Thorium (Th) is a silvery-white radioactive metallic element , one of the actinides. Its chief ore is monazite. Thorium is tetravalent, resembling zirconium and hafnium. It is used in magnesium alloys and to produce uranium-233 for atomic fuel.


Thorium has 13 radioactive isotopes only one of which, thorium-232, occurs naturally.


The refractory thorium (IV) oxide was used to make incandescent gas mantles; it is added in small amounts to the tungsten filaments in electric lights.


atomic number 90
relative atomic mass 232.04
relative density 14.66
melting point 1,700°C (3,100°F)
boiling point 4,000°C (7,200°F)