Osmium - chemistry.

Osmium - chemistry. Osmium, symbol Os, bluish-white, brittle metallic element. Osmium and iridium are generally considered the most dense elements. The atomic number of osmium is 76. The element is one of the transition elements of the periodic table (see Periodic Law). Osmium was discovered in 1803 by the British chemist Smithson Tennant. The element was named for the smell it gives off when powdered; osme is a Greek word meaning "smell." Osmium has a hardness of 7; it melts at 3033°C (5491°F) and boils at 5012°C (9054°F). The element has a specific gravity of 22.6, making osmium more than twice as dense as lead. Osmium also has the highest resistance to compression, or bulk modulus, of any known substance. The atomic weight of osmium is 190.2. Osmium is not attacked by ordinary acids, but dissolves in aqua regia or fuming nitric acid. Osmium forms salts in which it has valences of 1 through 8. The metal occurs naturally in platinum ores and as an alloy, osmiridium, with iridium. Osmium ranks about 74th in natural abundance among the elements in crustal rock. The chief use of the metal is in forming very hard alloys with other platinum group metals, such as osmiridium. Alloyed with platinum, it has been used for standard weights and measures. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.