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Actinium

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{ak-tin'-ee-uhm} The chemical element actinium is a radioactive metal, the first member of the ACTINIDE SERIES. Its symbol is Ac, its atomic number 89, and its atomic weight 227 (stablest isotope). Actinium was discovered in 1899 by French chemist Andre Debierne, who found it in pitchblende residues. Many radioactive isotopes of actinium are known, one of which, Ac-227 has a half-life of 21.6 years and is present in natural uranium. The other isotopes have half-lives of 10 days to less than 5 seconds.

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