ˈthōrēəm, ˈthȯr- noun
( -s )
Etymology: New Latin, from Thor, Norse god of thunder + New Latin -ium
: a radioactive tetravalent metallic element that occurs combined in minerals (as monazite and thorite) and usually associated with rare earths and principally as the isotope of mass number 232 having a half-life of 1.39 × 10 10 years and emitting alpha particles to form mesothorium 1, that is obtained by reduction of its compounds as a pyrophoric gray powder or a heavy malleable metal changing from silvery white to dark gray or black in air, and that is used chiefly with tungsten or nickel electrodes in gas-discharge lamps and for conversion to fissionable uranium of mass number 233 by the absorption of neutrons and gamma rays — symbol Th ; see actinium series , thorium series , uranium series ; see element table