Transition metal
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. Osmium has the highest density of any stable element and one of the lowest abundances in the Earth's crust. 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.
Osmium was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England. 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. The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.
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. Joseph Louis Proust thought that the residue was graphite. Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed iridium in the black platinum residue in 1803, but did not obtain enough material for further experiments. Later the two French chemists Fourcroy and Vauquelin identified a metal in a platinum residue they called ptène.
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 and obtained a volatile new oxide, which he believed was of this new metal—which he named ptene, from the Greek word πτηνος (ptènos) for winged. …
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. …
Osmium is one of the least abundant stable elements in Earth's crust, with an average mass fraction of 50 parts per trillion in the continental crust.
Osmium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (iridium rich), and iridosmium (osmium rich). In 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.
Within 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, 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. 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.
Because osmium is virtually unforgeable when fully dense and very fragile when sintered, it is rarely used in its pure state, but is instead often alloyed with other metals for high-wear applications. Osmium alloys such as osmiridium are very hard and, along with other platinum-group metals, are used in the tips of fountain pens, instrument pivots, and electrical contacts, as they can resist wear from frequent operation. They were also used for the tips of phonograph styli during the late 78 rpm and early "LP" and "45" record era, circa 1945 to 1955. Osmium-alloy tips were significantly more durable than steel and chromium needle points, but wore out far more rapidly than competing, and costlier, sapphire and diamond tips, so they were discontinued.
Only two osmium compounds have major applications: osmium tetroxide for staining tissue in electron microscopy and for the oxidation of alkenes in organic synthesis, and the non-volatile osmates for organic oxidation reactions.
Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for optical and electron microscopy. As a strong oxidant, it cross-links lipids mainly by reacting with unsaturated carbon–carbon bonds and thereby both fixes biological membranes in place in tissue samples and simultaneously stains them. Because osmium atoms are extremely electron-dense, osmium staining greatly enhances image contrast in transmission electron microscopy (TEM) studies of biological materials. …
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 1/2. 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.
The primary hazard presented by metallic osmium is the potential formation of osmium tetroxide (OsO4), which is volatile and very poisonous. This reaction is thermodynamically favorable at room temperature, but the rate depends on the temperature and surface area of the metal. As a result, bulk material is not considered hazardous while powders react quickly enough that samples can sometimes smell like OsO4 if they are handled in air.