A radioactive silvery-white metallic element that is recovered commercially from monazite. Its longest-lived isotope, the only one that occurs naturally, is Th-232 with a half-life of 14.1 billion years and is used as a nuclear fuel. Thorium is used in magnesium alloys, and its oxide is widely used in gas mantles of Welsbach burners. Atomic number 90; atomic weight 232.038; melting point 1,750°C; boiling point 4,788°C; specific gravity 11.72; valence 4. See Periodic Table.