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A hard, bluish-white, heavy metallic element. Osmium occurs naturally as the mineral osmiridium and as a trace element in all Platinum ores. It has an abundance in the earth's crust of 0.001 ppm and is mined commercially in Tasmania, Russia and Canada. Osmium was first discovered by Smithson Tennant in 1803. Osmium is the second densest element known, next to iridium. It forms octavalent compounds and is used as a catalyst, and in the manufacture of platinum alloys and electric light filaments. The mineral, osmiridium, is a natural alloy containing osmium and Iridium. This alloy, also called iridosmine, is used for fountain-pen points, surgical needles, phonograph needles, compass points and instrument pivots.

Synonyms and Related Terms

Os; iridosmine; osmiridium; osmio (It., Esp.); Ósmio (Port.)


  • Highly toxic by ingestion and inhalation.
  • Skin contact causes irritation.

Physical and Chemical Properties

Insoluble in acids. Soluble in alkalis.

Composition Os (atomic no. 76)
CAS 7440-04.2
Mohs Hardness 7.0
Melting Point ~2700 C
Density 22.61 g/ml
Molecular Weight atomic wt = 190.23
Boiling Point ~5500 C

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p.565
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 7021 (discovered by Tennant in 1804)
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
  • Chemical & Engineering News, American Chemical Society, Washington DC, 81 (36) , Sept. 8, 2003 Comment: Ian Shott, p. 144: discovered in 1803 by Smithson Tennant

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