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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