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
Origin of actiniumModern Latin from actino- + -ium
A radioactive element found in uranium ores, used in equilibrium with its decay products as a source of alpha rays. Its longest lived isotope is Ac-227 with a half-life of 21.77 years. Atomic number 89; melting point 1,050°C; boiling point 3,198°C; specific gravity (calculated) 10.07; valence 3. See Periodic Table.
Origin of actiniumFrom Greek aktīs aktīn- ray (from its radioactivity) ; see nekw-t- in Indo-European roots.
actin- + -ium