gallium găl´ēəm [key], metallic chemical element; symbol Ga; at. no. 31; at. wt. 69.723; 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.
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