imaging of [[cancer]]<ref>{{Cite journal|last1=Rodriguez|first1=Erik A.|last2=Wang|first2=Ye|last3=Crisp|first3=Jessica L.|last4=Vera|first4=David R.|last5=Tsien|first5=Roger Y.|last6=Ting|first6=Richard|date=2016-04-27|title=New Dioxaborolane Chemistry Enables [18F]-Positron-Emitting, Fluorescent [18F]-Multimodality Biomolecule Generation from the Solid Phase|journal=Bioconjugate Chemistry|language=EN|volume=27|issue=5|pages=1390–1399|doi=10.1021/acs.bioconjchem.6b00164|pmc=4916912|pmid=27064381}}</ref> and [[hemorrhages]],<ref>{{Cite journal|last1=Wang|first1=Ye|last2=An|first2=Fei-Fei|last3=Chan|first3=Mark|last4=Friedman|first4=Beth|last5=Rodriguez|first5=Erik A.|last6=Tsien|first6=Roger Y.|last7=Aras|first7=Omer|last8=Ting|first8=Richard|date=2017-01-05|title=18F-positron-emitting/fluorescent labeled erythrocytes allow imaging of internal hemorrhage in a murine intracranial hemorrhage model|journal=Journal of Cerebral Blood Flow & Metabolism|volume=37|issue=3|pages=776–786|language=en|doi=10.1177/0271678x16682510|pmid=28054494|pmc=5363488}}</ref> respectively. A <u>H</u>uman-<u>D</u>erived, <u>G</u>enetic, <u>P</u>ositron-emitting and <u>F</u>luorescent (HD-GPF) reporter system uses a human protein, [[Glutamate carboxypeptidase II|PSMA]] and non-immunogenic, and a small molecule that is positron-emitting (boron bound <sup>[[Fluorine-18|18]]</sup>[[Fluorine-18|F]]) and fluorescence for dual modality PET and fluorescent imaging of genome modified cells, e.g. [[cancer]], [[CRISPR/Cas9-mediated genome editing|CRISPR/Cas9]], or [[CAR-T cell|CAR T]]-cells, in an entire mouse.<ref>{{Cite journal|last1=Guo|first1=Hua|last2=Harikrishna|first2=Kommidi|last3=Vedvyas|first3=Yogindra|last4=McCloskey|first4=Jaclyn E|last5=Zhang|first5=Weiqi|last6=Chen|first6=Nandi|last7=Nurili|first7=Fuad|last8=Wu|first8=Amy P|last9=Sayman|first9=Haluk B.|date=2019-05-23|title=A fluorescent, [ 18 F]-positron-emitting agent for imaging PMSA allows genetic reporting in adoptively-transferred, genetically-modified cells|journal=ACS Chemical Biology|volume=14|issue=7|pages=1449–1459|language=en|doi=10.1021/acschembio.9b00160|pmid=31120734|pmc=6775626|issn=1554-8929}}</ref> The dual-modality small molecule targeting [[Glutamate carboxypeptidase II|PSMA]] was tested in humans and found the location of primary and [[Metastasis|metastatic]] [[prostate cancer]], fluorescence-guided removal of cancer, and detects single cancer cells in tissue margins.<ref>{{Cite journal|last1=Aras|first1=Omer|last2=Demirdag|first2=Cetin|last3=Kommidi|first3=Harikrishna|last4=Guo|first4=Hua|last5=Pavlova|first5=Ina|last6=Aygun|first6=Aslan|last7=Karayel|first7=Emre|last8=Pehlivanoglu|first8=Hüseyin|last9=Yeyin|first9=Nami|last10=Kyprianou|first10=Natasha|last11=Chen|first11=Nandi|date=March 2021|title=Small Molecule, Multimodal [18F]-PET and Fluorescence Imaging Agent Targeting Prostate Specific Membrane Antigen: First-in-Human Study|journal=Clinical Genitourinary Cancer|volume=19|issue=5|language=en|pages=405–416|doi=10.1016/j.clgc.2021.03.011|pmid=33879400|pmc=8449790|doi-access=free}}</ref>

In neutron capture therapy (BNCT) for malignant brain tumors, boron is researched to be used for selectively targeting and destroying tumor cells. The goal is to deliver higher concentrations of the non-radioactive boron isotope (<sup>10</sup>B) to the tumor cells than to the surrounding normal tissues. When these <sup>10</sup>B-containing cells are irradiated with low-energy thermal neutrons, they undergo nuclear capture reactions, releasing high linear energy transfer (LET) particles such as α-particles and lithium-7 nuclei within a limited path length. These high-LET particles can destroy the adjacent tumor cells without causing significant harm to nearby normal cells. Boron acts as a selective agent due to its ability to absorb thermal neutrons and produce short-range physical effects primarily affecting the targeted tissue region. This binary approach allows for precise tumor cell killing while sparing healthy tissues. The effective delivery of boron involves administering boron compounds or carriers capable of accumulating selectively in tumor cells compared to surrounding tissue. BSH and BPA have been used clinically, but research continues to identify more optimal carriers. Accelerator-based neutron sources have also been developed recently as an alternative to reactor-based sources, leading to improved efficiency and enhanced clinical outcomes in BNCT. By employing the properties of boron isotopes and targeted irradiation techniques, BNCT offers a potential approach to treating malignant brain tumors by selectively killing cancer cells while minimizing the damage caused by traditional radiation therapies.<ref name="pmid32676954">{{cite journal |vauthors=Miyatake SI, Wanibuchi M, Hu N, Ono K |title=Boron neutron capture therapy for malignant brain tumors |journal=J Neurooncol |volume=149 |issue=1 |pages=1–11 |date=August 2020 |pmid=32676954 |doi=10.1007/s11060-020-03586-6 |hdl=2433/226821 |s2cid=220577322 |url=|hdl-access=free }}</ref>

BNCT has shown promising results in clinical trials for various other malignancies, including glioblastoma, head and neck cancer, cutaneous melanoma, hepatocellular carcinoma, lung cancer, and extramammary Paget's disease. The treatment involves a nuclear reaction between nonradioactive boron-10 isotope and low-energy thermal or high-energy epithermal neutrons to generate α particles and lithium nuclei that selectively destroy DNA in tumor cells. The primary challenge lies in developing efficient boron agents with higher content and specific targeting properties tailored for BNCT. Integration of tumor-targeting strategies with BNCT could potentially establish it as a practical personalized treatment option for different types of cancers. Ongoing research explores new boron compounds, optimization strategies, theranostic agents, and radiobiological advances to overcome limitations and cost-effectively improve patient outcomes.<ref name="pmid37728998">{{cite journal |vauthors=Luo T, Huang W, Chu F, Zhu T, Feng B, Huang S, Hou J, Zhu L, Zhu S, Zeng W |title=The Dawn of a New Era: Tumor-Targeting Boron Agents for Neutron Capture Therapy |journal=Mol Pharm |volume=20 |issue=10 |pages=4942–4970 |date=October 2023 |pmid=37728998 |doi=10.1021/acs.molpharmaceut.3c00701 |s2cid=262086894 |url=}}</ref><ref name="pmid37102375">{{cite journal |vauthors=Coghi P, Li J, Hosmane NS, Zhu Y |title=Next generation of boron neutron capture therapy (BNCT) agents for cancer treatment |journal=Med Res Rev |volume=43 |issue=5 |pages=1809–1830 |date=September 2023 |pmid=37102375 |doi=10.1002/med.21964 |s2cid=258355021 |url=}}</ref><ref name="pmid35240726">{{cite journal |vauthors=Takahara K, Miyatake SI, Azuma H, Shiroki R |title=Boron neutron capture therapy for urological cancers |journal=Int J Urol |volume=29 |issue=7 |pages=610–616 |date=July 2022 |pmid=35240726 |doi=10.1111/iju.14855 |s2cid=247229359 |url=}}</ref>

===Research areas===
[[Magnesium diboride]] is an important [[superconductivity|superconducting material]] with the transition temperature of 39&nbsp;K. MgB<sub>2</sub> wires are produced with the [[powder-in-tube]] process and applied in superconducting magnets.<ref>{{Cite journal|title = Magnesium Diboride: Better Late than Never|last1 = Canfield|first1 = Paul C.|journal = Physics Today|volume = 56|issue = 3|pages = 34–41|date = 2003|url = http://www.cmp.ameslab.gov/personnel/canfield/pub/pt0303.pdf|doi = 10.1063/1.1570770|last2 = Crabtree|first2 = George W.|bibcode = 2003PhT....56c..34C|access-date = 22 September 2008|archive-url = https://web.archive.org/web/20081217171956/http://www.cmp.ameslab.gov/personnel/canfield/pub/pt0303.pdf|archive-date = 17 December 2008|url-status = dead}}</ref><ref>{{Cite journal|journal = Physica C: Superconductivity|volume = 456 |date = 2007|doi = 10.1016/j.physc.2007.01.030|title = Development of ex situ processed MgB<sub>2</sub> wires and their applications to magnets|first1 = Valeria|last1 = Braccini|pages = 209–217|last2 = Nardelli|first2 = D.|last3 = Penco|first3 = R.|last4 = Grasso|first4 = G.|issue = 1–2|bibcode=2007PhyC..456..209B}}</ref>

Amorphous boron is used as a melting point depressant in nickel-chromium braze alloys.<ref>{{Cite journal
|title = Evaluation of transient liquid phase bonding between nickel-based superalloys
|first1 = Xiaowei|last1 = Wu|journal = Journal of Materials Science|volume = 36|issue = 6|pages = 1539–1546|date = 2001|doi = 10.1023/A:1017513200502
|last2 = Chandel
|first2 = R. S.
|last3 = Li
|first3 = Hang|bibcode = 2001JMatS..36.1539W |s2cid = 134252793}}</ref>

Hexagonal [[boron nitride]] forms atomically thin layers, which have been used to enhance the [[electron mobility]] in [[graphene]] devices.<ref>{{Cite journal
|first1 = C. R.
|last1 = Dean
|first2 = A. F.
|last2 = Young
|first3 = I.
|last3 = Meric
|first4 = C.
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|first5 = L.
|last5 = Wang
|first6 = S.
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|first7 = K.
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|first10 = K. L.
|last10 = Shepard
|first11 = J.
|last11 = Hone
|title=Boron nitride substrates for high-quality graphene electronics|date=2010|journal=Nature Nanotechnology|volume=5|issue=10|pages=722–726|doi=10.1038/nnano.2010.172|bibcode=2010NatNa...5..722D|pmid=20729834|arxiv=1005.4917|s2cid = 1493242
}}</ref><ref>{{Cite journal|journal = Applied Physics Letters|volume = 98 |date = 2010|doi = 10.1063/1.3599708|title = Boron nitride substrates for high mobility chemical vapor deposited graphene|first1 = W.|last1 = Gannett|first2 = W.|last2 = Regan| first3 = K.| last3 = Watanabe|first4 = T.|last4 = Taniguchi|first5 = M. F.| last5 = Crommie|first6 = A.|last6 = Zettl |author6-link=Alex Zettl| page = 242105|issue = 24|bibcode = 2011ApPhL..98x2105G|arxiv = 1105.4938 |s2cid = 94765088 }}</ref> It also forms nanotubular structures ([[boron nitride#Boron nitride nanotubes|BNNTs]]), which have high strength, high chemical stability, and high [[thermal conductivity]], among its list of desirable properties.<ref>{{Cite journal|journal = Physics Today|volume = 63 |date = 2010|doi = 10.1063/1.3518210|title = The physics of boron nitride nanotubes|first1 = Alex|last1 = Zettl|last2 = Cohen|first2 = Marvin|s2cid = 19773801 |pages = 34–38|issue = 11|bibcode = 2010PhT....63k..34C}}</ref>

Boron has multiple applications in [[nuclear fusion]] research. It is commonly used for conditioning the walls in [[fusion reactors]] by depositing boron coatings on plasma-facing components and walls to reduce the release of hydrogen and impurities from the surfaces.<ref>{{cite journal
| last1 = Winter | first1 = J.
| title = Wall conditioning in fusion devices and its influence on plasma performance
| journal = Plasma Phys. Control. Fusion
| volume = 38
| issue = 9
| pages = 1503–1542
| date = 1996
| url = http://juser.fz-juelich.de/record/860761/files/J%C3%BCl_3217_Winter.pdf
| doi = 10.1088/0741-3335/38/9/001| s2cid = 250792253
}}</ref> It is also being used for the dissipation of energy in the fusion plasma boundary to suppress excessive energy bursts and heat fluxes to the walls.<ref>{{cite journal
| last1 = Gilson | first1 = Eric P.
| last2 = Lee | first2 = H.H.
| display-authors = 1
| title = Wall conditioning and ELM mitigation with boron nitride powder injection in KSTAR
| journal = Nuclear Materials and Energy
| volume = 28
| pages = 101043
| date = 2021
| doi = 10.1016/j.nme.2021.101043| doi-access = free
| url = https://www.osti.gov/biblio/1822213
}}</ref><ref>{{cite journal
| last1 = Effenberg | first1 = Florian
| last2 = Bortolon | first2 = Alessandro
| display-authors = 1
| title = Mitigation of plasma–wall interactions with low-Z powders in DIII-D high confinement plasmas
| journal = Nucl. Fusion
| volume = 62
| pages = 106015
| date = 2022
| issue = 10
| url = https://iopscience.iop.org/article/10.1088/1741-4326/ac899d/
| doi = 10.1088/1741-4326/ac899d| arxiv = 2203.15204
| bibcode = 2022NucFu..62j6015E
| s2cid = 247778852
}}</ref>

==Biological role==
{{see also|Boron deficiency (plant disorder)}}
Boron is an essential plant [[nutrient]], required primarily for maintaining the integrity of cell walls. However, high soil concentrations of greater than 1.0&nbsp;[[parts per million|ppm]] lead to marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8&nbsp;ppm produce these same symptoms in plants that are particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of soil boron, will show at least some symptoms of boron toxicity when soil boron content is greater than 1.8&nbsp;ppm. When this content exceeds 2.0&nbsp;ppm, few plants will perform well and some may not survive.<ref>{{Cite news|url = http://info.ag.uidaho.edu/Resources/PDFs/CIS1085.pdf|archive-url = https://web.archive.org/web/20091001005107/http://info.ag.uidaho.edu/Resources/PDFs/CIS1085.pdf|archive-date = 1 October 2009| publisher = University of Idaho| title = Essential Plant Micronutrients. Boron in Idaho| first = R. L.|last = Mahler| access-date= 2009-05-05}}</ref><ref>{{cite web|title = Functions of Boron in Plant Nutrition|url = http://www.borax.com/agriculture/files/an203.pdf|archive-url = https://web.archive.org/web/20090320175602/http://www.borax.com/agriculture/files/an203.pdf|archive-date = 20 March 2009|publisher = U.S. Borax Inc. }}</ref><ref>{{Cite journal|title = Functions of Boron in Plant Nutrition|first1 = Dale G.|last1 = Blevins|journal = Annual Review of Plant Physiology and Plant Molecular Biology|volume = 49|pages = 481–500|date = 1998|doi = 10.1146/annurev.arplant.49.1.481|pmid = 15012243|last2 = Lukaszewski|first2 = K. M.}}</ref>

It is thought that boron plays several essential roles in animals, including humans, but the exact physiological role is poorly understood.<ref>{{cite web|url = http://www.pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/bor_0040.shtml|title = Boron|access-date = 2008-09-18|publisher = PDRhealth |archive-url = https://web.archive.org/web/20071011101928/http://pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/bor_0040.shtml|archive-date=11 October 2007 }}</ref><ref name="utrace">{{Cite journal|title = Ultratrace elements in nutrition: Current knowledge and speculation| first = Forrest H.|last = Nielsen|journal =The Journal of Trace Elements in Experimental Medicine|volume = 11| issue = 2–3 |pages =251–274| doi = 10.1002/(SICI)1520-670X(1998)11:2/3<251::AID-JTRA15>3.0.CO;2-Q|date = 1998}}</ref> A small human trial published in 1987 reported on postmenopausal women first made boron deficient and then repleted with 3&nbsp;mg/day. Boron supplementation markedly reduced urinary calcium excretion and elevated the serum concentrations of 17 beta-estradiol and testosterone.<ref>{{cite journal |vauthors=Nielsen FH, Hunt CD, Mullen LM, Hunt JR |title=Effect of dietary boron on mineral, estrogen, and testosterone metabolism in postmenopausal women |journal=FASEB J. |volume=1 |issue=5 |pages=394–7 |year=1987 |pmid=3678698 |doi= 10.1096/fasebj.1.5.3678698|doi-access=free |s2cid=93497977 }}</ref>

Boron is not classified as an essential human nutrient because research has not established a clear biological function for boron.<ref name="pmid31639188">{{cite journal |vauthors=Nielsen FH, Eckhert CD |title=Boron |journal=Adv Nutr |volume=11 |issue=2 |pages=461–462 |date=March 2020 |pmid=31639188 |pmc=7442337 |doi=10.1093/advances/nmz110 |url=}}</ref><ref name="bhp">{{Cite web|url=https://ods.od.nih.gov/factsheets/Boron-HealthProfessional/|title=Office of Dietary Supplements - Boron|website=ods.od.nih.gov}}</ref> Still, studies suggest that boron may exert beneficial effects on reproduction and development, [[calcium metabolism]], [[Ossification|bone formation]], brain function, [[insulin]] and energy substrate metabolism, immunity, and [[steroid]] [[hormone]] (including [[estrogen]]) and [[vitamin D]] function, among other functions.<ref name="pmid26770156">{{cite journal |vauthors=Pizzorno L |title=Nothing Boring About Boron |journal=Integr Med (Encinitas) |volume=14 |issue=4 |pages=35–48 |date=August 2015 |pmid=26770156 |pmc=4712861 |doi= |url=}}</ref><ref name="bhp" /> The U.S. [[Food and Nutrition Board]] (FNB) found the existing data insufficient to derive a [[Recommended Dietary Allowance]] (RDA), [[Adequate Intake]] (AI), or [[Estimated Average Requirement]] (EAR) for boron.<ref name="pmid31639188" /><ref name="bhp" /> The U.S. [[Food and Drug Administration]] (FDA) has not established a Daily Value for boron for food and dietary supplement labeling purposes.<ref name="pmid31639188" /> While low boron status can be detrimental to health, probably increasing the risk of [[osteoporosis]], poor immune function, and cognitive decline; high boron levels are associated with cell damage and toxicity.<ref name="pmid29546541">{{cite journal |vauthors=Khaliq H, Juming Z, Ke-Mei P |title=The Physiological Role of Boron on Health |journal=Biol Trace Elem Res |volume=186 |issue=1 |pages=31–51 |date=November 2018 |pmid=29546541 |doi=10.1007/s12011-018-1284-3 |s2cid=255445828 |url=}}</ref> The exact mechanism by which boron exerts its physiological effects is not fully understood, but may involve interactions with [[adenosine monophosphate]] (ADP) and [[S-Adenosyl methionine|S-adenosyl methionine]] (SAM-e), two compounds involved in important cellular functions. Furthermore, boron appears to inhibit cyclic [[Adenosine diphosphate ribose|ADP-ribose]], thereby affecting the release of calcium ions from the [[endoplasmic reticulum]] and affecting various biological processes.<ref name="pmid29546541" /> Some studies suggest that boron may reduce levels of [[inflammatory biomarker]]s.<ref name="pmid26770156" />

In humans, boron is usually consumed with food that contains boron, such as fruits, [[Leaf vegetable|leafy vegetables]], and [[Nut (fruit)|nuts]].<ref name="pmid31639188"/> Foods that are particularly rich in boron include [[avocado]]s, dried fruits such as [[raisin]]s, [[peanut]]s, [[pecan]]s, [[prune]] juice, [[grape]] juice, [[wine]] and [[chocolate]] powder.<ref name="pmid26770156"/><ref name="pmid31639188"/> According to 2-day food records from the respondents to the [[National Health and Nutrition Examination Survey|Third National Health and Nutrition Examination Survey]] (NHANES III), adult dietary intake was recorded at 0.9 to 1.4&nbsp;mg/day.<ref name="nationalacademies">Boron. IN: [https://www.nap.edu/read/10026/chapter/15 Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Copper] {{Webarchive|url=https://web.archive.org/web/20170922174144/https://www.nap.edu/read/10026/chapter/15 |date=22 September 2017 }}. National Academy Press. 2001, pp. 510–521.</ref>

In 2013, a hypothesis suggested it was possible that boron and molybdenum catalyzed the production of [[RNA]] on [[Mars]] with life being transported to Earth via a meteorite around 3 billion years ago.<ref name="mars">{{cite news|url=https://www.newscientist.com/article/dn24120-primordial-broth-of-life-was-a-dry-martian-cupasoup.html|title=Primordial broth of life was a dry Martian cup-a-soup|work=New Scientist|date=29 August 2013|access-date=2013-08-29|archive-date=24 April 2015|archive-url=https://web.archive.org/web/20150424181341/http://www.newscientist.com/article/dn24120-primordial-broth-of-life-was-a-dry-martian-cupasoup.html|url-status=live}}</ref>

There exist several known boron-containing natural [[antibiotic]]s.<ref>{{cite journal|title = The tartrolons, new boron-containing antibiotics from a myxobacterium, ''Sorangium cellulosum''|journal = The Journal of Antibiotics|volume = 48|issue = 1|pages = 26–30|pmid = 7532644|vauthors = Irschik H, Schummer D, Gerth K, Höfle G, Reichenbach H|year = 1995|doi = 10.7164/antibiotics.48.26|url = https://www.jstage.jst.go.jp/article/antibiotics1968/48/1/48_1_26/_pdf|doi-access = free|access-date = 28 August 2019|archive-date = 10 May 2020|archive-url = https://web.archive.org/web/20200510223356/https://www.jstage.jst.go.jp/article/antibiotics1968/48/1/48_1_26/_pdf|url-status = live}}</ref> The first one found was [[boromycin]], isolated from [[streptomyces]] in the 1960s.<ref>{{Cite journal|last1=Hütter|first1=R.|last2=Keller-Schien|first2=W.|last3=Knüsel|first3=F.|last4=Prelog|first4=V.|last5=Rodgers Jr.|first5=G. C.|last6=Suter|first6=P.|last7=Vogel|first7=G.|last8=Voser|first8=W.|last9=Zähner|first9=H.|title=Stoffwechselprodukte von Mikroorganismen. 57. Mitteilung. Boromycin|date=1967|journal=[[Helvetica Chimica Acta]]|volume=50|issue=6|pages=1533–1539|doi=10.1002/hlca.19670500612|pmid=6081908}}</ref><ref>{{Cite journal | last1 = Dunitz | first1 = J. D. | last2 = Hawley | first2 = D. M. | last3 = Miklos | first3 = D. | last4 = White | first4 = D. N. J. | last5 = Berlin | first5 = Y. | last6 = Marusić | first6 = R. | last7 = Prelog | first7 = V. | doi = 10.1002/hlca.19710540624 | title = Structure of boromycin | journal = Helvetica Chimica Acta | volume = 54 | issue = 6 | pages = 1709–1713 | date = 1971 | pmid = 5131791 }}</ref> Others are [[tartrolon]]s, a group of antibiotics discovered in the 1990s from culture broth of the [[Myxobacteria|myxobacterium]] ''[[Sorangium cellulosum]]''.<ref name="wiley199419940310">{{cite journal |name-list-style = vanc|last1=Schummer |first1=Dietmar |last2=Irschik |first2=Herbert |last3=Reichenbach |first3=Hans |last4=Höfle |first4=Gerhard |date=1994-03-11 |title=Antibiotics from gliding bacteria, LVII. Tartrolons: New boron-containing macrodiolides fromSorangium cellulosum |url=https://onlinelibrary.wiley.com/doi/10.1002/jlac.199419940310 |journal=Liebigs Annalen der Chemie |language=de |volume=1994 |issue=3 |pages=283–289 |doi=10.1002/jlac.199419940310|url-access=subscription }}</ref>

[[Congenital endothelial dystrophy type 2]], a rare form of [[corneal dystrophy (human)|corneal dystrophy]], is linked to mutations in [[SLC4A11]] gene that encodes a transporter reportedly regulating the intracellular concentration of boron.<ref>{{Cite journal|doi=10.1038/ng1824|date=July 2006|author=Vithana, En|author2=Morgan, P|author3=Sundaresan, P|author4=Ebenezer, Nd|author5=Tan, Dt|author6=Mohamed, Md|author7=Anand, S|author8=Khine, Ko|author9=Venkataraman, D|author10=Yong, Vh|author11=Salto-Tellez, M|author12=Venkatraman, A|author13=Guo, K|author14=Hemadevi, B|author15=Srinivasan, M|author16=Prajna, V|author17=Khine, M|author18=Casey, Jr.|author19=Inglehearn, Cf|author20=Aung, T|title=Mutations in sodium-borate cotransporter SLC4A11 cause recessive congenital hereditary endothelial dystrophy (CHED2)|volume=38|issue=7|pages=755–7|issn=1061-4036|pmid=16767101|journal=Nature Genetics|s2cid=11112294}}</ref>

===Analytical quantification===
For determination of boron content in food or materials, the [[colorimetry|colorimetric]] ''[[curcumin]] method'' is used. Boron is converted to boric acid or [[borate]]s and on reaction with [[curcumin]] in acidic solution, a red colored boron-[[chelate]] complex, [[rosocyanine]], is formed.<ref>{{Cite journal| title = Corrections-Colorimetric Microdetermination of Boron by the Curcumin-Acetone Solution Method| first1 = L.|last1 = Silverman| journal = Anal. Chem.|date = 1953| volume = 25 |page = 1639| doi = 10.1021/ac60083a061| last2 = Trego| first2 = Katherine| issue = 11}}</ref>

===Health issues and toxicity===
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Elemental boron, [[boron trioxide|boron oxide]], [[boric acid]], borates, and many [[organoboron chemistry|organoboron compounds]] are relatively nontoxic to humans and animals (with toxicity similar to that of table salt). The [[median lethal dose|LD<sub>50</sub>]] (dose at which there is 50% mortality) for animals is about 6&nbsp;g per kg of body weight. Substances with LD<sub>50</sub> above 2&nbsp;g/kg are considered nontoxic. An intake of 4&nbsp;g/day of boric acid was reported without incident, but more than this is considered toxic in more than a few doses. Intakes of more than 0.5&nbsp;grams per day for 50 days cause minor digestive and other problems suggestive of toxicity.<ref>{{cite journal|doi=10.1023/A:1004276311956|date=1997|last1=Nielsen|first1=Forrest H.|journal=Plant and Soil|volume=193|issue=2|pages=199–208|title=Boron in human and animal nutrition|s2cid=12163109|url=https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=45215&content=PDF|access-date=29 April 2018|archive-date=12 March 2020|archive-url=https://web.archive.org/web/20200312091734/https://naldc-legacy.nal.usda.gov/naldc/download.xhtml?id=45215&content=PDF|url-status=live}}</ref> Dietary supplementation of boron may be helpful for bone growth, wound healing, and antioxidant activity,<ref>{{cite journal | title=Nothing boring about boron | journal=Integrative Medicine | volume=14 | issue=4 | pages=35–48 | last1=Pizzorno | first1=L | pmid=26770156 | pmc=4712861 | date=Aug 2015 }}</ref> and insufficient amount of boron in diet may result in [[boron deficiency (medicine)|boron deficiency]].

Single medical doses of 20&nbsp;g of [[boric acid]] for [[neutron capture therapy]] have been used without undue toxicity.

Boric acid is more toxic to insects than to mammals, and is routinely used as an insecticide.<ref name="Klotz 1994 1534–1536" />

The [[boranes]] (boron hydrogen compounds) and similar gaseous compounds are quite poisonous. As usual, boron is not an element that is intrinsically poisonous, but the toxicity of these compounds depends on structure (for another example of this phenomenon, see [[phosphine]]).<ref name="borates" /><ref name="boron" /> The boranes are also highly flammable and require special care when handling, some combinations of boranes and other compounds are highly explosive. Sodium borohydride presents a fire hazard owing to its reducing nature and the liberation of hydrogen on contact with acid. Boron halides are corrosive.<ref>{{cite web|url = http://www.inchem.org/documents/ehc/ehc/ehc204.htm|title = Environmental Health Criteria 204: Boron|date = 1998|publisher = the [[International Programme on Chemical Safety|IPCS]]|access-date = 2009-05-05|archive-date = 3 April 2019|archive-url = https://web.archive.org/web/20190403043829/http://www.inchem.org/documents/ehc/ehc/ehc204.htm|url-status = live}}</ref>

[[File:Boron toxicity (2313046082).jpg|thumb|Boron toxicity in rose leaves.]]
Boron is necessary for plant growth, but an excess of boron is toxic to plants, and occurs particularly in acidic soil.<ref name="Boron (B) and Chlorine (Cl) for Citrus Trees">{{cite web|last1=Zekri|first1=Mongi|last2=Obreza|first2=Tom|title=Boron (B) and Chlorine (Cl) for Citrus Trees|url=https://edis.ifas.ufl.edu/pdffiles/SS/SS61900.pdf|website=IFAS Extension|publisher=University of Florida|access-date=30 June 2017|archive-date=9 September 2016|archive-url=https://web.archive.org/web/20160909071409/http://edis.ifas.ufl.edu/pdffiles/SS/SS61900.pdf|url-status=live}}</ref><ref name="PeverillSparrow1999">{{cite book|author1=K. I. Peverill|author2=L. A. Sparrow|author3=Douglas J. Reuter|title=Soil Analysis: An Interpretation Manual|url=https://books.google.com/books?id=pWR1vUWbEhEC&pg=PA309|year=1999|publisher=Csiro Publishing|isbn=978-0-643-06376-1|pages=309–311|access-date=30 June 2017|archive-date=12 March 2020|archive-url=https://web.archive.org/web/20200312192335/https://books.google.com/books?id=pWR1vUWbEhEC&pg=PA309|url-status=live}}</ref> It presents as a yellowing from the tip inwards of the oldest leaves and black spots in barley leaves, but it can be confused with other stresses such as magnesium deficiency in other plants.<ref name="Reynolds2001">{{cite book|author=M. P. Reynolds|title=Application of Physiology in Wheat Breeding|url=https://books.google.com/books?id=PJ1a3yfTgg4C&pg=PA225|year=2001|publisher=CIMMYT|isbn=978-970-648-077-4|page=225|access-date=30 June 2017|archive-date=10 March 2020|archive-url=https://web.archive.org/web/20200310194730/https://books.google.com/books?id=PJ1a3yfTgg4C&pg=PA225|url-status=live}}</ref>

==See also==
{{Colbegin|colwidth=20em}}
* [[Allotropes of boron]]
* [[Boron deficiency (medicine)|Boron deficiency]]
* [[Boron oxide (disambiguation)|Boron oxide]]
* [[Boron nitride]]
* [[Boron neutron capture therapy]]
* [[Boronic acid]]
* [[Hydroboration-oxidation reaction]]
* [[Suzuki coupling]]
{{Colend}}
{{Subject bar
|portal=Chemistry
|book1=Boron
|book2=Period 2 elements
|book3=Boron group
|book4=Chemical elements (sorted&nbsp;alphabetically)
|book5=Chemical elements (sorted by number)
|commons=y
|wikt=y
|wikt-search=boron
|v=y
|v-search=Boron atom
}}

==References==
{{Reflist}}

==External links==
* [http://www.periodicvideos.com/videos/005.htm Boron] at [[The Periodic Table of Videos]] (University of Nottingham)
* J. B. Calvert: [http://mysite.du.edu:80/~jcalvert/phys/boron.htm Boron], 2004, private website ([https://web.archive.org/web/20030202000419/http://www.du.edu/~jcalvert/phys/boron.htm archived version])

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