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76 rheniumosmiumiridium


periodic table
Name, Symbol, Number osmium, Os, 76
Chemical series transition metals
Group, Period, Block 8, 6, d
Appearance silvery, blue cast
Atomic mass 190.23(3) g/mol
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/cm³
Liquid density at m.p. 20 g/cm³
Melting point 3306 K
(3033 °C, 5491 °F)
Boiling point 5285 K
(5012 °C, 9054 °F)
Heat of fusion 57.85 kJ/mol
Heat of vaporization 738 kJ/mol
Heat capacity (25 °C) 24.7 J/(mol·K)
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, 6, 4, 2, -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·K)
Thermal expansion (25 °C) 5.1 µm/(m·K)
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
Notable 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 1.58% 2.0×1015y α 2.822 182W
187Os 1.6% Os is stable with 111 neutrons
188Os 13.3% Os is stable with 112 neutrons
189Os 16.1% Os is stable with 113 neutrons
190Os 24.6% Os is stable with 114 neutrons
191Os syn 15.4 d β- 0.314 191Ir
192Os 41.0% Os is stable with 116 neutrons
193Os syn 30.11 d β- 1.141 193Ir
194Os syn 6 y β- 0.097 194Ir

Osmium (chemical symbol Os, atomic number 76) is a hard, brittle, blue-gray or blue-black transition metal in the platinum family and is found as an alloy in platinum ore. It is one of the densest natural elements.[1] Its alloys with iridium are used in electrical contacts and for applications where extreme durability and hardness are needed. Its alloy with platinum is used in surgical implants such as pacemakers and replacement pulmonary valves.

The compound osmium tetroxide is very toxic but has a number of important uses. For instance, it has been used in fingerprint detection, as an oxidant in chemical reactions, and as a stain for fatty tissue being examined under a microscope. It is also an important stain for transmission electron microscopy (TEM) studies of a variety of biological materials.

Occurrence and value

Turkey has the world's largest known reserve of osmium, estimated at 127,000 tons. Bulgaria also has substantial reserves, of about 2,500 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. Osmium also occurs in nickel-bearing ores found in the Sudbury, Ontario region, with other platinum group metals. Although the proportion of platinum metals in these ores is small, the large volume of nickel ores processed makes commercial recovery possible.

Osmium is quite valuable, costing about US $100 per gram (g). One of the stable isotopes, 187Os, is worth about $25,000 per gram.[2]


Osmium (from the Greek word osme, meaning "a smell") was discovered in 1803 by Smithson Tennant, while working with William Hyde Wollaston in London, England.

They were looking for a way to purify platinum by dissolving native platinum ore in aqua regia (a mixture of concentrated nitric and hydrochloric acids). A large amount of insoluble black powder remained as a byproduct of this operation.

Wollaston focused on analyzing 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.

Notable characteristics

Osmium is a transition metal that lies between rhenium and iridium in period 6 of the periodic table. It is thus a member of the platinum group of metals. In addition, it is located in group 8 (former group 8B), just below ruthenium.

In its metallic form, osmium is blue white, brittle, and lustrous even at high temperatures, but it is extremely difficult to make. It is easier to make osmium in a powdered form, but when this form is exposed to air, it is converted to osmium tetroxide (OsO4), which is toxic. The oxide is also a powerful oxidizing agent, emits a strong smell, and boils at 130°C.

The measured density of osmium is higher than that of any other element, with a value slightly higher than that of iridium. Osmium is therefore often listed as the densest element known. On the other hand, when density is calculated based on the space lattice structures of these elements, one obtains a value of 22,650 kilograms per cubic meter (kg/m³) for iridium, versus 22,610 kg/m³ for osmium. Based on these data, it is currently not possible to arrive at a firm conclusion about which of them is denser. If one were to distinguish between different isotopes, then the heaviest ordinary substance would be 192Os.

Osmium has the highest melting point and the lowest vapor pressure of the platinum family. It also has a very low compressibility value. Common oxidation states of osmium are +4 and +3, but observed oxidation states range from +1 to +8.


Osmium has seven naturally occurring isotopes, five of which are stable: 187Os, 188Os, 189Os, 190Os, and (most abundant) 192Os. Two radioactive isotopes, 184Os and 186Os, have enormously long half-lives and can be considered stable for all practical purposes.

The isotope 187Os is the daughter product of 187Re (rhenium-187, half-life = 4.56 x 1010 years) and is most often measured in terms of the ratio 187Os/188Os. This ratio, as well as the ratio 187Re/187Os, have been used extensively in dating terrestrial and meteoric rocks. It has also been used to measure the intensity of continental weathering over geologic time.

The most notable application of osmium in dating has been in conjunction with iridium, to analyze the layer of shocked quartz along the "K-T boundary." This temporal boundary between the Cretaceous and Tertiary eras marks the extinction of dinosaurs 65 million years ago (see iridium).

Alloys and compounds

  • Iridiosmium, iridosmium, or osmium iridian (Os, Ir): This is an alloy of osmium and iridium. It occurs naturally as small, extremely hard, flat metallic grains with hexagonal crystal structure, and sometimes contains traces of platinum, palladium, rhodium, and ruthenium. Iridiosmium has been used in making fountain pen nibs.
  • Osmiridium: This is an alloy of osmium and iridium, with traces of platinum and rhodium. It is found in small amounts in nature, in mines of other platinum group metals. It can also be made artificially. It can be isolated by adding a piece to aqua regia, which has the ability to dissolve gold and platinum but not osmiridium. This alloy is used in making surgical equipment and other high-wear devices. It was once used for fountain pen nibs.
  • Osmium tetroxide, osmium tetraoxide, osmium(VIII) oxide, or osmic acid (OsO4): This compound is highly poisonous, even at low exposure levels, and must be handled with appropriate precautions. Nonetheless, it is useful for a variety of applications. When pure, it is colorless, but it is usually contaminated with a small amount of yellow-brown osmium dioxide (OsO2), giving it a yellowish hue. OsO4 sublimes (changes from solid to gas) at room temperature and has a characteristic odor similar to that of ozone. In organic synthetic reactions, OsO4 is widely used to oxidize alkenes to the dialcohols. In addition, it is used as a staining agent in transmission electron microscopy, scanning electron microscopy, and optical microscopy.


Because of the extreme toxicity of its oxide, osmium is rarely used in its pure state. Instead, it is often alloyed with other metals. Osmium alloys such as osmiridium are very hard and are used in high-wear applications and electrical contacts. Osmiridium was once used in fountain pens nibs and phonograph needles.

An alloy of 90 percent platinum and 10 percent osmium is used in surgical implants such as pacemakers and replacement pulmonary valves.

Osmium tetroxide, despite being very toxic, has been used for a number applications, including fingerprint detection and staining fatty tissue for microscope slides. As a strong oxidant, it cross-links lipids by fixing biological membranes in place. Futhermore, osmium atoms are extremely electron dense, making OsO4 an important stain for transmission electron microscopy (TEM) studies of a wide range of biological materials. This oxide is also an important oxidant for chemical syntheses.

In 1898, Austrian chemist Carl Auer von Welsbach developed the "Oslamp," with a filament made of osmium, which he introduced commercially in 1902. After a few years, however, osmium was replaced by the more stable metal tungsten.


  1. The two competitors for densest natural element are osmium and iridium. Currently, the dispute over which one is denser has not been resolved by the scientific community.
  2. http://www.ecplaza.net/ecmarket/list.asp?cmd=search&keywords=osmium+187. Retrieved November 25, 2007.

ISBN links support NWE through referral fees

  • Los Alamos National Laboratory - Osmium. Retrieved November 25, 2007.
  • National Synchrotron Light Source - Science Highlights. Retrieved November 25, 2007.
  • Cynn, H., J. E. Klepeis, C. S. Yeo and D. A. Young. 2002. "Osmium has the Lowest Experimentally-Determined Compressibility," Phys. Rev. Lett. 88 #13.
  • Sahu, B. R. and L. Kleinman. 2005. "Osmium Is Not Harder Than Diamond," Phys. Rev. B 72.
  • Cotton, S. A. 1997. Chemistry of Precious Metals. Chapman and Hall (London). ISBN 0-7514-0413-6
  • Berrisford, D. J., C. Bolm, and K. B. Sharpless. 1995. "Ligand-Accelerated Catalysis," Angewandte Chemie, International Edition English, volume 34, pp. 1059–1070.

External links

All links retrieved November 17, 2022.


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