Plutonium, a radioactive, metallic chemical element. Although very small quantities of plutonium occur in nature in some uranium ores, virtually all plutonium in existence has been produced in nuclear reactors. It is silvery white and tarnishes readily on exposure to air. At ordinary temperatures the metal is hard and brittle. When solid plutonium is heated to its melting point, it undergoes several changes in crystal structure; when it is cooled to ordinary temperatures, these changes are reversed.

The longest-lived plutonium isotope is plutonium 244, with a half-life period of about 80,000,000 years. The most important is plutonium 239 (half-life about 24,400 years), used as a fuel in some nuclear reactors and as an explosive ingredient in nuclear weapons. Another isotope, plutonium 238, is a heat source used in small thermoelectric generators (devices that convert heat directly into electricity).

Plutonium was discovered by Glenn T. Seaborg, E. M. McMillan, J. W. Kennedy, and A. C. Wahl in 1940. It was named for the planet Pluto.

Symbol: Pu. Atomic number: 94. Atomic weight: 244. Specific gravity: 19.84. Melting point: 1,184 F. (640 C.). Boiling point: 5,855 F. (3,235 C.). Plutonium has 15 isotopes: Pu-232 to Pu-246. Plutonium is a transuranium element belonging to the actinide series of the Periodic Table and may have a valence of +3, 4, 5, or 6.