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Element Fact
Iridium is the most corrosion-resistant metal known. Together with platinum, it is used to make the international prototype of the kilogram, which is the standard definition for the unit of mass.

Osmium, Os

osmium sample

Atomic number: 76
Atomic mass: 190.23(3)
Natural abundance: 0.005 ppm
184Os (183.952491) - 0.02%
186Os (185.953838) - 1.58%
187Os (186.955748) - 1.6%
188Os (187.955836) - 13.3%
189Os (188.958145) - 16.1%
190Os (189.958445) - 26.4%
192Os (191.961479) - 41.0%

History and Production

From Greek word osme, meaning a smell. It was discovered by S. Tennant in 1803 in the residue left after crude platinum has been dissolved in aqua regia. The name was given due to the pungent smell of the oxide (OsO4), which is now known to be very toxic. It is generally obtained from the anode slime in the electrolytic refinement of nickel and copper. After a series of chemical treatments the metal is obtained as powder or sponge. The metal is mainly used to produce hard alloys with other platinum metals such as instrument pivots, fountain pen tips and electrical contacts.

Physical Data

It has a lustrous, bluish-white appearance and extremely hard. It has the highest melting point among the platinum metals. It is also being one of the metal with the highest density, with a value that is very close to that of iridium. It is usually found (0.005 ppm in abundance) in elemental state along with other platinum metals that frequently associate with nickel and copper sulfide ores.

Interatomic distance: 267.6 pm
Melting point: 3033C
Boiling point: 5012C
Thermal conductivity/Wm-1K-1: 87.6 (27C)
Density/kgm-3: 22590 (20C), 20100 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 791.0 kJ/mol
Gibbs free energy of formation: 745 kJ/mol
Entropy: 192.6 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Xe] 4f14 5d6 6s2
Term symbol: 5D4
Electron affinity: 106.1339 kJ/mol   Electronegativity (Pauline): 2.20
Ionization energy (first, second, third): 755.818, -, - kJ/mol


A factual and humorous look at osmium

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