ruthenium (rōthēˈnēəm) [key], metallic chemical element; symbol Ru; at. no. 44; at. wt. 101.07; m.p. about 2,310°C; b.p. about 3,900°C; sp. gr. 12.41 at 20°C; valence commonly +2, +3, +4, +6, or +8. Ruthenium is a hard, lustrous, silver-gray metal with a close-packed hexagonal crystalline structure. It is found directly above osmium in Group 8 of the periodic table. Below about 100°C ruthenium is insoluble in acids, including aqua regia, but reacts violently if potassium chlorate is added. It reacts with the halogens and with fused alkali hydroxides. When heated above 800°C it oxidizes, but it does not react with air at room temperature. It forms compounds with silicon and sulfur. Its compounds resemble those of osmium. Ruthenium is found in nature with other metals of the so-called platinum group in river sands, in minerals such as laurite (the sulfide) and osmiridium, and in association with certain ores containing platinum, copper, and nickel. It is obtained commercially as a byproduct of the refining of nickel sulfide ores mined near Sudbury, Ont., Canada. Ruthenium metal powder is obtained as the final product of a complex chemical process. It is used as a catalyst in hydrogenation, isomerization, nitrogenation, oxidation, and reforming reactions. The metal is used as a hardener in electrical contact alloys and filaments, in jewelry, in pen nibs, and in instrument pivots. It is usually alloyed with other metals. It is a very good hardener for palladium and platinum, and vastly improves the corrosion resistance of titanium when added in small amounts. It is also used in alloys with cobalt, molybdenum, nickel, tungsten, and other metals. Ruthenium compounds are used to color ceramics and glass. Ruthenium was discovered in 1827 in an impure form by G. W. Osann in residues of crude platinum ores from the Ural Mts. of Russia. Pure ruthenium was first prepared by K. K. Klaus in 1845; he showed that Osann's sample contained at least one other metal.