76 Os 190.23

Os - Osmium

See metal norms for Osmium

Chemical Element Osmium Melting Point °C 3054
Chemical Symbol Os Boiling Point °C 5027
Atomic Number 76 Density g/cm3 22.6
Atomic Weight 190.23 Oxide OsO4, OsO2


Osmium is a lustrous, bluish–white metal belonging to the platinum group. Osmium is the 81st most abundant element in the Earth’s crust, making it extremely rare. Of all the known elements, osmium is the densest, closely followed by iridium. Osmium is unaffected by water or acids, but dissolves in molten alkalis. Pure osmium is difficult to obtain, therefore it is often fabricated as a powder, which is then formed into a solid by heating.


An Englishman, Smithson Tennant, is credited with the discovery of osmium in 1803. He came across it whilst analysing a specimen of crude platinum. This crude platinum was dissolved in dilute aqua regia, and the outcome was that not all of the metal went into solution; some remained as a powder. This powder was then tested by using a combination of acid and alkali treatments, eventually separating into two new metals which Tennant called osmium and iridium.


Osmium can be found as an un-combined metal, but is mainly found in nickel and platinum bearing ores and in alloys with iridium. These alloys are either known as iridosmine or osmiridium. Osmium ores are very rare but deposits can be extracted from ores in countries such as Russia, Canada, South Africa and the USA. Overall, less than 100kgs of osmium are produced each year.


Some former uses of osmium are no longer in demand, such as fountain pen tips, compass needles, clock bearings and gramaphone needles, where its extreme hardness and corrosion resistance prevented wear from long-term use.  Today it is used as a catalyst in industry, and in the development certain anti-cancer drugs.  Osmium metal has very little use in itself, as it is very rare and therefore expensive to acquire, and also the gas of its oxide is poisonous making it an impractical substance.  More often than not, osmium as an alloying agent is quite safe and is used to make hard alloys with metals such as platinum and iridium.

  • Emsley, John. Nature’s Building Blocks, An A-Z Guide to the Elements, New Edition, Oxford University Press, 2011
  • Gray, Theodore. The Elements, A Visual Exploration of Every Known Atom in the Universe, Black Dog & Leventhal Publishers, Inc, NY, 2009
  • Stwertka, Albert. A Guide to the Elements, 3rd Edition, Oxford University Press, 2012