a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no: 89; half-life of most stable isotope,227Ac: 21.6 years; relative density: 10.07; melting pt: 1051°C; boiling pt: 3200 ± 300°C
Word origin
C19: New Latin, from actino- + -ium
actinium in American English
(ækˈtɪniəm)
noun
a white, radioactive, metallic chemical element, the first member of the actinide series, found in pitchblende and other minerals or formed in reactors by the neutron irradiation of radium: symbol, Ac; at. no., 89