terbium (tûrˈbēəm) [key] [from Ytterby, a village in Sweden], metallic chemical element; symbol Tb; at. no. 65; at. wt. 158.92535; m.p. 1,356°C; b.p. 3,123°C; sp. gr. about 8.25; valence +3 or +4. Terbium is a soft, malleable, ductile, silver-gray metal. It is one of the rare-earth metals of the lanthanide series in Group 3 of the periodic table. It does not tarnish rapidly in air. Its oxide, terbia, Tb2O3, is white; its peroxide, Tb4O7, is dark brown to black. Most of the salts are colorless or white and all contain trivalent terbium. The element and its compounds have limited commercial importance; some minor uses are in lasers, semiconductor devices, and phosphors for color television picture tubes. Terbium is found in gadolinite, cerite, and other rare-earth minerals and is recovered from euxenite, monazite, and zenotime. It is difficult to separate it from the other rare-earth metals; several methods are used. The pure metal may be produced by chemical reduction of the halide with calcium. The element was discovered in 1843 by C. G. Mosander as its oxide, which he called erbia. The element has been known as terbium since 1877.
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