osmium (ŏzˈmēəm) [key], metallic chemical element; symbol Os; at. no. 76; at. wt. 190.23; m.p. 3,045±30°C; b.p. 5,027±100°C; sp. gr. 22.57 at 20°C; valence usually +0 to +8. Osmium is a very hard, brittle, lustrous bluish-white metal with a close-packed hexagonal crystalline structure. It immediately precedes iridium in Group 8 of the periodic table. The measured densities of osmium and iridium indicate that osmium is slightly more dense than iridium, and osmium is generally credited as the heaviest element. Osmium does not oxidize readily in air except when heated or in powdered form; it then forms the unpleasant smelling, highly toxic tetroxide, OsO4. The tetroxide is used in microscopy as a stain, in fingerprint detection, and as a catalyst. Osmium is not affected by common acids but is oxidized to the tetroxide by hot nitric acid, hot sulfuric acid, or aqua regia. Osmium reacts with fluorine or chlorine gas at high temperatures to give the tetrafluoride or tetrachloride. In addition to the valences noted above, osmium assumes other valences between 0 and +8 in various compounds. Osmium is found in platinum ores and in the mineral osmiridium. It is recovered commercially as a byproduct of the refining of nickel ores mined near Sudbury, Ont., Canada. The metal is used largely for the production of hard alloys for use in fountain pen points, phonograph needles, and instrument bearings. Osmium was discovered by Smithson Tennant in 1804 in a residue left after dissolving crude platinum in aqua regia.