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Osmium (pronounced /'?zmi?m/) is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element. The density of osmium is 22.61&_160;?g/cm3, slightly greater than the density of iridium, the second densest element. Osmium is used in alloys with platinum, iridium and other platinum group metals. Osmium is found in nature as an alloy in platinum ore. Alloys of osmium are employed in fountain pen tips, electrical contacts and in other applications where extreme durability and hardness are needed. Osmium (Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England.[1] The discovery of osmium is intertwined with that of platinum and the other metals of the platinum group. Platinum reached Europe as platina ("small silver"), first encountered in the late 17th century in silver mines around the Chocó Department, in Colombia.[2] The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.[3] Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue.[4] Joseph Louis Proust thought that the residue was graphite.[4] The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.[4] In 1803, British scientist Smithson Tennant analysed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids[5] and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word pt???? (ptènos) for winged.[6][7] However, Tennant, who had the advantage of a much greater amount of residue, continued his research and identified two previously undiscovered elements in the black residue, iridium and osmium.[4][5] He obtained a yellow solution (probably of cis–[Os(OH)2O4]2-) by reactions with sodium hydroxide at red heat. After acidification he was able to distill the formed OsO4.[7] He named osmium after Greek osme meaning "a smell", because of the smell of the volatile osmium tetroxide.[8] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.[4][9] Uranium and osmium were early successful catalysts in the Haber process, the nitrogen fixation reaction of nitrogen and hydrogen to produce ammonia, giving enough yield to make the process economically successful. However, in 1908 cheaper catalysts based on iron and iron oxides were introduced for the first pilot plants.[10]
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