Plutonium

Plutonium is a transuranic radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon and hydrogen. When exposed to moist air, it forms oxides and hydrides that expand the sample up to 70% in volume, which in turn flake off as a powder that can spontaneously ignite. It is radioactive and can accumulate in the bones. These properties make the handling of plutonium dangerous.
Symbol: Pu
Electron configuration: Rn 5f6 7s2
Discovered: 1940
Atomic number: 94
Atomic mass: 244 u
Discoverers: Joseph W. Kennedy, Glenn T. Seaborg, Edwin McMillan, Arthur Wahl