An element, atomic no. 89, atomic wt. 227.05; it possesses no stable isotopes and exists in nature only as a disintegration product of uranium and thorium. [G. aktis, a ray]
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ac·tin·i·um ak-'tin-ē-əm n a radioactive trivalent metallic element that resembles lanthanum in chemical properties and that is found esp. in pitchblende symbol Ac see ELEMENT (table)
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ac·tin·i·um (Ac) (ak-tinґe-əm) [Gr. aktis ray] a rare metallic chemical element occurring in the ores of uranium and having radioactive properties; atomic number 89, atomic weight 227.Medical dictionary. 2011.