Thorium is a radioactive chemical element used to make fuel for nuclear reactors. It is a soft metal with a silvery shine. Tiny amounts of thorium appear naturally in nearly all rocks, soils, water, plants, and animals. Larger concentrations appear in the minerals monazite and thorite. Those minerals are mined chiefly in Brazil, India, and South Africa.
Scientists can convert thorium into uranium 233, a nuclear fuel, by bombarding it with neutrons. Engineers mix thorium with other metals to make strong alloys (combinations of two or more metals). Manufacturers use thorium in devices called photoelectric cells that measure ultraviolet light. Some camping lanterns use mantles (light-producing screens) made of thorium because it gives off a bright white light when heated.
Thorium has 12 isotopes. The isotopes of an element have the same number of protons but different numbers of neutrons. Thorium’s most stable isotope has an atomic mass number (total number of protons and neutrons) of 232. Thorium 232 has a half-life of 14 billion years-that is, due to radioactive decay, only half the atoms in a sample of thorium 232 would still be atoms of that isotope after 14 billion years.
Thorium’s chemical symbol is Th. Its atomic number (number of protons in its nucleus) is 90. Its relative atomic mass is 232.0381. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most stable isotope of carbon. Thorium melts at 1750 °C and boils at about 4790 °C. For information on the position of thorium on the periodic table, see the article Periodic table.
The Swedish chemist Jons Jakob Berzelius discovered thorium in 1828. The element is named for Thor, the god of thunder in Norse mythology.
See also Monazite; Radiation (Naturally radioactive substances).
