palladium, chemical element

palladium [Gr. Pallas, goddess of wisdom], metallic chemical element; symbol Pd; at. no. 46; at. wt. 106.42; m.p. 1,554℃; b.p. 2,970℃; sp. gr. 12.02 at 20℃; valence +2, +3, or +4. Palladium is a lustrous silver-white metal with a face-centered cubic crystalline structure. Directly above platinum, it is one of the platinum metals in Group 10 of the periodic table. It is strongly resistant to corrosion in air and to the action of acids (except nitric acid) at ordinary temperatures. However, it is attacked by hot acids, and it dissolves in aqua regia. It forms many compounds, including oxides, chlorides, fluorides, sulfides, phosphides, and several complex salts. Palladium has a great ability to absorb hydrogen; when finely divided, one volume of palladium absorbs as many as 900 volumes of the gas. When heated, it allows hydrogen to diffuse rapidly through it; it is thus used to purify hydrogen gas. Palladium is found in nature with platinum minerals and in association with certain nickel ores; the primary palladium-mining countries are South Africa and Russia.

Because of its corrosion resistance, one important use of palladium is in alloys used in low voltage electrical contacts. Palladium is used extensively in jewelry-making in certain alloys called “white gold.” It may be alloyed with platinum or substituted for it. It is used in watch bearings, springs, and balance wheels, in surgical instruments, and also for mirrors in scientific instruments. For use in dentistry it is alloyed with silver, gold, and copper. In chemistry it is a catalyst in sulfuric acid manufacture and in hydrogenation processes; it used as a catalyst also in motor vehicle catalytic converters, the use that now consumes more than three fourths of the palladium produced. Palladium salts are used in electroplating. Palladium is not as abundant as platinum, but it was long less expensive. Palladium was discovered in 1803 by W. H. Wollaston.

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