OSMIUM

Listing description

Osmium ( /ˈɒzmiəm/ OZ-mee-əm) 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. Osmium is twice as dense as lead. The density of osmium is 22.59 g/cm³, slightly greater than that of iridium, the second densest element. Osmium is found in nature as an alloy, mostly in platinum ores. Osmium is also used in alloys, with platinum, iridium and other platinum group metals. Those alloys are employed in fountain pen tips, electrical contacts and in other applications where extreme durability and hardness are needed.^[2]

Detailed description

Osmium is an extremely dense, blue-gray, hard but brittle metal that remains lustrous even at high temperatures. Due to its hardness, brittleness, low vapor pressure (the lowest of the platinum group metals), and very high melting point (the fourth highest of all elements), solid osmium is difficult to machine, form, or work. Osmium is generally considered to be the densest known element, slightly denser than iridium.^[3] Calculations of density from the space lattice may produce the most reliable data for these elements, giving a density of 22.562±0.009 g/cm³ for iridium versus 22.587±0.009 g/cm³ for osmium.^[4] The extraordinary density of osmium is a consequence of the lanthanide contraction.^[4]

Osmium possesses quite remarkable chemical and physical properties. It has the highest melting point and the lowest vapor pressure in the platinum family. Osmium has a very low compressibility. Correspondingly, its bulk modulus is extremely high, reported between 395 and 462 GPa, which rivals that of diamond (443 GPa). However, the hardness of osmium is lower than diamond, only 4 GPa.^[5][6][7]

Osmium heptafluoride (OsF₇) and osmium pentafluoride (OsF₅) are known, but osmium trifluoride (OsF₃) has not been synthesized yet. The lower oxidation states are stabilized by the larger halogens. Therefore, the trichloride, tribromide, triiodide and even osmium diiodide are known. The oxidation state +1 is only known for the osmium iodide (OsI), whereas several carbonyl complexes of osmium, such as triosmium dodecacarbonyl (Os₃(CO)₁₂), represent the oxidation state 0.^{[12][13][16][17]}

In general, the lower oxidation states of osmium are stabilized by ligands that are good σ-donors (such as amines) and π-acceptors (heterocycles containing nitrogen). The higher oxidation states are stabilized by strong σ- and π-donors, such as O²⁻ and N³⁻.^[18]

History

Osmium (from Greek osme (ὀσμή) meaning "smell") was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England.^[22] 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.^[23] The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.^[24] 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.^[25] Joseph Louis Proust thought that the residue was graphite.^[25] Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough material for further experiments.^[25]

In 1803, Smithson Tennant analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids^[26] and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word πτηνος (ptènos) for winged.^[27][28] However, Tennant, who had the advantage of a much larger amount of residue, continued his research and identified two previously undiscovered elements in the black residue, iridium and osmium.^[25][26] 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.^[27] He named osmium after Greek osme meaning "a smell", because of the smell of the volatile osmium tetroxide.^[29] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.^[25][30]

Occurrence

Osmium is one of the least abundant elements in the Earth's crust with an average mass fraction of 0.05 ppb in the continental crust.^[33]

Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich).^[26] In the nickel and copper deposits, the platinum group metals occur as sulfides (i.e., (Pt,Pd)S)), tellurides (e.g., PtBiTe), antimonides (e.g., PdSb), and arsenides (e.g., PtAs₂); in all these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum group metals, osmium can be found naturally in alloys with nickel or copper.^[34]

Within the Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa,^[35] though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin in Canada are also significant sources of osmium. Smaller reserves can be found in the United States.^[35] The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum group metals. The second large alluvial deposit was found in the Ural Mountains, Russia, which is still mined.

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