DENSITY: 11.724 g/cm −3

Thorium is a radioactive chemical element that belongs to the actinide series. Its ground state electronic configuration is [Rn]5f 0 6d 2 7s 2 . Thorium was discovered by Jöns Jacob Berzelius in 1828. Its name is derived from "Thor," the god of war in the Scandinavian mythology. Thorium chemistry is dominated by the tetravalent thorium ion (Th 4+ ). Its ionic radius is very similar to that of the trivalent cerium ion (Ce 3+ ). For that reason it is no surprise that thorium occurs in nature together with the rare earth elements . Thorium is recovered commercially from the rare earth ore monazite (mainly CePO 4 ), which contains up to 9 percent ThO 2 . Other thorium-containing minerals are thorite and thorianite. The most abundant isotope is 232 Th, with a half-life of 1.4 × 10 10 years. None of the twenty-five known isotopes of thorium (with atomic masses ranging between 212 and 236) is stable.

Pure thorium is a silvery-white metal (melting point 1,750°C) that tarnishes upon exposure to air. Its density is 11.724 g/cm −3 at 25°C (77°F), similar to that of lead. The best-known application of thorium is its use in incandescent mantles for gas lamps. These mantles consist of a metal oxide skeleton (99% ThO 2 and 1% CeO 2 ). Thorium(IV) oxide is used by chemists

Thorium is used in incandescent mantles for gas lamps.
Thorium is used in incandescent mantles for gas lamps.

as a catalyst in different organic reactions, and in the conversion of ammonia to nitric acid. Thorium is about three times as abundant as uranium, and therefore it may become an important nuclear fuel in the future. Because one of the disintegration products of thorium is the radioactive noble gas radon ( 220 Rn), good ventilation of areas and places where thorium is stored or handled is necessary.

SEE ALSO Actinium ; Berkelium ; Einsteinium ; Fermium ; Lawrencium ; Mendelevium ; Neptunium ; Nobelium ; Plutonium ; Protactinium ; Rutherfordium ; Uranium .

Koen Binnemans


Cotton, Simon (1991). Lanthanides and Actinides. New York: Oxford University Press.

Katz, Joseph J.; Seaborg, Glenn T.; and Morss, Lester R. (1986). The Chemistry of the Actinide Elements, 2nd edition. New York: Chapman and Hall.

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