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A naturally occurring, radioactive element found in uranium ores. Actinium has a half-life of 21.77 years. Metallic actinium is silvery white and produces a faint blue glow which is visible in the dark. Actinium was first isolated and named in 1899 by André Debierne; in 1902, it was independently discovered by Friedrich Giesel and named emanium. The two elements were reconciled as one in 1904. Actinium is a highly reactive material with a strong basic character that produces water-insoluble salts with most anions. It is very difficult to extract and separate from uranium ores and has limited commercial applications.

Synonyms and Related Terms

Ac; emanium; aktinium (Ces., Sven.); Actinium (Dan., Deut., Fr., Ned., Nor.); actinio (Esp.); attinio (It.); (Ned.); aktyn (Pol.); actínio (Port.)


  • Extremely hazardous.
  • Radioactive bone-seeking poison.
  • Akzo Nobel: SDS

Physical and Chemical Properties

Reacts with water to produce hydrogen gas.

Composition Ac (atomic no. 89)
CAS 7440-34-8
Melting Point 1050 C
Density 10.07 g/ml
Molecular Weight atomic wt=227
Boiling Point about 3300 C

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 649
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983
  • 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: "Actinium" by Greg Wall, p. 162