Osmium

Vapor pressure
76	rhenium ← osmium → iridium
	periodic table
General
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)
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
Miscellaneous
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

Previous (Osman I)

Next (Osmosis)

For other uses, see Osmium (disambiguation).

Osmium (chemical symbol Os, atomic number 76) is a hard, brittle, blue-gray or blue-black transition metal in the platinum family. It is one of the densest natural elements^[1] It is used in some alloys with platinum and iridium. Osmium is found native as an alloy in platinum ore and its tetroxide has been used to stain tissues and in fingerprinting. Alloys of osmium are employed in fountain pen tips, electrical contacts, and other applications where extreme durability and hardness are needed.

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).^[2] One of the stable isotopes, ¹⁸⁷Os, is worth about $25,000 per gram.^[3]

History

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 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 toxic. The oxide is also a powerful oxidizing agent, emits a strong smell and boils at 130°C.

The extraordinary density of osmium is a consequence of the lanthanide contraction.

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 heaviest ordinary substance would be ¹⁹²Os.

Osmium also has a very low compressibility. Correspondingly, its bulk modulus is extremely high — commonly quoted as 462GPa, which is higher than that of diamond but lower than that of ADNR or 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 published in 2002 reported that osmium had this bulk modulus, based on an experimental result; but several subsequent authors have cast doubt upon this (for example Sahu et al in Sept 2005, who refer also to other such papers - see References section).

This 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.

Isotopes

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 lifes 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. 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.

Applications

Because of the 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 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. 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.

Alloys and compounds

Iridiosmium, osmium iridian, or iridosmium (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 is 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 fountain pen nibs, surgical equipment, and other high-wear devices.

Osmium tetroxide, osmium tetraoxide, osmium(VIII) oxide, or osmic acid (OsO₄): When pure, the compound is colorless, but it is usually contaminated with a small amount of yellow-brown osmium dioxide (OsO₂), giving it a yellowish hue. OsO₄ is sublimes (changes from solid to gas) at room temperature and has a characteristic odor similar to that of ozone. OsO₄ is highly poisonous, even at low exposure levels, and must be handled with appropriate precautions. 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.

Footnotes

↑ 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.
↑ http://www.engelhard.com/eibprices/DPCharts.aspx?MetalName=Osmium
↑ http://www.ecplaza.net/ecmarket/list.asp?cmd=search&keywords=osmium+187

References
ISBN links support NWE through referral fees

Los Alamos National Laboratory - Osmium
National Synchrotron Light Source - Science Highlights
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).
B R Sahu and L Kleinman, "Osmium Is Not Harder Than Diamond", Phys. Rev. B 72 (2005).

Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0-7514-0413-6.
Berrisford, D. J.; Bolm, C.; Sharpless, K. B., "Ligand Accelerated Catalysis", Angewandte Chemie, International Edition English, 1995, volume 34, pp. 1059-1070.

External links

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[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] ttp://www.engelhard.com/eibprices/DPCharts.aspx?MetalName=Osmium

[3] ttp://www.ecplaza.net/ecmarket/list.asp?cmd=search&keywords=osmium+187

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