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Osmium is the chemical element with symbol Os and atomic number 76.


Chemical properties

Osmium forms compounds with oxidation state from -2 to 8. It is slightly attacked by aqua regia, and by fuming nitric acid, and it is attacked by molten hydroxides. It forms the toxic osmium tetroxide (OsO4), the most common compound with +8 oxidation state, in contact with air. A lower oxide, osmium dioxide, is known. It forms coordination complexes easily.

Physical properties

Osmium is a hard, brittle, white-bluish transition metal and usually is found in traces in platinum ores. It is the densest non-radioactive element known, with a density of 22.6 g/cm3 (22.59 g/cm3 according to most sources), slightly denser than iridium. It's almost twice as dense as lead. It melts at 3033 ºC, which makes it unfeasible for the amateur chemist to cast.


It is one of the rarest elements so it is quite expensive. It can be bought online from Metallium and sometimes from eBay.


  • Osmium tetroxide synthesis (Dangerous! Toxic!)
  • Organoosmium compounds
  • Element collection



Osmium metal poses very little toxicity as it does not react with most reagents. Osmium powder, however, will slowly oxidise in air to form osmium tetroxide, which is toxic and a powerful oxidant. Osmium powder is also pyrophoric. The bulk metal, however, is safe to keep in air.


Bulk osmium does not require any special storage. Osmium powder should be kept in closed containers, in a well ventilated area, preferable under a gas other than air (such as carbon dioxide).


Due to osmium's high price, it's best to try and recycle it as much possible.


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