Plutonium

Chemical element with atomic number 94 (Pu)

Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states.

From Wikipedia, under CC BY-SA. More on occurri.