cadmium

cadmium (kădˈmēəm) [key] [from cadmia, Lat. for calamine, with which cadmium is found associated], metallic chemical element; symbol Cd; at. no. 48; at. wt. 112.411; m.p. 321°C; b.p. 765°C; sp. gr. 8.65 at 20°C; valence +2. Cadmium is a lustrous, silver-white, ductile, very malleable metal. It belongs to Group 12 of the periodic table, and resembles zinc in its chemical properties. Like zinc, it tarnishes in moist air. Cadmium oxide, a brown powder formed by burning the metal in air, is used in electroplating; it is also made by heating cadmium hydroxide. Cadmium forms a carbonate, a chloride, and several complex ions. Cadmium yellow (the sulfide) is a very durable yellow pigment used in paints. The major use of cadmium is as a coating that is electroplated on iron and steel to prevent corrosion; it is preferable to zinc for protection from alkalies. Cadmium is also used in so-called fusible metals, which are low-melting alloys such as Wood's metal, used in automatic fire sprinklers and alarm systems. Cadmium is used in alkaline nickel-cadmium electric storage cells, which have a greater storage capacity than an equal weight of lead-acid storage cells. It has also found some use in the control of nuclear reactions, since it absorbs neutrons. Cadmium does not occur uncombined in nature; greenockite, a cadmium sulfide mineral first found in Scotland, is the only commercial ore. Cadmium is obtained principally as a byproduct of the smelting and refining of ores of zinc, especially zinc sulfides, and of lead and copper. The element was discovered in 1817 by Friedrich Stromeyer.

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