Plutonium () is a radioactive, metallic and toxic chemical element. It has the symbol Pu and the atomic number 94. It is a fissile element used in most modern nuclear weapons. The most significant isotope of plutonium is 239Pu, with a half-life of 24,100 years. It can be made from natural uranium. The most stable isotope is 244Pu, with a half-life of about 80 million years, long enough to be found in extremely small quantities in nature, making 244Pu the nucleon-richest atom that naturally occurs in the Earth's crust, albeit in small traces.
Plutonium has been called "the most complex metal" and "a physicist's dream but an engineer's nightmare" for its peculiar physical and chemical properties. It has six allotropes normally and a seventh under pressure, each of which have very similar energy levels but with significantly varying densities, making it very sensitive to changes in temperature, pressure, or chemistry, and allowing for dramatic vol