Osmium (from Ancient Greek ὀσμή (osmḗ) 'smell') is a chemical element; it has symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group. It was discovered in 1803 by Smithson Tennant, who named it for the characteristic smell of its volatile oxide. Osmium has the highest density of any stable element and one of the lowest abundances in the Earth's crust, where it is mainly found alongside other platinum group metals and in ores of copper and nickel. While its metallic applications are limited, manufacturers use alloys of osmium with platinum, iridium, and other platinum-group metals for fountain pen nib tipping, electrical contacts, and other applications that require extreme durability and hardness. Its chemical applications are more prominent, with major applications in electron microscopy and laboratory-scale production of vicinal diols.
Characteristics
Physical properties
Osmium is a hard, brittle, blue-gray metal, and the densest stable element—about twice as dense as lead. The density of osmium is slightly greater than that of iridium; the two are so similar (22.587 versus 22.562 g/cm3 at 20 °C) that each was at one time considered to be the densest element. Only in the 1990s were measurements made accurately enough (by means of X-ray crystallography) to be certain that osmium is the denser of the two. Osmium has a blue-gray tint. The reflectivity of single crystals of osmium is complex and strongly direction-dependent, with light in the red and near-infrared wavelengths being more strongly absorbed when polarized parallel to the c crystal axis than when polarized perpendicular to the c axis; the c-parallel polarization is also slightly more reflected in the mid-ultraviolet range. Reflectivity reaches a sharp minimum at around 1.5 eV (near-infrared) for the c-parallel polarization and at 2.0 eV (orange) for the c-perpendicular polarization, and peaks for both in the visible spectrum at around 3.0 eV (blue-violet). Osmium is a hard but brittle metal that remains lustrous even at high temperatures. It 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). The hardness of osmium is moderately high at 4 GPa. Because of its hardness, brittleness, low vapor pressure (the lowest of the platinum-group metals), and very high melting point (the fourth highest of all elements, after carbon, tungsten, and rhenium), solid osmium is difficult to machine, form, or work.
Chemical properties
Osmium forms compounds with oxidation states ranging from −2 to +8. The most common oxidation states are +2, +3, +4, and +8. Examples of the −1 and −2 oxidation states are Na2[Os4(CO)13] (mixed 0 and −1 oxidation states) and Na2[Os(CO)4], respectively; these reactive compounds are used to synthesize osmium cluster compounds. Another example of the −1 oxidation state of osmium is K2[Os2(CO)8].
The most common compound exhibiting the +8 oxidation state, osmium tetroxide (OsO4), is a toxic, volatile, and water-soluble solid with a "pronounced and nauseating" smell. Osmium tetroxide forms red perosmates OsO4(OH)22− upon reaction with alkali. With ammonia, it forms the nitrido-osmates OsO3N−. The +4 oxide, osmium dioxide (OsO2), is a darkly-colored, non-volatile, and much less reactive compound. Osmium pentafluoride (OsF5) is known, but osmium trifluoride (OsF3) has not yet been synthesized. The lower oxidation states are stabilized by the larger halogens, so that the trichloride, tribromide, triiodide, and even diiodide are known. The oxidation state +1 is known only for osmium monoiodide (OsI), whereas several carbonyl complexes of osmium, such as triosmium dodecacarbonyl (Os3(CO)12), represent oxidation state 0. 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−. Despite its broad range of compounds in numerous oxidation states, osmium in bulk form at ordinary temperatures and pressures is inert. It resists attack by most acids and bases including aqua regia, but is attacked by F2 and Cl2 at high temperatures, and by hot concentrated nitric acid to produce OsO4. It can be dissolved by molten alkalis fused with an oxidizer such as sodium peroxide (Na2O2) or potassium chlorate (KClO3) to give osmates such as K2[OsO2(OH)4].
Isotopes
Osmium has seven naturally occurring isotopes, five of which are stable: 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os. At least 37 artificial radioisotopes and 20 nuclear isomers exist, with mass numbers ranging from 160 to 203; the most stable of these is 194Os with a half-life of 6.0 years. The two primordial radioisotopes 184Os and 186Os are known to undergo alpha decay with such long half-lives - the current best values being (1.12±0.23)×1013 and (2.0±1.1)×1015 years, approximately 140000 times the age of the universe - that for practical purposes it can be considered stable. Alpha decay is predicted for all the other naturally occurring isotopes, but has not been observed, presumably due to very long half-lives; 184Os and 192Os are also predicted to undergo double beta decay, but this not been observed either. 189Os has a spin of 3/2 but 187Os has a nuclear spin ½. Its low natural abundance (1.64 %) and low nuclear magnetic moment means that it is one of the most difficult natural abundance isotopes for NMR spectroscopy. 187Os is the descendant of 187Re (half-life 4.12×1010 years) and is used extensively in dating terrestrial as well as meteoric rocks (see Rhenium–osmium dating). It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons. This decay is a reason why rhenium-rich minerals are abnormally rich in 187Os.
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