A chemical element with atomic number 90. Thorium is a gray, slightly radioactive metal. It is three times more abundant on Earth than uranium. The main source of thorium are monazite sands. Thorium is not fissile, but thorium-232 can be converted via neutron capture into fissile uranium-233. It can therefore be turned into nuclear fuel in fast breeder reactors. The use of thorium as an energy source is still under research.

Thorium is the starting element of the thorium decay series.