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From The Collaborative International Dictionary of English v.0.44 [gcide]:
Actinium \Ac*tin"i*um\, noun [Gr. ?, ?, ray.] (Chem.) A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light.
From WordNet (r) 2.0 [wn]:
actinium
noun: a radioactive element of the actinide series; found in uranium ores [syn: {Ac}, {atomic number 89}]
From Elements database 20001107 [elements]:
actinium Symbol: Ac Atomic number: 89 Atomic weight: (227) Silvery radioactive metallic element, belongs to group 3 of the periodic table. The most stable isotope, Ac-227, has a half-life of 217 years. Ac-228 (half-life of 6.13 hours) also occurs in nature. There are 22 other artificial isotopes, all radioactive and having very short half-lives. Chemistry similar to lanthanum. Used as a source of alpha particles. Discovered by A. Debierne in 1899.
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