1s² 2s²p⁶ 3s²p⁶d¹⁰ 4s²p¹

Characteristics

Gallium is one of four metals--Mercury (Hg), Cesium (Cs) and Rubidium (Rb)--which melts near room temperature and has one of the largest liquid ranges of any metal, so it has found use in high temperature thermometers.  At room temperature Gallium is as soft as lead and can be cut with a knife.  Its melting point is abnormally low and it will begin to melt in the palm of a warm hand.   Gallium is one of a small number of metals that expands when freezing.

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 conchiodally 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 Germanium, Bismuth and water.

Gallium also attack most other metals by diffusing into their metal lattice.   Gallium for example diffuses into the grain boundaries of Al/Zn alloys or steel.   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/freezing point, thus necessitating seeding in order to solidify.  Gallium is one of the metals (with Caesium, Francium and Mercury) which are liquid at or near normal room temperature, and can therefore be used in metal-in-glass high-temperature thermometers.  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 crystallize in any of the simple crystal structures.  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 ppm) 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 Ga₂ dimers are seen as the fundamental building blocks of the crystal.   The compound with Arsenic, Gallium Arsenide is a semiconductor commonly used in light-emitting diodes.

High-purity Gallium is attacked slowly by mineral acids. 

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 USGS estimates Gallium reserves based on 50 ppm by weight concentration in known reserves of bauxite and zinc ores.  Some flue dusts from burning coal have been shown to contain as much as 1.5 percent gallium.

Most gallium is extracted from the crude Aluminum Hydroxide solution of the Bayer Process for producing alumina and Aluminum.  A 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.

Applications

Semiconductor and electronic industry.  The semiconductor applications are the main reason for the low-cost commercial availability of the extremely high-purity (99.9999+%) metal:

• As a component of the semiconductor Gallium Arsenide, the most common application for Gallium is analog intetrated circuits, with the second largest use being optoelectronic devices (mostly laser diodes and light-emitting diodes).
• Gallium is used widely as a dopant to dope semiconductors and produce solid-state devices like transistors.
• 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.

Wetting & Alloy Agent

• Since gallium wets glass or porcelain, Gallium can be used to create brilliant mirrors.  
• Gallium readily alloys with most metals, and has been used as a component in low-melting alloys.  The Plutonium used in nuclear weapons pits is machined by alloying with Gallium to stabilize the allotropes of Plutonium.
• Gallium added in quantities up to 2% in common solder can aid wetting and flow characteristics.

Liquid Alloys

• 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.  Gallium has a Specific Heat Capacity of 0.37 J/(g·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 supercomputers.
• Gallium is used in some high temperature thermometers.

Biomedical Applications

• A low temperature liquid 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 (with the "-stan" referring to the Tin), has a freezing point of -20°C.
• Gallium salts 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⁺³ which is active). For these applications, a radioactive isotope such as ⁶⁷Ga is used.  The body handles Ga⁺³ 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.  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.
• Much research is being devoted to Gallium alloys as substitutes for Mercury dental amalgams, but these compounds have yet to see wide acceptance.

Miscellaneous

• Magnesium Gallate containing impurities (such as Mn⁺²), is beginning to be used in ultraviolet-activated phosphor powder.
• Neutrino Detection:  Possibly the largest amount of pure Gallium ever collected in a single spot was the GALLEX neutrino detector operated in the early 1990's in an Italian mountain tunnel.  The detector contained 12.2 tons of watered Gallium-71.  Solar neutrinos caused a few atoms of Ga-71 to become radioactive Ge-71, which were detected.   The solar neutrino flux deduced was found to have a deficit of 40% from theory.   This was not explained until better solar neutrino detectors and theories were constructed.

Compounds

Gallium is often found as a trace element in Diaspore, Sphalerite, Germanite, Bauxite and Coal.  Gallium arsenide (GaAs) can produce laser light directly from electricity. Large amounts of gallium trichloride (GaCl₃) have been gathered to build the Gallium Neutrino Observatory, an observatory located in Italy built to study particles called neutrinos which are produced inside the sun during the process of nuclear fusion.

Isotopes

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.

(L. Gallia, France; also from Latin, gallus, a translation of Lecoq, a cock) Predicted and described by Mendeleev as ekaaluminum, and discovered spectroscopically by Lecoq de Boisbaudran in 1875, who in the same year obtained the free metal by electrolysis of a solution of the hydroxide in KOH. Gallium is often found as a trace element in diaspore, sphalerite, germanite, bauxite, and coal. Some flue dusts from burning coal have been shown to contain as much 1.5% gallium. It is the only metal, except for mercury, cesium, and rubidium, which can be liquid near room temperatures; this makes possible its use in high-temperature thermometers. It has one of the longest liquid ranges of any metal and has a low vapor pressure even at high temperatures. There is a strong tendency for gallium to supercool below its freezing point. Therefore, seeding may be necessary to initiate solidification. Ultra-pure gallium has a beautiful, silvery appearance, and the solid metal exhibits a conchoidal fracture similar to glass. The metal expands 3.1% on solidifying; therefore, it should not be stored in glass or metal containers, as they may break as the metal solidifies. Gallium wets glass or porcelain and forms a brilliant mirror when it is painted on glass. It is widely used in doping semiconductors and producing solid-state devices such as transistors. High-purity gallium is attacked only slowly by mineral acids. Magnesium gallate containing divalent impurities such as Mn⁺² is finding use in commercial ultraviolet activated powder phosphors. Gallium arsenide is capable of converting electricity directly into coherent light. Gallium readily alloys with most metals, and has been used as a component in low-melting alloys. Its toxicity appears to be of a low order, but should be handled with care until more data are forthcoming. The metal can be supplied in ultrapure form (99.99999+%). The cost is about $3/g.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond