noun Chemistry.

  1. a grayish-white, lustrous, somewhat ductile and malleable, radioactive metallic element present in monazite: used as a source of nuclear energy, as a coating on sun-lamp and vacuum-tube filament coatings, and in alloys. Symbol: Th; atomic weight: 232.038; atomic number: 90; specific gravity: 11.7.


  1. 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, 232 Th: 1.41 × 10 10 years; valency: 4; relative density: 11.72; melting pt: 1755°C; boiling pt: 4788°C

n.rare metallic element, 1832, Modern Latin, named 1828-9 by its discoverer, Swedish chemist Jöns Jakob Berzelius (1779-1848) in honor of the Scandinavian god Thor. n. Symbol Th

  1. A radioactive metallic element that is used in magnesium alloys; its longest-lived isotope, Th 232, has a half-life of 1.41 X 1010 years. Atomic number 90.


  1. A silvery-white, radioactive metallic element of the actinide series. It is used for fuel in some nuclear reactors and for improving the high-temperature strength of magnesium alloys. The only naturally occurring isotope of thorium, Th 232, is also its most stable, having a half-life of 14.1 billion years. Atomic number 90; atomic weight 232.038; approximate melting point 1,750°C; approximate boiling point 4,500°C; approximate specific gravity 11.7; valence 4. See Periodic Table.
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