Plutonium, whose name derives of the planet Pluto, was the second transuranian element to be discovered; isotope Pu 238 was produced, in 1940, by Seaborg, McMillan, Kennedy and Wahl, in the University of California - Berkeley, through the bombardment of uranium with deuterons, in a 60 inches cyclotron. The most important of its isotopes is Pu 239, produced in great amounts in nuclear reactions as a uranium by-product.

In August 1942, B.B. Cunningham and L.B. Werner succeeded to isolate about a microgram of Pu 239, in the Metallurgical Laboratory of the University of Chicago. This made plutonium the first synthetic element to be obtained in visible amounts. Those scientists isolated a sample of 2,77 micrograms, in September 10 of that year.