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)