Thorium, a radioactive, metallic chemical element. It is soft and can be cut with a knife. When freshly cut, thorium is silver-white; when exposed to air, it slowly turns a dull gray, then black. It is capable of being drawn into wire and can be shaped by other metal-forming operations. Naturally occurring thorium is only mildly radioactive and has a half-life of 14,000,000 years.

Thorium metal is used chiefly in magnesium alloys. These alloys remain strong at high temperatures and are only about two-thirds as heavy as aluminum. Magnesium-thorium alloys are used in aircraft engines and in structural parts of missiles and high-speed aircraft.

Thorium compounds have several important uses. Thorium dioxide, or thoria, is used in laboratory crucibles and in molds for casting metals and alloys that have a high melting point. Thorium nitrate is used in the manufacture of the mantles for incandescent lanterns and, mixed with tungsten, in the manufacture of electrodes for electric-arc welding.

Thorium is potentially a very important source of nuclear energy. Although thorium is not itself a nuclear fuel, the naturally occurring form of thorium—thorium 232—can be readily converted in a nuclear reactor to uranium 233, which is a nuclear fuel. Thorium has been used successfully in a number of reactors; however, at present it is little used as a source of nuclear energy because the technology for the commercial use of other nuclear fuels has been much more extensively developed.

Thorium is widely distributed in the earth's crust. The most important ore of thorium is monazite, a mineral composed of thorium dioxide and a variety of rare-earth elements. Thorium also occurs combined with uranium in the minerals thorite and thorianite. The major producers of monazite are Australia, India, Malaysia, Brazil, and South Africa. A large variety of processes have been developed for obtaining thorium from its ores.

Thorium was discovered in 1828 by the Swedish chemist Jns Jakob Berzelius. He named it for Thor, the Norse god of war.

Symbol: Th. Atomic number: 90. Atomic weight of most stable isotope: 232. Specific gravity: 11.7. Melting point: 3,182 F. (1,750 C.). Boiling point: about 8,670 F. (4,800 C.). Thorium has 14 known isotopes: Th-223 through Th-236. It belongs to the Actinide Series of the Periodic Table and has a valence of +4.