Plutonium is a highly reactive, silvery-white metal element denoted by the atomic symbol, Pu. Its atomic number is 94 and it has an atomic weight of approximately 244. It is a transuranium element in the actinide series. Fifteen isotopes of plutonium are known.

Plutonium is a man-made element first created during the mid-twentieth century. After his discovery of neptunium in 1940, Edwin McMillan felt sure that a second transuranium element was present among the products of the nuclear fission of uranium that he had been studying. When he left Berkeley, California, in November 1940, McMillan suggested to Glenn Seaborg that he look for that element. Only a month later, with colleagues A. C. Wahl and J. W. Kennedy, Seaborg isolated the element. They chose to name their discovery plutonium (chemical symbol: Pu) after the planet Pluto. The decision followed the pattern of naming uranium after Uranus and neptunium after Neptune. News of...