Discovered by Seaborg (see element 106) in 1941, plutonium was named for the 9th planet, completing the series of elements named for planets which begins with uranium. Evidence for the existence of plutonium was obtained from work with samples of neptunium and the eventual synthesis in fact results from the beta decay of neptunium-239.
The longest-lived isotope of plutonium is Pu-244 with a half-life of 82 million years. However the isotope of chief interest is Pu-239 which, like U-235, is fissionable. Most of the nuclear weapons built by the "great powers" today are based on Pu-239 which is derived from U-238 in special "breeder" reactors. Pu-239 is also a by-product of normal fission power reactors and accounts for a good deal of the concern over nuclear waste by-products since it is both highly radioactive and exceptionally toxic. Also, since the critical mass of plutonium is only about one third that of U-235, the possibility for terrorist diversion of the material is considered a serious matter.
A piece of plutonium about the size of a softball would feel hot to the touch because of the high level of alpha particle radiation given off. A somewhat larger piece of the metal would boil water within minutes. Plutonium is occasionally used in deep-space probes as a source of energy (too far from the sun for effective solar power), the heat being directly converted into electricity by a special device.
Stephen R. Marsden (ChemTopics)