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{{Infobox protactinium}}

'''Protactinium''' is a [[chemical element]] with symbol '''Pa''' and [[atomic number]] 91. It is a dense, silvery-gray metal which readily reacts with [[oxygen]], water vapor and inorganic [[acid]]s. It forms various [[chemical compound]]s where protactinium is usually present in the [[oxidation state]] +5, but can also assume +4 and even +2 or +3 states. The average concentrations of protactinium in the Earth's crust is typically on the order of a few parts per trillion, but may reach up to a few parts per million in some [[uraninite]] ore deposits. Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside of scientific research, and for this purpose, protactinium is mostly extracted from spent [[nuclear fuel]].

Protactinium was first identified in 1913 by [[Kasimir Fajans]] and [[Oswald Helmuth Göhring]] and named ''brevium'' because of the short [[half-life]] of the specific [[isotope]] studied, namely protactinium-234. A more stable isotope (<sup>231</sup>Pa) of protactinium was discovered in 1917/18 by [[Otto Hahn]] and [[Lise Meitner]], and they chose the name proto-actinium, but then the [[IUPAC]] named it finally protactinium in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant "parent of [[actinium]]" and reflected the fact that [[actinium]] is a product of radioactive decay of protactinium.

The longest-lived and most abundant (nearly 100%) naturally occurring [[isotope]] of protactinium, protactinium-231, has a [[half-life]] of 32,760 years and is a decay product of [[uranium-235]]. Much smaller trace amounts of the short-lived [[nuclear isomer]] protactinium-234m occur in the decay chain of [[uranium-238]]. Protactinium-233 results from the decay of [[thorium]]-233 as part of the chain of events used to produce [[uranium-233]] by neutron irradiation of thorium-232. It is an undesired intermediate product in thorium-based [[nuclear reactor]]s and is therefore removed from the active zone of the reactor during the breeding process. Analysis of the relative concentrations of various uranium, thorium and protactinium isotopes in water and minerals is used in [[radiometric dating]] of [[sediment]]s which are up to 175,000 years old and in modeling of various geological processes.

==History==
In 1871, [[Dmitri Mendeleev]] [[Mendeleev's predicted elements|predicted]] the existence of an element between [[thorium]] and [[uranium]].<ref name="Emsley"/> The actinide element group was unknown at the time. Therefore, uranium was positioned below [[tungsten]], and thorium below zirconium, leaving the space below tantalum empty and, until the 1950s, [[periodic table]]s were published with this structure.<ref>{{cite journal|doi = 10.1007/s10698-004-5959-9|title=A Revised Periodic Table: With the Lanthanides Repositioned|author=Laing, Michael |journal=Foundations of Chemistry|volume=7|issue=3|date=2005|page=203}}</ref> For a long time chemists searched for eka-tantalum as an element with similar chemical properties to tantalum, making a discovery of protactinium nearly impossible.

In 1900, [[William Crookes]] isolated protactinium as an intensely radioactive material from uranium; however, he could not characterize it as a new chemical element and thus named it uranium-X (UX).<ref name="Emsley">{{cite book|title = Nature's Building Blocks: An A-Z Guide to the Elements|last = Emsley|first = John|publisher = Oxford University Press|date = 2001|location = Oxford, England, UK|isbn = 0-19-850340-7|chapter = Protactinium|pages = 347–349|url = http://books.google.com/?id=j-Xu07p3cKwC&pg=PA348|date = 2003}}</ref><ref>http://books.google.co.uk/books?id=-zgrAAAAYAAJ&pg=PA180</ref><ref>{{cite journal|doi = 10.1098/rspl.1899.0120|last1 = Crookes|first1 = W.|title = Radio-Activity of Uranium|url = http://books.google.com/books?id=hmZDAAAAYAAJ&pg=PA409-IA6|journal = Proceedings of the Royal Society of London|volume = 66|pages = 409–423|date = 1899}}</ref><!--http://www.jstor.org/pss/96048 --> Crookes dissolved [[uranium nitrate]] in [[ether]], the residual aqueous phase contains most of the {{Nuclide2|Th|234}} and  {{Nuclide2|Pa|234}}. His method was still  used in the 1950s to isolate {{Nuclide2|Th|234}} and  {{Nuclide2|Pa|234}} from uranium compounds.<ref>{{cite journal|last1 = Johansson|first1 = Sven|title = Decay of UX1, UX2, and UZ|journal = Physical Review|volume = 96|pages = 1075|date = 1954|doi = 10.1103/PhysRev.96.1075|issue = 4|bibcode = 1954PhRv...96.1075J }}</ref> Protactinium was first identified in 1913, when [[Kasimir Fajans]] and Oswald Helmuth Göhring encountered the isotope <sup>234</sup>Pa during their studies of the decay chains of [[uranium-238]]: {{Nuclide2|U|238}} → {{Nuclide2|Th|234}} → {{Nuclide2|Pa|234}} → {{Nuclide2|U|234}}. They named the new element ''brevium'' (from the Latin word, ''brevis'', meaning brief or short) because of its short half-life, 6.7&nbsp;hours for {{Nuclide2|Pa|234}}.<ref name=g1250>[[#Greenwood|Greenwood]], p. 1250</ref><ref name=g1254>[[#Greenwood|Greenwood]], p. 1254</ref><ref>{{cite journal|author = Fajans, K.|author2 = Gohring, O.|last-author-amp = yes|title = Über die komplexe Natur des Ur X|journal = Naturwissenschaften|date = 1913|volume =14|pages = 339|url =http://www.digizeitschriften.de/no_cache/home/jkdigitools/loader/?tx_jkDigiTools_pi1%5BIDDOC%5D=201162&tx_jkDigiTools_pi1%5Bpp%5D=425 |doi = 10.1007/BF01495360|issue = 14|bibcode = 1913NW......1..339F }}</ref><ref>{{cite journal|author = Fajans, K.|author2 = Gohring, O.|last-author-amp = yes |title = Über das Uran X<sub>2</sub>-das neue Element der Uranreihe|journal = Physikalische Zeitschrift|date = 1913|volume = 14|pages = 877–84}}</ref> In 1917/18, two groups of scientists, [[Otto Hahn]] and [[Lise Meitner]] of [[Germany]] and [[Frederick Soddy]] and John Cranston of [[Great Britain]], independently discovered another isotope of protactinium, <sup>231</sup>Pa having much longer half-life of about 32,000&nbsp;years. Thus the name ''brevium'' was changed to ''protoactinium'' as the new element was part of the decay chain of uranium-235 before the actinium (from {{lang-gr|πρῶτος}} = ''protos'' meaning ''first'', ''before''). For ease of pronunciation, the name was shortened to ''protactinium'' by the  [[International Union of Pure and Applied Chemistry|IUPAC]] in 1949.<ref name=CRC/><ref name=g1251>[[#Greenwood|Greenwood]], p. 1251</ref> The discovery of protactinium completed the last gap in the early versions of the periodic table, proposed by Mendeleev in 1869, and it brought to fame the involved scientists.<ref>Shea, William R. (1983) [http://books.google.com/books?id=W7xyvXc-hgEC&pg=PA213 Otto Hahn and the rise of nuclear physics], Springer, p. 213, ISBN 90-277-1584-X.</ref>

[[Aristid von Grosse]] produced 2&nbsp;milligrams of Pa<sub>2</sub>O<sub>5</sub> in 1927,<ref>{{cite journal|author = von Grosse, Aristid |title = Das Element 91; seine Eigenschaften und seine Gewinnung |pages = 233–245|journal = [[Berichte der deutschen chemischen Gesellschaft]]|doi = 10.1002/cber.19280610137|volume = 61|issue = 1|date = 1928}}</ref> and in 1934 first isolated elemental protactinium from 0.1&nbsp;milligrams of Pa<sub>2</sub>O<sub>5</sub>.<ref>{{cite journal|doi = 10.1002/ange.19340473706|title = Die technische Gewinnung des Protactiniums|date = 1934|last1 = Graue|first1 = G.|last2 = Käding|first2 = H.|journal = Angewandte Chemie|volume = 47|issue = 37|pages = 650–653}}</ref> He used two different procedures: in the first one, protactinium oxide was irradiated by 35&nbsp;keV electrons in vacuum. In another method, called the [[crystal bar process|van Arkel–de Boer process]], the oxide was chemically converted to a [[halide]] ([[chloride]], [[bromide]] or [[iodide]]) and then reduced in a vacuum with an electrically heated metallic filament:<ref name=CRC/><ref>{{cite journal| last1=Grosse| first1=A. V.| journal=Journal of the American Chemical Society| volume=56|pages=2200| date=1934| doi=10.1021/ja01325a508| issue=10| title=Metallic Element 91}}</ref>

: 2 PaI<sub>5</sub> → 2 Pa + 5 I<sub>2</sub>

In 1961, the [[United Kingdom Atomic Energy Authority]] (UKAEA) produced 125&nbsp;grams of 99.9% pure protactinium by processing 60&nbsp;tonnes of waste material in a 12-stage process, at a cost of about 500,000&nbsp;USD.<ref name=CRC/> For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies.<ref name="Emsley"/> [[Oak Ridge National Laboratory]] in the US provided protactinium at a cost of about 280&nbsp;USD/gram.<ref>{{Wayback|url=http://periodic.lanl.gov/91.shtml|title=Protactinium|date=20110928025549}} from [[Los Alamos National Laboratory]].</ref>

[[File:Mendelejevs periodiska system 1871.png|thumb|450px|Mendeleev's 1871 periodic table with a gap for protactinium on the bottom row of the chart, between thorium and uranium]]

==Occurrence==
Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes – <sup>231</sup>Pa and <sup>234</sup>Pa, with the isotope <sup>234</sup>Pa occurring in two different energy states. Nearly all natural protactinium is protactinium-231. It is an [[Alpha particle|alpha emitter]] and is formed by the decay of uranium-235, whereas the [[beta particle|beta radiating]] protactinium-234 is produced as a result of [[:File:Decay chain(4n+2, Uranium series).PNG|uranium-238 decay]]. Nearly all uranium-238 (99.8%) decays first to the <sup>234m</sup>Pa isomer.<ref name=ANL>[http://www.ead.anl.gov/pub/doc/protactinium.pdf Protactinium], Argonne National Laboratory, Human Health Fact Sheet, August 2005</ref>

Protactinium occurs in [[uraninite]] (pitchblende) at concentrations of about 0.3-3 [[parts-per notation|parts]] <sup>231</sup>Pa per million parts (ppm) of ore.<ref name="Emsley"/> Whereas the usual content is closer to 0.3 ppm<ref name=brit/> (e.g. in [[Jáchymov]], [[Czech Republic]]<ref>{{cite journal|last1=Grosse|first1=A. V.|last2=Agruss|first2=M. S.|journal=Journal of the American Chemical Society|volume=56|pages=2200|date=1934|doi=10.1021/ja01325a507|issue=10|title=The Isolation of 0.1 Gram of the Oxide of Element 91 (Protactinium)}}</ref>), some ores from the [[Democratic Republic of the Congo]] have about 3 ppm.<ref name=CRC/> Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, that corresponds to the radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even the water present in the same sample of soil. Much higher ratios of 2,000 and above are measured in [[loam]] soils and clays, such as [[bentonite]].<ref name=ANL/><ref>Cornelis, Rita (2005) [http://books.google.com/books?id=1PmjurlE6KkC&pg=PA520 Handbook of elemental speciation II: species in the environment, food, medicine & occupational health, Vol. 2], John Wiley and Sons, pp. 520–521, ISBN 0-470-85598-3.</ref>

===In nuclear reactors===
Two major protactinium isotopes, <sup>231</sup>Pa and <sup>233</sup>Pa, are produced from thorium in [[nuclear reactor]]s; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular, <sup>232</sup>Th via (''n'',2''n'') reactions produces <sup>231</sup>Th which quickly (half-life 25.5 hours) decays to <sup>231</sup>Pa. The last isotope, while not a transuranic waste, has a long half-life of 32,760 years and is a major contributor to the long term [[radiotoxic]]ity of spent nuclear fuel.<ref name=b1/>

Protactinium-233 is formed upon neutron capture by <sup>232</sup>Th. It further either decays to uranium-233 or captures another neutron and converts into the non-fissile uranium-234.<ref>{{cite book|author=Hébert, Alain|title=Applied Reactor Physics|url=http://books.google.com/books?id=sibA5ECQ8LoC&pg=PA265|date=July 2009|publisher=Presses inter Polytechnique|isbn=978-2-553-01436-9|pages=265–}}</ref> <sup>233</sup>Pa has a relatively long half-life of 27 days and high [[cross section (physics)|cross section]] for neutron capture (the so-called "[[neutron poison]]"). Thus instead of rapidly decaying to the useful <sup>233</sup>U, a significant fraction of <sup>233</sup>Pa converts to non-fissile isotopes and consumes neutrons, degrading [[neutron economy|the reactor efficiency]]. To avoid this, <sup>233</sup>Pa is extracted from the active zone of thorium [[molten salt reactor]]s, during their operation, so that it only decays to <sup>233</sup>U. This is achieved using several meters tall columns of molten [[bismuth]] with lithium dissolved in it. In a simplified scenario, lithium selectively reduces protactinium salts to protactinium metal which is then extracted from the molten-salt cycle, and bismuth is merely a carrier. It is chosen because of its low [[melting point]] (271&nbsp;°C), low vapor pressure, good solubility for lithium and actinides, and immiscibility with molten [[halide]]s.<ref name=b1>Groult, Henri (2005) [http://books.google.com/books?id=dR2DA50PUV4C&pg=PA562 Fluorinated materials for energy conversion], Elsevier, pp. 562–565, ISBN 0-08-044472-5.</ref>

==Preparation==
[[File:Uraninite-39029.jpg|thumb|left|upright|Protactinium occurs in [[uraninite]] ores.]]
Before the advent of nuclear reactors, protactinium was separated for scientific experiments from uranium ores. Nowadays, it is mostly produced as an intermediate product of [[nuclear fission]] in thorium high-temperature reactors:

:<math>\mathrm{^{232}_{\ 90}Th \ + \ ^{1}_{0}n \ \longrightarrow \ ^{233}_{\ 90}Th \ \xrightarrow[22.3\ min]{\beta^-} \ ^{233}_{\ 91}Pa \ \xrightarrow[26.967\ d]{\beta^-} \ ^{233}_{\ 92}U}</math>

:<small>The times are half-lives.</small>

Protactinium metal can be prepared by reduction of its [[fluoride]] with [[calcium fluoride]],<ref name=exp>{{cite journal|last1=Marples|first1=J. A. C.|title=On the thermal expansion of protactinium metal|journal=Acta Crystallographica|volume=18|pages=815|date=1965|doi=10.1107/S0365110X65001871|issue=4}}</ref> [[lithium]] or [[barium]] at a temperature of 1300–1400&nbsp;°C.<ref name=super/><ref name=pao2/>

==Physical and chemical properties==
Protactinium is an [[actinide]] which is positioned in the [[periodic table]] to the left of [[uranium]] and to the right of [[thorium]], and many of its physical properties are intermediate between those two actinides. So, protactinium is more dense and rigid than thorium but is lighter than uranium, and its melting point is lower than that of thorium and higher than that of uranium. The thermal expansion, electrical and thermal conductivities of these three elements are comparable and are typical of [[post-transition metal]]s. The estimated [[shear modulus]] of protactinium is similar to that of [[titanium]].<ref>Seitz, Frederick and Turnbull, David (1964) [http://books.google.com/books?id=F9V3a-0V3r8C&pg=PA289 Solid state physics: advances in research and applications], Academic Press, pp. 289–291, ISBN 0-12-607716-9.</ref> Protactinium is a metal with silvery-gray luster that is preserved for some time in air.<ref name=CRC>{{cite book| author = Hammond, C. R. |title = The Elements, in Handbook of Chemistry and Physics |edition = 81st| publisher =CRC press| isbn = 0-8493-0485-7}}</ref><ref>{{cite book |last1=Myasoedov |first1= B. F. |last2=Kirby |first2=H. W.|last3=Tananaev |first3= I. G. |editor1-first=L. R. |editor1-last=Morss |editor2-first=N. M. |editor2-last=Edelstein |editor3-first=J. |editor3-last=Fuger |title=The Chemistry of the Actinide and Transactinide Elements |edition=3rd |date=2006 |publisher=Springer |location=Dordrecht, The Netherlands |chapter=Chapter 4: Protactinium |isbn=978-1-4020-3555-5}}</ref> Protactinium easily reacts with oxygen, water vapor and acids, but not with alkalis.<ref name="Emsley"/>

At room temperature, protactinium crystallizes in body-centered [[Tetragonal crystal system|tetragonal]] structure which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa. The structure changes to face-centered cubic (''fcc'') upon cooling from high temperature, at about 1200&nbsp;°C.<ref name=exp/><ref>Young, David A. (1991) [http://books.google.com/books?id=F2HVYh6wLBcC&pg=PA222 Phase diagrams of the elements], University of California Press, p. 222, ISBN 0-520-07483-1.</ref> The thermal expansion coefficient of the tetragonal phase between room temperature and 700&nbsp;°C is 9.9{{e|-6}}/°C.<ref name=exp/>

Protactinium is [[paramagnetism|paramagnetic]] and no magnetic transitions are known for it at any temperature.<ref>Buschow, K. H. J. (2005) [http://books.google.com/books?id=N9mvytGEBtwC&pg=PA129 Concise encyclopedia of magnetic and superconducting materials], Elsevier, pp. 129–130, ISBN 0-08-044586-1.</ref> It becomes [[Superconductivity|superconductive]] at temperatures below 1.4 K.<ref name="Emsley"/><ref name=super>{{cite journal| display-authors =4| author =Fowler, R. D. |title = Superconductivity of Protactinium| journal = Phys. Rev. Lett.| volume = 15 |page = 860|date = 1965| doi = 10.1103/PhysRevLett.15.860| bibcode=1965PhRvL..15..860F| issue =22| last2 =Matthias| first2 =B.| last3 =Asprey| first3 =L.| last4 =Hill| first4 =H.| last5 =Lindsay| first5 =J.| last6 =Olsen| first6 =C.| last7 =White| first7 =R.}}</ref> Protactinium tetrachloride is paramagnetic at room temperature but turns [[ferromagnetism|ferromagnetic]] upon cooling to 182 K.<ref>{{cite journal|last1=Hendricks|first1=M. E.|title=Magnetic Properties of Protactinium Tetrachloride|journal=The Journal of Chemical Physics|volume=55|pages=2993|date=1971|doi=10.1063/1.1676528|issue=6|bibcode = 1971JChPh..55.2993H }}</ref>

Protactinium exists in two major [[oxidation state]]s, +4 and +5, both in solids and solutions, and the +3 and +2 states were observed in some solid phases. As the electron configuration of the neutral atom is [Rn]5f<sup>2</sup>6d<sup>1</sup>7s<sup>2</sup>, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f<sup>0</sup> configuration. Both +4 and +5 states easily form [[hydroxide]]s in water with the predominant ions being Pa(OH)<sup>3+</sup>, {{chem|Pa(OH)|2|2+}}, {{chem|Pa(OH)|3|+}} and Pa(OH)<sub>4</sub>, all colorless.<ref name=g1265>[[#Greenwood|Greenwood]], p. 1265</ref> Other known protactinium ions include {{chem|PaCl|2|2+}}, {{chem|PaSO|4|2+}}, PaF<sup>3+</sup>, {{chem|PaF|2|2+}}, {{chem|PaF|6|-}}, {{chem|PaF|7|2-}} and {{chem|PaF|8|3-}}.<ref name=g1275>[[#Greenwood|Greenwood]], p. 1275</ref><ref name=trif/>

==Chemical compounds==
{| Class = "wikitable" style = "text-align: center"
! Formula
! color
! symmetry
! [[space group]]
! No
! [[Pearson symbol]]
! ''a'' (pm)
! ''b'' (pm)
! ''c'' (pm)
! ''Z''
! density, g/cm<sup>3</sup>
|-
| Pa
| silvery-gray
| [[Tetragonal crystal system|tetragonal]]<ref name=str/>
| I4/mmm
| 139
| tI2
| 392.5
| 392.5
| 323.8
| 2
| 15.37
|-
| PaO
|
| rocksalt<ref name=pao2>{{cite journal|doi = 10.1021/ja01652a011|date = 1954|last1 = Sellers|first1 = Philip A.|last2 = Fried|first2 = Sherman|last3 = Elson|first3 = Robert E.|last4 = Zachariasen|first4 = W. H.|journal = Journal of the American Chemical Society|volume = 76|pages = 5935|title = The Preparation of Some Protactinium Compounds and the Metal|issue = 23}}</ref>
| Fm{{overline|3}}m
| 225
| cF8
| 496.1
|
|
| 4
| 13.44
|-
| [[Protactinium(IV) oxide|PaO<sub>2</sub>]]
| black
| ''fcc''<ref name=pao2/>
| Fm{{overline|3}}m
| 225
| cF12
| 550.5
|
|
| 4
| 10.47
|-
| [[Protactinium(V) oxide|Pa<sub>2</sub>O<sub>5</sub>]]
| white
|
| Fm{{overline|3}}m<ref name=pao2/>
| 225
| cF16
| 547.6
| 547.6
| 547.6
| 4
| 10.96
|-
| Pa<sub>2</sub>O<sub>5</sub>
| white
|
| orthorhombic<ref name=pao2/>
|
|
| 692
| 402
| 418
|
|
|-
| PaH<sub>3</sub>
| black
| cubic<ref name=pao2/>
| Pm{{overline|3}}n
| 223
| cP32
| 664.8
| 664.8
| 664.8
| 8
| 10.58
|-
| PaF<sub>4</sub>
| brown-red
| monoclinic<ref name=pao2/>
| C2/c
| 15
| mS60
|
|
|
| 2
|
|-
| PaCl<sub>4</sub>
| green-yellow
| [[Tetragonal crystal system|tetragonal]]<ref>{{cite journal|journal=J. Chem. Soc., Dalton Trans|date=1973|title=Structural parameters and unit cell dimensions for the tetragonal actinide tetrachlorides(Th, Pa, U, and Np) and tetrabromides (Th and Pa)|pages=686–691|author=Brown D.|author2=Hall T.L.|author3=Moseley P.T|doi=10.1039/DT9730000686|issue=6}}</ref>
| I4<sub>1</sub>/amd
| 141
| tI20
| 837.7
| 837.7
| 748.1
| 4
| 4.72
|-
| PaBr<sub>4</sub>
| brown
| tetragonal<ref name=pabr4>{{cite journal|display-authors=4|last1=Tahri|first1=Y|last2=Chermette|first2=H|last3=El Khatib|first3=N|last4=Krupa|first4=J|last5=Simoni|first5=E|title=Electronic structures of thorium and protactinium halide clusters of [ThX8]4− type|journal=Journal of the Less Common Metals|volume=158|pages=105|date=1990|doi=10.1016/0022-5088(90)90436-N}}</ref><ref name=pabr5b/>
| I4<sub>1</sub>/amd
| 141
| tI20
| 882.4
| 882.4
| 795.7
|
|
|-
| [[Protactinium(V) chloride|PaCl<sub>5</sub>]]
| yellow
| [[Monoclinic crystal system|monoclinic]]<ref name=pacl5>{{cite journal|doi=10.1107/S0365110X67000155|last1=Dodge|first1=R. P.|last2=Smith|first2=G. S.|last3=Johnson|first3=Q.|last4=Elson|first4=R. E.|title=The crystal structure of protactinium pentachloride|journal=Acta Cryst|date=1967|volume=22|pages=85–89}}</ref>
| C2/c
| 15
| mS24
| 797
| 1135
| 836
| 4
| 3.74
|-
| PaBr<sub>5</sub>
| red
| monoclinic<ref name=pabr5b/><ref name=pabr5>{{cite journal|last1=Brown|first1=D.|last2=Petcher|first2=T. J.|last3=Smith|first3=A. J.|title=The crystal structure of β-protactinium pentabromide|journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry|volume=25|pages=178|date=1969|doi=10.1107/S0567740869007357|issue=2}}</ref>
| P2<sub>1</sub>/c
| 14
| mP24
| 838.5
| 1120.5
| 1214.6
| 4
| 4.98
|-
| PaOBr<sub>3</sub>
|
| monoclinic<ref name=pabr5b/>
| C2
|
|
| 1691.1
| 387.1
| 933.4
|
|
|-
| Pa(PO<sub>3</sub>)<sub>4</sub>
|
| orthorhombic<ref name=papo3>{{cite journal|doi=10.1016/j.jssc.2004.08.009|last1=Brandel|first1=V.|date=2004|pages=4743|volume=177|journal=Journal of Solid State Chemistry|last2=Dacheux|first2=N. |title=Chemistry of tetravalent actinide phosphates—Part I|issue=12|bibcode = 2004JSSCh.177.4743B }}</ref>
|
|
|
| 696.9
| 895.9
| 1500.9
|
|
|-
| Pa<sub>2</sub>P<sub>2</sub>O<sub>7</sub>
|
| cubic<ref name=papo3/>
| Pa3
|
|
| 865
| 865
| 865
|
|
|-
| Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>
| golden-yellow
| monoclinic<ref name=cene>{{cite journal|doi=10.1021/ic50136a011|last1=Starks|date=1974|first1=David F.|pages=1307|volume=13|last2=Parsons|journal=Inorganic Chemistry|first2=Thomas C.|last3=Streitwieser|first3=Andrew|last4=Edelstein|first4=Norman|title=Bis(π-cyclooctatetraene) protactinium|issue=6|authorlink3=Andrew Streitwieser}}</ref>
|
|
|
| 709
| 875
| 1062
|
|
|}

Here ''a'', ''b'' and ''c'' are lattice constants in picometers, No is space group number and ''Z'' is the number of [[formula unit]]s per [[unit cell]]; ''fcc'' stands for the [[Cubic crystal system|face-centered cubic]] symmetry. Density was not measured directly but calculated from the lattice parameters.

===Oxides and oxygen-containing salts===
Protactinium oxides are known for the metal oxidation states +2, +4 and +5. The most stable is white pentoxide Pa<sub>2</sub>O<sub>5</sub>, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500&nbsp;°C.<ref name=g1268>[[#Greenwood|Greenwood]], p. 1268</ref> Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO<sub>2.25</sub>. Another phase of this oxide with orthorhombic symmetry has also been reported.<ref name=pao2/><ref name=pacl4b/> The black dioxide PaO<sub>2</sub> is obtained from the pentoxide by reducing it at 1550&nbsp;°C with hydrogen. It is not readily soluble in either dilute or concentrated [[nitric acid|nitric]], [[hydrochloric acid|hydrochloric]] or [[sulfuric acid]]s, but easily dissolves in [[hydrofluoric acid]].<ref name=pao2/> The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100&nbsp;°C.<ref name=pacl4b>{{cite journal|last1=Elson|first1=R.|last2=Fried|first2=Sherman|last3=Sellers|first3=Philip|last4=Zachariasen|first4=W. H.|title=The tetravalent and pentavalent states of protactinium|journal=Journal of the American Chemical Society|volume=72|pages=5791|date=1950|doi=10.1021/ja01168a547|issue=12}}</ref> The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.<ref name=pao2/>

Protactinium forms mixed binary oxides with various metals. With alkali metals ''A'', the crystals have a chemical formula APaO<sub>3</sub> and [[perovskite structure]], or A<sub>3</sub>PaO<sub>4</sub> and distorted rock-salt structure, or A<sub>7</sub>PaO<sub>6</sub> where oxygen atoms for a hexagonal close-packed lattice. In all these materials, protactinium ions are octahedrally coordinated.<ref name=g1269>[[#Greenwood|Greenwood]], p. 1269</ref><ref>{{cite journal|doi=10.1107/S056774087100284X|last1=Iyer|first1=P. N.|date=1971|pages=731|volume=27|journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum or protactinium. IV. Further systems involving alkali metals|issue=4}}</ref> The pentoxide Pa<sub>2</sub>O<sub>5</sub> combines with rare-earth metal oxides R<sub>2</sub>O<sub>3</sub> to form various nonstoichiometric mixed-oxides, also of perovskite structure.<ref>{{cite journal|last1=Iyer|first1=P. N.|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum, or protactinium. III. Systems involving the rare earths|journal=Acta Crystallographica|volume=23|pages=740|date=1967|doi=10.1107/S0365110X67003639|issue=5}}</ref>

Protactinium oxides are [[Basic oxide|basic]]; they easily convert to hydroxides and can form various salts, such as [[sulfate]]s, [[phosphate]]s, [[nitrate]]s, etc. The nitrate is usually white but can be brown due to [[radiolysis|radiolytic]] decomposition. Heating the nitrate in air at 400&nbsp;°C converts it to the white protactinium pentoxide.<ref name=target/> The polytrioxophosphate Pa(PO<sub>3</sub>)<sub>4</sub> can be produced by reacting difluoride sulfate PaF<sub>2</sub>SO<sub>4</sub> with [[phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>) under inert gas atmosphere. Heating the product to about 900&nbsp;°C eliminates the reaction by-products such as [[hydrofluoric acid]], [[sulfur trioxide]] and phosphoric anhydride. Heating to higher temperatures in an inert atmosphere decomposes Pa(PO<sub>3</sub>)<sub>4</sub> into the diphosphate PaP<sub>2</sub>O<sub>7</sub>, which is analogous to diphosphates of other actinides. In the diphosphate, the PO<sub>3</sub> groups form pyramids of C<sub>2v</sub> symmetry. Heating PaP<sub>2</sub>O<sub>7</sub> in air to 1400&nbsp;°C decomposes it into the pentoxides of phosphorus and protactinium.<ref name=papo3/>

===Halides===
Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and [[Coordination number|coordinated]] by 7 other ions. The coordination is the same in protactinium(V) chloride, but the color is yellow. The coordination changes to octahedral in the brown protactinium(V) bromide and is unknown for protactinium(V) iodide. The protactinium coordination in all its tetrahalides is 8, but the arrangement is square antiprismatic in protactinium(IV) fluoride and dodecahedral in the chloride and bromide. Brown-colored protactinium(III) iodide has been reported where protactinium ions are 8-coordinated in a bicapped trigonal prismatic arrangement.<ref name=g1270>[[#Greenwood|Greenwood]], p. 1270</ref>

[[File:PaCl5.svg|200px|thumb|right|Coordination of protactinium (solid circles) and halogen atoms (open circles) in protactinium(V) fluoride or chloride.]]
Protactinium(V) fluoride and protactinium(V) chloride have a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all the halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The coordination 7 of protactinium originates from the 5 halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water.<ref name=g1271/> The pentachloride melts at 300&nbsp;°C and sublimates at even lower temperatures.

Protactinium(V) fluoride can be prepared by reacting protactinium oxide with either [[bromine pentafluoride]] or [[bromine trifluoride]] at about 600&nbsp;°C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and [[hydrogen fluoride]] at 600&nbsp;°C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction.<ref name=pao2/>

Protactinium(V) chloride is prepared by reacting protactinium oxide with [[carbon tetrachloride]] at temperature of 200–300&nbsp;°C.<ref name=pao2/> The by-products (such as PaOCl<sub>3</sub>) are removed by fractional sublimation.<ref name=pacl5/> Reduction of protactinium(V) chloride with hydrogen at about 800&nbsp;°C yields protactinium(IV) chloride – a yellow-green solid which sublimes in vacuum at 400&nbsp;°C; it can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400&nbsp;°C.<ref name=pao2/>

Protactinium bromides are produced by the action of [[aluminum bromide]], [[hydrogen bromide]], [[carbon tetrabromide]] or a mixture of hydrogen bromide and [[thionyl bromide]] on protactinium oxide. An alternative reaction is between protactinium pentachloride and hydrogen bromide or thionyl bromide.<ref name=pao2/> Protactinium(V) bromide has two similar monoclinic forms, one is obtained by sublimation at 400–410&nbsp;°C and another by sublimation at slightly lower temperature of 390–400&nbsp;°C.<ref name=pabr5b>{{cite journal|doi=10.1038/217737a0|last1=Brown|first1=D.|last2=Petcher|first2=T. J.|last3=Smith|first3=A. J.|title=Crystal Structures of some Protactinium Bromides|date=1968|pages=737|volume=217|journal=Nature|issue=5130|bibcode = 1968Natur.217..737B }}</ref><ref name=pabr5/>

Protactinium iodides result from the oxides and [[aluminum iodide]] or [[ammonium iodide]] heated to 600&nbsp;°C.<ref name=pao2/> Protactinium(III) iodide was obtained by heating protactinium(V) iodide in vacuum.<ref name=g1271/> As with oxides, protactinium forms mixed halides with alkali metals. Among those, most remarkable is Na<sub>3</sub>PaF<sub>8</sub> where protactinium ion is symmetrically surrounded by 8 F<sup>−</sup> ions which form a nearly perfect cube.<ref name=g1275/>

More complex protactinium fluorides are also known such as Pa<sub>2</sub>F<sub>9</sub><ref name=g1271>[[#Greenwood|Greenwood]], p. 1271</ref> and ternary fluorides of the types MPaF<sub>6</sub> (M = Li, Na, K, Rb, Cs or NH<sub>4</sub>), M<sub>2</sub>PaF<sub>7</sub> (M = K, Rb, Cs or NH<sub>4</sub>) and M<sub>3</sub>PaF<sub>8</sub> (M = Li, Na, Rb, Cs), all being white crystalline solids. The MPaF<sub>6</sub> formula can be represented as a combination of MF and PaF<sub>5</sub>. These compounds can be obtained by evaporating a hydrofluoric acid solution containing these both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it lowers to orthorphombic for larger cations K<sup>+</sup>, Rb<sup>+</sup>, Cs<sup>+</sup> or NH<sub>4</sub><sup>+</sup>. A similar variation was observed for the M<sub>2</sub>PaF<sub>7</sub> fluorides, namely the crystal symmetry was dependent on the cation and differed for Cs<sub>2</sub>PaF<sub>7</sub> and M<sub>2</sub>PaF<sub>7</sub> (M = K, Rb or NH<sub>4</sub>).<ref name=trif>{{cite journal|last1=Asprey|first1=L. B.|last2=Kruse|first2=F. H.|last3=Rosenzweig|first3=A.|last4=Penneman|first4=R. A.|title=Synthesis and X-Ray Properties of Alkali Fluoride-Protactinium Pentafluoride Complexes|journal=Inorganic Chemistry|volume=5|pages=659|date=1966|doi=10.1021/ic50038a034|issue=4}}</ref>

===Other inorganic compounds===
Oxyhalides and oxysulfides of protactinium are known. PaOBr<sub>3</sub> has a monoclinic structure composed of double-chain units where protactinium has coordination 7 and is arranged into pentagonal bipyramids. The chains are interconnected through oxygen and bromine atoms, and each oxygen atom is related to three protactinium atoms.<ref name=pabr5b/> PaOS is a light-yellow non-volatile solid with a cubic crystal lattice isostructural to that of other actinide oxysulfides. It is obtained by reacting protactinium(V) chloride with a mixture of [[hydrogen sulfide]] and [[carbon disulfide]] at 900&nbsp;°C.<ref name=pao2/>

In hydrides and nitrides, protactinium has a low oxidation state of about +3. The hydride is obtained by direct action of hydrogen on the metal at 250&nbsp;°C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is stable to heating to 800&nbsp;°C in vacuum. Protactinium carbide PaC is formed by reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400&nbsp;°C.<ref name=pao2/> Protactinium forms borohydrides which include Pa(BH<sub>4</sub>)<sub>4</sub>. It has an unusual polymeric structure with helical chains where the protactinium atom has coordination number of 12 and is surrounded by six BH<sub>4</sub><sup>−</sup> ions.<ref name=g1277>[[#Greenwood|Greenwood]], p. 1277</ref>

[[File:Uranocene-3D-balls.png|thumb|upright|The proposed structure of the Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub> molecule]]

===Organometallic compounds===
Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C<sub>5</sub>H<sub>5</sub>)<sub>4</sub>) with four [[Cyclopentadienyl complex|cyclopentadienyl]] rings, which can be synthesized by reacting protactinium(IV) chloride with molten Be(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>. One ring can be substituted with a halide atom.<ref name=g1278>[[#Greenwood|Greenwood]], pp. 1278–1279</ref> Another organometallic complex is golden-yellow bis(π-cyclooctatetraene) protactinium, Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>, which is analogous in structure to [[uranocene]]. There, the metal atom is sandwiched between two [[cyclooctatetraene]] ligands. Similar to uranocene, it can be prepared by reacting protactinium tetrachloride with dipotassium [[cyclooctatetraene|cyclooctatetraenide]], K<sub>2</sub>C<sub>8</sub>H<sub>8</sub>, in [[tetrahydrofuran]].<ref name=cene/>

==Isotopes==<!-- This section is linked from [[uranium]] -->
{{main|Isotopes of protactinium}}
Twenty-nine [[radioisotope]]s of protactinium have been discovered, the most stable being <sup>231</sup>Pa with a [[half-life]] of 32,760 years, <sup>233</sup>Pa with a half-life of 27 days, and <sup>230</sup>Pa with a half-life of 17.4 days. All of the remaining isotopes have half-lives shorter than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. Protactinium also has two [[nuclear isomer]]s, <sup>217m</sup>Pa (half-life 1.2 milliseconds) and <sup>234m</sup>Pa (half-life 1.17 minutes).<ref name="nubase">{{cite journal|last1=Audi|first1=G|doi=10.1016/j.nuclphysa.2003.11.001|title=The NUBASE evaluation of nuclear and decay properties|date=2003|pages=3|volume=729|journal=Nuclear Physics A|url=http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf|bibcode=2003NuPhA.729....3A|last2=Bersillon|first2=O.|last3=Blachot|first3=J.|last4=Wapstra|first4=A.H.}}</ref>

The primary [[decay mode]] for isotopes of protactinium lighter than (and including) the most stable isotope <sup>231</sup>Pa (i.e., <sup>212</sup>Pa to <sup>231</sup>Pa) is [[alpha decay]] and the primary mode for the heavier isotopes (i.e., <sup>232</sup>Pa to <sup>240</sup>Pa) is [[beta decay]]. The primary [[decay product]]s of isotopes of protactinium lighter than (and including) <sup>231</sup>Pa are [[actinium]] isotopes and the primary decay products for the heavier isotopes of protactinium are [[uranium]] isotopes.<ref name="nubase"/>

==Applications==
Although protactinium is located in the periodic table between uranium and thorium, which both have numerous applications, owing to its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside of scientific research.<ref name=ANL/>

Protactinium-231 arises from the decay of uranium-235 formed in nuclear reactors, and by the reaction <sup>232</sup>Th&nbsp;+&nbsp;n&nbsp;→&nbsp;<sup>231</sup>Th&nbsp;+&nbsp;2n and subsequent [[beta decay]]. It may support a nuclear chain reaction, which could in principle be used to build [[nuclear weapon]]s. The [[physicist]] Walter Seifritz once estimated the associated [[critical mass]] as {{val|750|180|u=kg}},<ref>Seifritz, Walter ''Nukleare Sprengkörper – Bedrohung oder Energieversorgung für die Menschheit'', Thiemig-Verlag (1984), ISBN 3-521-06143-4.</ref> but this possibility (of a chain reaction) has been ruled out by other nuclear physicists since then.<ref>Ganesan, S. ''et al.'' [http://www.iisc.ernet.in/currsci/sept10/researcharticle.pdf ''A Re-calculation of Criticality Property of <sup>231</sup>Pa Using New Nuclear Data''], Current Science, '''1999''', ''77&nbsp;(5) 667–677.</ref>

With the advent of highly sensitive [[Mass spectrometry|mass spectrometers]], an application of <sup>231</sup>Pa as a tracer in geology and [[paleoceanography]] has become possible. So, the ratio of protactinium-231 to thorium-230 is used for [[radiometric dating]] of sediments which are up to 175,000 years old and in modeling of the formation of minerals.<ref name=brit/> In particular, its evaluation in oceanic sediments allowed to reconstruct the movements of [[North Atlantic]] water bodies during the last melting of [[Ice Age]] [[glacier]]s.<ref>{{cite journal|doi = 10.1038/nature02494|display-authors = 4|author = McManus, J. F.|author2 = Francois, R.|author3 = Gherardi, J.-M.|author4 = Keigwin, L. D.|author5 = Brown-Leger, S.|last-author-amp = yes |title=Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes|journal=[[Nature (journal)|Nature]]|date=2004|volume=428|issue = 6985|pages=834–837|pmid = 15103371|url=http://www.seas.harvard.edu/climate/pdf/mcmanus-2004.pdf|bibcode = 2004Natur.428..834M }}</ref><!--10.1016/S0016-7037(98)00255-5--> Some of the protactinium-related dating variations rely on the analysis of the relative concentrations for several long-living members of the uranium decay chain – uranium, thorium and protactinium, for example. These elements have 6, 5 and 4 f-electrons in the outer shell and thus favor +6, +5 and +4 oxidation states, respectively, and show different physical and chemical properties. So, thorium and protactinium, but not uranium compounds are poorly soluble in aqueous solutions, and precipitate into sediments; the precipitation rate is faster for thorium than for protactinium. Besides, the concentration analysis for both protactinium-231 (half-life 32,750 years) and thorium-230 (half-life 75,380 years) allows to improve the accuracy compared to when only one isotope is measured; this double-isotope method is also weakly sensitive to inhomogeneities in the spatial distribution of the isotopes and to variations in their precipitation rate.<ref name=brit>Articles "Protactinium" and "Protactinium-231 – thorium-230 dating" in Encyclopædia Britannica, 15th edition, 1995, p. 737</ref><ref>{{cite journal|last1=Cheng|first1=H|title=Uranium-thorium-protactinium dating systematics|journal=Geochimica et Cosmochimica Acta|volume=62|pages=3437|date=1998|doi=10.1016/S0016-7037(98)00255-5|bibcode=1998GeCoA..62.3437C|issue=21–22|last2=Edwards|first2=R.Lawrence|last3=Murrell|first3=M.T.|last4=Benjamin|first4=T.M.}}</ref>

==Precautions==
Protactinium is both toxic and highly radioactive and thus all manipulations with it are performed in a sealed [[glove box]]. Its major isotope <sup>231</sup>Pa has a [[specific activity]] of {{convert|0.048|Ci|GBq|lk=on}} per gram and primarily emits alpha-particles with an energy of 5&nbsp;MeV, which can be stopped by a thin layer of any material. However, it slowly decays, with a half-life of 32,760 years, into <sup>227</sup>Ac, which has a specific activity of {{convert|74|Ci|GBq}} per gram, emits both alpha and beta radiation, and has a much shorter half-life of 22 years. <sup>227</sup>Ac, in turn, decays into lighter isotopes with even shorter half-lives and much greater specific activities (SA), as summarized in the table below showing the decay chain of protactinium-231.<ref name=ANL/>

{|class="wikitable" style="text-align:center"
!Isotope
|<sup>231</sup>Pa|| <sup>227</sup>Ac|| <sup>227</sup>Th|| <sup>223</sup>Ra|| <sup>219</sup>Rn|| <sup>215</sup>Po||<sup>211</sup>Pb|| <sup>211</sup>Bi || <sup>207</sup>Tl
|-
!SA ([[Curie|Ci]]/g)
| 0.048|| 73|| 3.1{{e|4}}|| 5.2{{e|4}}|| 1.3{{e|10}}||3{{e|13}}|| 2.5{{e|7}}|| 4.2{{e|8}}||1.9{{e|8}}
|-
!Decay
|α || α, β||α || α ||α ||α ||β||α, β||β
|-
![[Half-life]]
| 33 ka|| 22 a|| 19 days || 11 days|| 4 s|| 1.8 ms|| 36 min|| 2.1 min|| 4.8 min
|}

As protactinium is present in small amounts in most natural products and materials, it is ingested with food or water and inhaled with air. Only about 0.05% of ingested protactinium is absorbed into the blood and the remainder is excreted. From the blood, about 40% of the protactinium
deposits in the bones, about 15% goes to the liver, 2% to the kidneys, and the rest leaves the body. The biological half-life of protactinium is about 50 years in the bones, whereas in other organs the kinetics has a fast and slow component. So in the liver 70% of protactinium have a half-life of 10 days and 30% remain for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In all these organs, protactinium promotes cancer via its radioactivity.<ref name=ANL/><ref name=target/> The maximum safe dose of Pa in the human body is {{convert|0.03|µCi|kBq|abbr=on}}, which corresponds to 0.5 micrograms of <sup>231</sup>Pa. This isotope is 2.5{{e|8}} times more toxic than [[hydrocyanic acid]].<ref>{{cite book|author = Palshin, E.S. |author2 = et al.|title = Analytical chemistry of protactinium| place =Moscow|publisher = Nauka|date = 1968}}</ref> The maximum allowed concentrations of <sup>231</sup>Pa in the air in Germany is {{val|3|e=-4|u=Bq/m<sup>3</sup>}}.<ref name=target>{{cite journal|doi=10.1016/j.nima.2008.02.084|last1=Grossmann|date=2008|first1=R|pages=122|volume=590|last2=Maier|journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment|first2=H|last3=Szerypo|first3=J|last4=Friebel|first4=H|title=Preparation of 231Pa targets|bibcode = 2008NIMPA.590..122G }}</ref>

==See also==
* [[Ada Hitchins]], who helped Soddy to discover the element protactinium

==References==
{{reflist|35em}}

==Bibliography==
*{{cite book
 |last1=Greenwood |first1=Norman  N.
 |authorlink1=Norman Greenwood
 |last2=Earnshaw |first2=Alan
 |date=1997
 |title=Chemistry of the Elements
 |edition=2nd|ref=Greenwood
 |publisher=[[Butterworth–Heinemann]]
 |isbn=0080379419
}}

==External links==
{{Commons|Protactinium}}
{{wiktionary|protactinium}}
* [http://www.periodicvideos.com/videos/091.htm Protactinium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

{{clear}}
{{Compact periodic table}}
{{Protactinium compounds}}
{{Use dmy dates|date=December 2011}}

[[Category:Protactinium| ]]
[[Category:Actinides]]
[[Category:Chemical elements]]