gallium

 

(găl'ēəm) , metallic chemical element; symbol Ga; at. no. 31; at. wt. 69.72; m.p. 29.78°C; b.p. 2,403°C; sp. gr. 5.904 at 29.6°C (solid), 6.095 at 29.8°C (liquid); valence +2 or +3. Solid gallium is a blue-gray metal with orthorhombic crystalline structure. The liquid metal has a beautiful silver color. Although gallium is solid at normal room temperatures, it becomes liquid when heated slightly. It is the only metal other than mercury, cesium, and rubidium that has this property. Gallium is a liquid over a wide temperature range and has a low vapor pressure even at high temperatures; it has found limited use in thermometers and manometers for high-temperature measurements. Gallium expands about 3% when solidified. The metal is relatively unreactive. It does not react with air or water at room temperature and is only slightly attacked by mineral acids; it is oxidized slowly when red-hot and reacts with water at high temperatures. Liquid gallium wets porcelain and glass surfaces; it forms a bright, highly reflective surface when coated on glass. It is used to form low-melting alloys. Gallium is chemically similar to aluminum, the element above it in Group 13 of the periodic table. It forms many compounds, among them oxides, hydroxides, halides, alums, and numerous organometallic compounds. Gallium arsenide and gallium phosphide are used in rectifiers and transistors as semiconductors and in lasers, light-emitting transistors, photocells, and electronic refrigeration. Although gallium is widely distributed in nature, it does not occur in appreciable concentrations even in germanite, the ore richest in gallium. Gallium is produced commercially as a byproduct in the production of zinc and aluminum. In Europe and Great Britain it is recovered from flue dust, a residue from the burning of coal. D. I. Mendeleev predicted the properties of gallium, which he called ekaaluminum, before it was discovered spectroscopically in 1875 by P. E. Lecoq de Boisbaudran.

31 zinc ← gallium → germanium Al ↑ Ga ↓ In Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	gallium, Ga, 31
Chemical series	poor metals
Group, Period, Block	13, 4, p
Appearance	silvery white
Standard atomic weight	69.723(1)  g·mol−1
Electron configuration	[Ar] 3d10 4s2 4p1
Electrons per shell	2, 8, 18, 3
Physical properties
Phase	solid
Density (near r.t.)	5.91  g·cm−3
Liquid density at m.p.	6.095  g·cm−3
Melting point	302.9146 K (29.7646 °C, 85.5763 °F)
Boiling point	2477 K (2204 °C, 3999 °F)
Heat of fusion	5.59  kJ·mol−1
Heat of vaporization	254  kJ·mol−1
Heat capacity	(25 °C) 25.86  J·mol−1·K−1
Vapor pressure P(Pa) 1 10 100 1 k 10 k 100 k at T(K) 1310 1448 1620 1838 2125 2518
Atomic properties
Crystal structure	orthorhombic
Oxidation states	3, 1 (amphoteric oxide)
Electronegativity	1.81 (scale Pauling)
Ionization energies (more)	1st:  578.8  kJ·mol−1
	2nd:  1979.3  kJ·mol−1
	3rd:  2963  kJ·mol−1
Atomic radius	130  pm
Atomic radius (calc.)	136  pm
Covalent radius	126  pm
Van der Waals radius	187 pm
Miscellaneous
Magnetic ordering	no data
Thermal conductivity	(300 K) 40.6  W·m−1·K−1
Speed of sound (thin rod)	(20 °C) 2740 m/s
Mohs hardness	1.5
Brinell hardness	60  MPa
CAS registry number	7440-55-3
Selected isotopes
Main article: Isotopes of gallium iso NA half-life DM DE (MeV) DP 69Ga 60.11% Ga is stable with 38 neutrons 71Ga 39.89% Ga is stable with 40 neutrons
References

Gallium (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 compounds gallium nitride and gallium arsenide, used as a semiconductor, most notably in light-emitting diodes (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 conchoidally 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, antimony and water.

Gallium also attacks most other metals by diffusing into their metal lattice. Gallium for example diffuses into the grain boundaries of Al/Zn alloys^[1] or steel.^[2], making them very brittle. Also, Gallium metal easily alloys with many metals,^{[citation needed]} and was used in small quantities in the core of the first atomic bomb to help stabilize the plutonium crystal structure.^{[citation needed]}

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, rubidium, 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 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 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.

History

Gallium (Latin Gallia meaning Gaul (essentially modern France); also gallus, meaning "rooster") was discovered spectroscopically by Lecoq de Boisbaudran in 1875 by its characteristic spectrum (two 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 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 KOH solution. He named the element "gallia" after his native land of France. It was later claimed that, in one of those multilingual puns 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 (USGC) 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 small quantities of gallium, typically less than 1 % by weight.^[3][4][5][6]

Most gallium is extracted from the crude aluminium 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.

As of 2006, the current price for 1 kg gallium of 99.9999% purity seems to be at about US$ 400.^{[citation needed]}

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 integrated 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.

As a wetting, and alloy improvement agent:

• Because 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 weapon pits is machined by alloying with gallium to stabilize the allotropes of plutonium.
• Gallium added in quantities up to 2% in common solders can aid wetting and flow characteristics.

As part of an energy storage mechanism:

• When gallium is alloyed with aluminium it can be used to break the bond between hydrogen and oxygen in water. A reaction occurs when water is added to the alloy which produces hydrogen and aluminium oxide. This could potentially provide a solid hydrogen source for transportation purposes, which would be more convenient than a pressurized hydrogen tank.^[7] Resmelting the resultant aluminum oxide and gallium mixture to metallic aluminum and gallium and reforming these into electrodes would constitute most of the energy input into the system, while electricity produced by a hydrogen fuel cell could constitute an energy output.^[8]The thermodynamic efficiency of the aluminum smelting process is said to be approximately 50 percent. Therefore, at most no more than half the energy that goes into smelting aluminum could be recovered by a fuel cell.

For 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. [1]
• 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 Ga³⁺ which is active). For these applications, a radioactive isotope such as ⁶⁷Ga is used. The body handles Ga³⁺ 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 fluorodeoxyglucose (FDG) for positron emission tomography, "PET" scan.
• Gallium nitrate, both oral and topical, is finding use in treating arthritis.^[9]
• Much research is being devoted to gallium alloys as substitutes for mercury dental amalgams, but these compounds have yet to see wide acceptance.
• Research is being conducted to determine whether gallium can be used to fight bacterial infections in people with cystic fibrosis. Gallium is similar in size to iron, an essential nutrient for respiration. When gallium is mistakenly picked up by bacteria such as Pseudomonas, the bacteria's ability to respire is interfered with and the bacteria die. The mechanism behind this is that iron is redox active, which allows for the transfer of electrons during respiration, but gallium is redox inactive. ^[10][11]

Miscellaneous:

• Magnesium gallate containing impurities (such as Mn²⁺), 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 (see SNO).[2]
• As a liquid metal ion source for a focused ion beam.

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

• Gallium compounds

References

• Los Alamos National Laboratory – Gallium
• Webelements: detailed information on gallium

1. ^ W. L. Tsai, Y. Hwu, C. H. Chen, L. W. Chang, J. H. Je, H. M. Lin, G. Margaritondo (2003). "Grain boundary imaging, gallium diffusion and the fracture behavior of Al–Zn Alloy – An in situ study". Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 199: 457-463. DOI:10.1016/S0168-583X(02)01533-1. 
2. ^ Vigilante, G. N., Trolano, E., Mossey, C. (Jun 1999). Liquid Metal Embrittlement of ASTM A723 Gun Steel by Indium and Gallium. Defense Technical Information Center.
3. ^ Shan Xiao-quan, Wang Wen and Wen Bei (1992). "Determination of gallium in coal and coal fly ash by electrothermal atomic absorption spectrometry using slurry sampling and nickel chemical modification". J. Anal. At. Spectrom. 7: 761 - 764. DOI:10.1039/JA9920700761. 
4. ^ Gallium in West Virginia Coals. West Virginia Geological and Economic Survey (2 Mar 2002).
5. ^ O. Font, X. Querol, R. Juan, R. Casado, C. R. Ruiz, A. Lopez-Soler, P. Coca and F. G. Pena (2007). "Recovery of gallium and vanadium from gasification fly ash". Journal of Hazardous Materials 139 (3): 413-423. DOI:10.1016/j.jhazmat.2006.02.041. 
6. ^ A. J. W. Headlee and Richard G. Hunter (1953). "Elements in Coal Ash and Their Industrial Significance". Industrial and Engineering Chemistry 45 (3): 548 - 551. DOI:10.1021/ie50519a028. 
7. ^ "Purdue Energy Center symposium to pave the road to a hydrogen economy" (press release), Purdue University, April 10, 2007. 
8. ^ "New process generates hydrogen from aluminum alloy to run engines, fuel cells", PhysOrg.com, 16 May 2007. 
9. ^ G. Eby (2005). "Elimination of arthritis pain and inflammation for over 2 years with a single 90 min, topical 14% gallium nitrate treatment: Case reports and review of actions of gallium III". Medical Hypotheses 65 (6): 1136-1141. DOI:10.1016/j.mehy.2005.06.021. 
10. ^ A Trojan-horse strategy selected to fight bacteria
11. ^ Gallium May Have Antibiotic-Like Properties

External links

• WebElements.com – textbook information on gallium
• Picture in the Element collection from Heinrich Pniok
• Material safety data sheet at acialloys.com
• www.lenntech.com – textbook information regarding gallium
• environmental effects of gallium
• Price development of gallium 1959-1998
• Technology produces hydrogen by adding water to an alloy of aluminum and gallium

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