My watch list  


76 rheniumosmiumiridium


Name, Symbol, Number osmium, Os, 76
Chemical series transition metals
Group, Period, Block 8, 6, d
Appearance silvery, blue cast
Standard atomic weight 190.23(3)  g·mol−1
Electron configuration [Xe] 4f14 5d6 6s2
Electrons per shell 2, 8, 18, 32, 14, 2
Physical properties
Phase solid
Density (near r.t.) 22.61  g·cm−3
Liquid density at m.p. 20  g·cm−3
Melting point 3306 K
(3033 °C, 5491 °F)
Boiling point 5285 K
(5012 °C, 9054 °F)
Heat of fusion 57.85  kJ·mol−1
Heat of vaporization 738  kJ·mol−1
Heat capacity (25 °C) 24.7  J·mol−1·K−1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 3160 3423 3751 4148 4638 5256
Atomic properties
Crystal structure hexagonal
Oxidation states 8, 7, 6, 5, 4, 3, 2, 1, −1, −2
(mildly acidic oxide)
Electronegativity 2.2 (Pauling scale)
Ionization energies 1st: 840 kJ/mol
2nd: 1600 kJ/mol
Atomic radius 130  pm
Atomic radius (calc.) 185  pm
Covalent radius 128  pm
Magnetic ordering  ?
Electrical resistivity (0 °C) 81.2 nΩ·m
Thermal conductivity (300 K) 87.6  W·m−1·K−1
Thermal expansion (25 °C) 5.1  µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 4940 m/s
Shear modulus 222  GPa
Poisson ratio 0.25
Bulk modulus 462  GPa
Mohs hardness 7.0
Brinell hardness 3920  MPa
CAS registry number 7440-04-2
Selected isotopes
Main article: Isotopes of osmium
iso NA half-life DM DE (MeV) DP
184Os 0.02% >5.6×1013y α 2.966 180W
185Os syn 93.6 d ε 1.013 185Re
186Os 1.59% 2.0×1015y α 2.822 182W
187Os 1.96% Os is stable with 111 neutrons
188Os 13.24% Os is stable with 112 neutrons
189Os 16.15% Os is stable with 113 neutrons
190Os 26.26% Os is stable with 114 neutrons
191Os syn 15.4 d β- 0.314 191Ir
192Os 40.78% Os is stable with 116 neutrons
193Os syn 30.11 d β- 1.141 193Ir
194Os syn 6 y β- 0.097 194Ir

Osmium (pronounced /ˈɒzmiəm/) 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 one of the densest natural elements, competing for this status with iridium. 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.


Notable characteristics

Osmium in a metallic form is extremely dense, blue-white, brittle, and lustrous even at high temperatures, but proves to be extremely difficult to make. Powdered osmium is easier to make, but powdered osmium exposed to air leads to the formation of osmium tetroxide (OsO4), which is very toxic. The tetroxide is a powerful oxidizing agent, very volatile, water-soluble, pale yellow, crystalline solid with a strong smell that boils at 130°C. By contrast osmium dioxide (OsO2) is black, non-volatile and much less reactive and toxic.

Due to its very high density osmium is generally considered to be the densest known element, narrowly defeating iridium. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 kg/m3 for iridium versus 22610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time. If one distinguishes different isotopes, then the highest density ordinary substance would be 192Os. The extraordinary density of osmium is a consequence of the lanthanide contraction.

Osmium has a very low compressibility. Correspondingly, its bulk modulus is extremely high—commonly quoted as 462 GPa, which is higher than that of diamond but lower than that of aggregated diamond nanorods—although there is some debate in the academic community about whether it is in fact this high. A paper by Cynn et al [1] reported that osmium had this bulk modulus, based on an experimental result, but other authors have cast doubt upon this ([2] and references therein).

Osmium metal has the highest melting point and the lowest vapor pressure of the platinum family. Common oxidation states of osmium are +4 and +3, but oxidation states from +1 to +8 are observed.


Because of the volatility and extreme toxicity of its oxide, osmium is rarely used in its pure state, and is instead often alloyed with other metals that are used in high-wear applications. Osmium alloys such as osmiridium are very hard and, along with other platinum group metals, is almost entirely used in alloys employed in the tips of fountain pens, phonograph needles, instrument pivots, and electrical contacts, as they can resist wear from frequent use.

Osmium tetroxide has been used in fingerprint detection and in staining fatty tissue for microscope slides. 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, since osmium atoms are extremely electron dense, making OsO4 an important stain for transmission electron microscopy (TEM) studies of many biological materials. An alloy of 90% platinum and 10% osmium (90/10) is used in surgical implants such as pacemakers and replacement pulmonary valves.

The tetroxide (and a related compound, potassium osmate) are important oxidants for chemical synthesis, despite being very poisonous.

In 1898 an Austrian chemist, Auer von Welsbach, developed the Oslamp with a filament made of osmium, which he introduced commercially in 1902. After only a few years, osmium was replaced by the more stable metal tungsten (originally known as wolfram). Tungsten has the highest melting point of any metal, and using it in light bulbs increases the luminous efficacy and life of incandescent lamps.

The light bulb manufacturer OSRAM (founded in 1906 when three German companies; Auer-Gesellschaft, AEG and Siemens & Halske combined their lamp production facilities), derived its name from the elements of OSmium and wolfRAM.


Osmium (Greek osme meaning "a smell") was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England.

Wollaston and Tennant were looking for a way to purify platinum by dissolution of native platinum ore in aqua regia. Large amounts of insoluble black powder remained as a byproduct of this operation.

Wollaston concentrated on the soluble portion and discovered palladium (in 1802) and rhodium (in 1804), while Tennant examined the insoluble residue. In the summer of 1803, Tennant identified two new elements; osmium and iridium. Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.


Turkey, with 127,000 tons, has the world's largest known reserve of osmium. Bulgaria also has substantial reserves of about 2500 tons. This transition metal is also found in iridiosmium, a naturally occurring alloy of iridium and osmium, and in platinum-bearing river sands in the Ural Mountains, and North and South America. It also occurs in nickel-bearing ores found in the Sudbury, Ontario region with other platinum group metals. Even though the quantity of platinum metals found in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.


See also osmium compounds. Osmium tetroxide OsO4


Main article: isotopes of osmium

Osmium has seven naturally-occurring isotopes, 5 of which are stable: 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os. The other two, 184Os and 186Os, have enormously long half-lives and for practical purposes can be considered to be stable as well. 187Os is the daughter of 187Re (half-life 4.56 x 1010 years) and is most often measured in an 187Os/188Os ratio. This ratio, as well as the 187Re/187Os ratio, have been used extensively in dating terrestrial as well as meteoric rocks. 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. However, the most notable application of Os in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the K-T boundary that marks the extinction of the dinosaurs 65 million years ago.


Osmium tetroxide is highly volatile, penetrates skin readily, and is very toxic by inhalation, in contact with skin and if swallowed. Airborne low concentrations of osmium vapour can cause lung congestion, skin or eye damage, and it should therefore be used in a fume hood. Osmium tetroxide is rapidly reduced to relatively inert compounds by poly-unsaturated vegetable oils such as corn oil.


  1. ^ H Cynn, J E Klepeis, C S Yeo and D A Young, "Osmium has the Lowest Experimentally-Determined Compressibility", Phys. Rev. Lett. 88 #13 (2002).
  2. ^ B R Sahu and L Kleinman, "Osmium Is Not Harder Than Diamond", Phys. Rev. B 72 (2005)
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Osmium". A list of authors is available in Wikipedia.
Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE