Thorium was discovered in 1828. It is a relatively heavy, radioactive metal occurring in monazite sands, and is associated with the rare-earth elements of the lanthanide series. Thorium is a soft, malleable and ductile metal. It appears shiny when freshly cut, but develops a dark grey or black oxide film.

Thorium can be used instead of uranuim as a reactor fuel because it readily converts to uranium. Thorium occurs in greater volume in the earth's crust than does uranium and is thus important for future supply of nuclear fuel. It is an important alloying agent for improving the high-temperature strength of metals such as magnesium. It also has applications in electronic photosensors, photoelectric cells, x-ray tubes etc. Cathodes of thoriated tungstenare used in electronic devices. The oxide of thorium is a common ingredient in high-quality lenses.