A button of plutonium above a calcium chloride salt cake.

Plutonium is the chemical element with the atomic number 94 and chemical symbol Pu. It is a highly radioactive metal, and is the metal used in most nuclear weapons. The chemical element plutonium has some different isotopes. The most important isotope of plutonium is ²³⁹Pu (or plutonium-239). It takes 24,110 years for half of a sample of plutonium-239 to decay, which is called its half-life. It can be made from natural uranium. It is fissile and a nuclear fuel.