Thorium

Thorium, symbol Th, radioactive metallic element with an atomic number of 90. Thorium is a member of the actinide series of the periodic table.

Thorium was discovered (1828) by the Swedish chemist Baron Jöns Jakob Berzelius. The element is dark in colour, slowly attacked by water, soluble in hydrochloric and sulphuric acids, and slightly soluble in nitric acid. It ranks 39th in abundance among the elements in the crust of the Earth. Thorium melts at about 1750° C (about 3182° F), boils at about 4850° C (about 8762° F), and has a relative density of 11.8. The atomic weight of thorium is 232.038.

Small quantities of thorium are found in thorite, or thorium silicate; in orangite, a variety of thorite; and in thorianite, a radioactive mineral composed of thorium oxide and uranium. The larger deposits occur mainly as thorium oxide, ThO₂, in the monazite sands of India and Brazil.

Thorium has isotopes ranging in mass number from 212 to 236. Thorium-232 occurs naturally, has a half-life of about 14 billion years, and is the first member of the radioactive-decay series, ending with the stable lead isotope lead-208. Thorium is currently important as a potential atomic-fuel source, because bombardment of thorium-232 by slow neutrons yields the fissile isotope uranium-233. This process is comparable to the process by which fast neutrons “breed” fissile plutonium-239 from nonfissile uranium-238 (see Nuclear Energy). The thorium-uranium fuel cycle is being studied by scientists as an alternative to the uranium-plutonium fuel cycle. Two types of reactors, the molten-salt breeder reactor and the light-water breeder reactor, are being considered. Thorium metal is used in magnesiumalloys and as a stabilizing component of electronic tubes. Thorium oxide is used in light filaments and electrodes and also as a catalyst.