Plutonium is a radioactive chemical element. Its atomic number is 94.
Its most stable isotope occurs in very small quantities on Earth. It is a solid at room temperature and its melting point is 640°C. Its density is 19.84 g cm-3.
Plutonium was discovered by Glenn T. Seaborg, Edwin M. McMillan, J. W. Kennedy and A. C. Wahl (United States) in 1940.