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A dense, hard metallic element discovered by Smithson Tennant in 1803. Iridium is a rare element that has an abundance of 0.001 ppm in the earth's crust. It is found as a native metal alloyed with Osmium, Platinum, or Gold. Iridium deposits have been found in Canada, South Africa, Russia and the U.S. (Alaska). The white, hard metal is insoluble in acids and does not tarnish in air. Iridium is used to harden platinum. It is also used in jewelry, weight standards, precision instruments, surgical tools, pen points, electrical contacts, and wear-resistant bearings.

Synonyms and Related Terms

Ir; iridio (It., Esp.); Irídio (Port.)


  • Flammable solid.
  • Pyrophoric.
  • Spontaneously flammable in air.
  • Contact causes irritation.
  • Fisher Scientific: MSDS

Physical and Chemical Properties

Resistant to acids. Slowly soluble in fused alkalis.

Composition Ir (atomic no. 77)
CAS 7439-88-5
Mohs Hardness 6.5
Melting Point 2410 C
Density 22.42 g/ml
Molecular Weight atomic wt = 192.217
Boiling Point 4310 C

Resources and Citations

  • 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 5098; discovered 1804 by Tennant
  • 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: Richard Eisenberg, p. 148: discovered in 1803 by Smithson Tennant