Encyclopediarhodium
rhodium (rō'dēum) [key], metallic chemical element; symbol Rh; at. no. 45; at. wt. 102.9055; m.p. about 1,966°C; b.p. 3,727±100°C; sp. gr. 12.41 at 20°C; valence +2, +3, +4, +5, or +6. Rhodium is a lustrous, silver-white, chemically resistant metal in the so-called platinum group of metals in Group 9 of the periodic table. It has a face-centered cubic crystalline structure. It is insoluble in most acids, including aqua regia, but is dissolved in hot concentrated sulfuric acid. Rhodium compounds include halides, oxides, sulfates, sulfites, a nitrate, and a sulfide. The salts form rose-colored aqueous solutions. Rhodium is found associated with other platinum metals in river sands and in compounds in such minerals as rhodite and sperrylite. It is obtained as a byproduct in the refining of nickel sulfide ores mined near Sudbury, Ont., Canada. The major use of the metal is in alloys with platinum and iridium; it gives improved high-temperature strength and oxidation resistance. These alloys are used in pen nibs, phonograph needles, high-temperature thermocouple and resistance wires, bearings, and electrical contacts and as a catalyst. The metal itself, because of its brilliance and resistance to tarnish, is used to plate jewelry and the reflectors of searchlights. Rhodium was discovered in 1804 by W. H. Wollaston in crude platinum ore.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007, Columbia University Press. All rights reserved.
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