Plutonium (Pu) is a radioactive element, capable of self-maintained explosive fission. It was isolated from uranium 238 during research on the atomic bomb in 1940 by neutron bombardment. It has 13 isotopes, from which Pu 239 and Pu 244, with a half-life of 82 million years, are the most important. The process of plutonium formation is as follows: Uranium238 absorbs neutrons emitted by the fission of uranium235, forming uranium239, which radiates a beta particle, decaying to neptunium239; the neptunium239 emits another beta particle, becoming plutonium239. Symbol: Pu; valence 3, 4, 5, 6.