gallium

/gal"ee euhm/, n. Chem.

a rare, steel-gray, trivalent metallic element used in high-temperature thermometers because of its high boiling point (1983°C) and low melting point (30°C). Symbol: Ga; at. wt.: 69.72; at. no.: 31; sp. gr.: 5.91 at 20°C.

[1870-75; < NL, equiv. to L gall(us) cock (trans. of F coq, from Lecoq de Boisbaudran, 19th-century French chemist) + NL -ium -IUM]

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▪ chemical element

 chemical element, metal of main Group 13 (IIIa, or boron group) of the periodic table. It liquefies just above room temperature.

      Gallium was discovered (1875) by Paul-Émile Lecoq de Boisbaudran (Lecoq de Boisbaudran, Paul-Émile), who observed its principal spectral lines while examining material separated from zinc blende. Soon afterward he isolated the metal and studied its properties, which coincided with those that Dmitry Ivanovich Mendeleyev had predicted a few years earlier for eka-aluminum, the then-undiscovered element lying between aluminum and indium in his periodic table.

      Though widely distributed at the Earth's surface, gallium does not occur free or concentrated in independent minerals, except for gallite, CuGaS₂, rare and economically insignificant. It is extracted as a by-product from zinc blende, iron pyrites, bauxite, and germanite.

      Silvery white and soft enough to be cut with a knife, gallium takes on a bluish tinge because of superficial oxidation. Unusual for its low melting point (about 30° C [86° F]), gallium also expands upon solidification and supercools readily, remaining a liquid at temperatures as low as 0° C (32° F). Gallium remains in the liquid phase over a temperature range of about 2,000° C (about 3,600° F) with a very low vapour pressure up to about 1,500° C (about 2,700° F), the longest useful liquid range of any element. The liquid metal clings to (wets) glass and similar surfaces. The crystal structure of gallium is orthorhombic. Natural gallium consists of a mixture of two stable isotopes: gallium-69 (60.4 percent) and gallium-71 (39.6 percent).

      Gallium has been considered as a possible heat-exchange medium in nuclear reactors, although it has a high neutron-capture cross section. Radioactive gallium-72 shows some promise in the study of bone cancer; a compound of this isotope is absorbed by the cancerous portion of the bone.

      The metal gallium is stable in dry air. Somewhat similar to aluminum chemically, gallium slowly oxidizes in moist air until a protective film forms. On burning in air or oxygen it forms the white oxide, formula Ga₂O₃. This oxide can be reduced to the metal when heated strongly in hydrogen, and with gallium metal at 700° C it gives the lower oxide Ga₂O. It does not dissolve in cold nitric acid because, as with moist air, a protective film of gallium oxide forms. Gallium does not react with water at temperatures up to 100° C (212° F) but reacts slowly with hydrochloric and other mineral acids to give the gallium ion, Ga³⁺. The metal does dissolve in other acids to give gallium salts, and it dissolves in alkalies, with the evolution of hydrogen, to give gallates, such as [Ga(OH)₄]⁻, in which gallium appears in the anion. Gallium is amphoteric (i.e., it reacts either as an acid or a base depending on the circumstance), reacting with sodium and potassium hydroxide solutions to yield a gallate and hydrogen gas. The halogens attack it vigorously.

      In most of its compounds gallium has an oxidation state of +3 and, in a few, +1 (for example, the oxide, Ga₂O). There is no evidence for authentic compounds of gallium in its +2 state. The “dihalides,” for example, contain Ga⁺ and Ga³⁺ in a one-to-one ratio. With the Group 15 (Va) elements phosphorus, arsenic, and antimony and the Group 13 elements aluminum and indium, gallium forms compounds—e.g., gallium arsenide, GaAs, and indium gallium arsenide phosphide, InGaAsP—that have valuable semiconductor and optoelectronic properties. Some of these compounds are used in solid-state devices such as transistors and rectifiers, and some form the basis for light-emitting diodes and semiconductor lasers. Of the halides, only gallium trifluoride is ionic; the others have molecular lattices containing dimeric molecules, with formula Ga₂X₆. The sulfide (GaS), selenide (GaSe), and telluride (GaTe), made directly by combination of the elements at high temperature, are diamagnetic and contain gallium−gallium units with four positive charges (Ga−Ga)⁴⁺, in a layer lattice. The hydroxide, formula Ga(OH)₃, is amphoteric; it is precipitated from solutions of gallium salts by alkali hydroxides.

atomic number

31

atomic weight

69.72

melting point

29.78° C

boiling point

2,403° C

specific gravity

5.904 (29.6° C)

oxidation state

+3

electronic config.

[Ar]3d¹⁰4s²4p¹

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