View source for Gallium

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| colspan="2" align="center" | [[image:Ga,31.jpg|125px|Typical (melted blob)]]&nbsp;[[Image:Gallium1_640x480.jpg|130px|Crystallized]]
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'''Gallium''' ([[International Phonetic Alphabet|IPA]]: {{IPA|/ˈgaliəm/}}) is a [[chemical element]] that has the symbol '''Ga''' and [[atomic number]] 31. A soft silvery metallic [[poor metal]], gallium is a brittle solid at low temperatures but liquefies slightly above [[room temperature]] and will melt in the hand. It occurs in trace amounts in [[bauxite]] and [[zinc]] ores. An important application is in the compound [[gallium arsenide]], used as a [[semiconductor]], most notably in [[light-emitting diode]]s (LEDs). 

== Notable characteristics ==
Elemental gallium is not found in nature, but it is easily obtained by [[smelting]].
Very pure gallium metal has a brilliant silvery color and its solid metal fractures [[Conchoidal fracture|conchoid]]ally like [[glass]]. Gallium metal expands by 3.1 percent when it solidifies, and therefore storage in either glass or metal containers is avoided, due to the possibility of container rupture with freezing. Gallium shares the higher-density liquid state with only a few materials like water and [[bismuth]]. 

Gallium also [[Liquid metal embrittlement|attacks]] most other metals by [[diffusion|diffusing]] into their metal [[crystal structure|lattice]] — another reason why it is important to keep gallium away from metal containers such as steel or aluminum. Gallium metal easily alloys with many metals, and was used in small quantities in the core of the first atomic bomb to help stabilize the plutonium crystal structure.

The [[melting point]] temperature of 30 °C allows the metal to be melted in one's hand. This metal has a strong tendency to [[supercool]] below its [[melting point]], thus necessitating [[seed crystal|seeding]] in order to solidify. Gallium is one of the metals (with [[caesium]], [[francium]] and [[mercury (element)|mercury]]) which are liquid at or near normal room temperature, and can therefore be used in metal-in-glass high-temperature [[thermometer]]s. It is also notable for having one of the largest liquid ranges for a metal, and (unlike mercury) for having a low [[vapor pressure]] at high temperatures. Unlike mercury, liquid gallium metal wets glass and skin, making it mechanically more difficult to handle (even though it is substantially less toxic and requires far fewer precautions). For this reason as well as the metal contamination problem and freezing-expansion problems noted above, samples of gallium metal are usually supplied in polyethylene packets within other containers.

Gallium does not [[crystal]]lize in any of the simple [[crystal structure]]s. The stable phase under normal conditions is [[orthorhombic]] with 8 atoms in the conventional [[unit cell]]. Each atom has only one nearest neighbor (at a distance of 244 [[picometre|pm]]) and six other neighbors within additional 39 pm. Many stable and [[metastable]] phases are found as function of temperature and pressure.

The bonding between the nearest neighbors is found to be of [[covalent]] character, hence Ga<sub>2</sub> [[dimer]]s are seen as the fundamental building blocks of the crystal. The compound with [[arsenic]], [[gallium(III) arsenide|gallium arsenide]] is a [[semiconductor]] commonly used in [[light-emitting diode]]s.

High-purity gallium is attacked slowly by [[mineral acid]]s.

== History ==
Gallium ([[Latin]] ''Gallia'' meaning [[Gaul]] (essentially modern [[France]]); also ''gallus'', meaning "rooster") was discovered [[spectroscopy|spectroscopically]] by [[Lecoq de Boisbaudran]] in [[1875]] by its characteristic spectrum (two [[violet (color)|violet]] lines) in an examination of a [[zinc blende]] from the [[Pyrenees]]. Before its discovery, most of its properties had been predicted and described by [[Dmitri Mendeleev]] (who called the hypothetical element ''[[Mendeleev's predicted elements|eka-aluminium]]'') on the basis of its  position in his [[periodic table]]. Later, in 1875, Boisbaudran obtained the free metal through the [[electrolysis]] of its [[hydroxide]] in [[potassium hydroxide|KOH]] solution. He named the element "gallia" after his native land of [[France]]. It was later claimed that, in one of those multilingual [[pun]]s so beloved of men of science of the early [[19th century]], he also named it after himself, as 'Lecoq' = the [[rooster]], and [[Latin]] for rooster is "gallus"; however, he denied this in an 1877 article.

== Occurrence ==
Gallium does not exist in free form in nature, nor do any high-gallium minerals exist to serve as a primary source of extraction of the element or its compounds. Gallium is found and extracted as a trace component in [[bauxite]], [[coal]], [[diaspore]], [[germanite]], and [[sphalerite]].  The [[United States Geological Survey|USGS]] estimates gallium reserves based on 50 ppm by weight concentration in known reserves of bauxite and zinc ores.  Some [[flue]] [[dust]]s from burning coal have been shown to contain as much as 1.5 percent gallium.

Most gallium is extracted from the crude [[aluminium hydroxide]] solution of the [[Bayer process]] for producing alumina and aluminum. A [[mercury (element)|mercury]] cell [[electrolysis]] and [[hydrolysis]] of the [[amalgam]] with [[sodium hydroxide]] leads to sodium gallate. Electrolysis then gives gallium metal. For [[semiconductor]] use, further purification is carried out using [[zone melting]], or else single crystal extraction from a melt ([[Czochralski process]]). Purities of 99.9999% are routinely achieved and commercially widely available.

The current price for 1 kg gallium of 99.9999% purity seems to be at about 400 US$.

== Applications ==
As a component of the semiconductor [[Gallium(III) arsenide|Gallium arsenide]], the most common application for gallium is analog [[integrated circuit]]s,  with the second largest use being [[optoelectronic]] devices (mostly [[laser diode]]s and [[LED|light-emitting diodes]].)

Other uses include:
*Since gallium [[wetting|wets]] glass or [[porcelain]], gallium can be used to create brilliant [[mirror]]s.
*Used widely as a [[dopant]] to [[doping (semiconductor)|dope]] [[semiconductor]]s and produce solid-state devices like [[transistor]]s.
*Gallium readily [[alloy]]s with most metals, and has been used as a component in [[fusible alloy|low-melting alloys]].  The [[plutonium]] used in [[Nuclear weapon design#Pit|nuclear weapon pits]] is machined by alloying with gallium to stabilize the [[allotropy|allotrope]]s of plutonium.  Much research is being devoted to gallium alloys as substitutes for mercury [[dental amalgam]]s, but such compounds have yet to see wide acceptance.  
*Gallium added in quantities up to 2% in common [[solder]]s can aid wetting and flow characteristics.
*Gallium is used in some high temperature thermometers.
*A [[eutectic]] alloy of gallium, [[indium]], and [[tin]], is widely available in medical thermometers (fever thermometers), replacing problematic mercury.  This alloy, with the trade name ''[[Galinstan]]'', has a freezing point of &minus;20 °C.
*[[Magnesium]] [[gallate]] containing impurities (such as Mn<sup>+2</sup>), is beginning to be used in [[ultraviolet]]-activated [[phosphor]] powder.
*It has been suggested that a liquid gallium-[[tin]] alloy could be used to cool computer chips in place of water. As it conducts heat approximately 65 times better than water it can make a comparable [[coolant]]. [http://www.newscientist.com/article.ns?id=dn7348] Gallium has a [[specific heat capacity]] of 0.37 J/(g&nbsp;K). Gallium's high [[specific gravity]] of 5.91 gives it a [[volumetric heat capacity]] of 0.37 x 5.91 = 2.187 J/cm³, meaning that a volume of gallium will heat by 4.184/2.187 = 1.9 times more than an equal volume of water in a cooling device. However given water's benign handling characteristics and plentiful abundance in most developed countries, gallium alloys are only really likely to see use in specialised applications such as cooling [[supercomputer]]s.
*Gallium [[salt]]s such as gallium [[citrate]] and gallium [[nitrate]] are used as [[radiopharmaceutical]] agents in [[nuclear medicine]] imaging. (The form or salt is not important, since it is the free dissolved gallium ion Ga<sup>+3</sup> which is active). For these applications, a [[radionuclide|radioactive isotope]] such as <sup>67</sup>Ga is used. The body handles Ga<sup>+3</sup> in many ways as though it were iron, and thus it is bound (and concentrates) in areas of inflammation, such as infection, and also areas of rapid cell division. This allows such sites to be imaged by nuclear scan techniques. See [[gallium scan]]. This use has largely been replaced by FDG [[Positron emission tomography]], "PET"  scan.
*Gallium nitrate, both oral and topical, is finding use in treating arthritis.[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16122880&query_hl=1&itool=pubmed_docsum]
*Gallium is the rarest component of new [[photovoltaic]] compounds (such as copper indium gallium selenium sulphide or Cu(In,Ga)(Se,S)2, recently announced by South African researchers) for use in solar panels as an alternative to [[crystalline silicon]], which is currently in short supply.

== Precautions ==
While not considered toxic, the data about gallium is inconclusive. Some sources suggest that it may cause [[dermatitis]] from prolonged exposure; other tests have not caused a positive reaction. Like most metals, finely divided gallium loses its luster. Powdered gallium appears gray. When gallium is handled with bare hands, the extremely fine dispersion of liquid gallium droplets which results from wetting skin with the metal may appear as a gray skin stain.

==See also==
* [[:category:Gallium compounds|Gallium compounds]]

== References ==
*[http://periodic.lanl.gov/elements/31.html Los Alamos National Laboratory &ndash; Gallium]
*[http://www.webelements.com/webelements/elements/text/Ga/key.html Webelements: detailed information on gallium]

== External links ==
{{Commons|Gallium}}
{{wiktionary|gallium}}
*[http://www.webelements.com/webelements/elements/text/Ga/index.html WebElements.com &ndash; textbook information on gallium]
*[http://www.pniok.de/ga.htm Picture in the Element collection from Heinrich Pniok]
*[[Material safety data sheet]] at [http://www.acialloys.com/msds/ga.html acialloys.com]
*[http://www.lenntech.com/Periodic-chart-elements/Ga-en.htm www.lenntech.com &ndash; textbook information regarding gallium]
*[http://minerals.usgs.gov/minerals/pubs/commodity/gallium/index.html environmental effects of gallium]
*[http://minerals.usgs.gov/minerals/pubs/commodity/gallium/460798.pdf Price development of gallium 1959-1998]


[[Category:Chemical elements]]
[[Category:Poor metals]]
[[Category:Coolants]]
[[Category:Gallium]]

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