a soft ductile silvery-white metallic element. It is radioactive and occurs in thorite and monazite: used in gas mantles, magnesium alloys, electronic equipment, and as a nuclear power source. Symbol: Th; atomic no: 90; atomic wt: 232.0381; half-life of most stable isotope,232Th: 1.41 × 1010 years; valency: 4; relative density: 11.72; melting pt: 1755°C; boiling pt: 4788°C