Joseph William Kennedy was an American chemist who co-discovered plutonium, along with Glenn T. Seaborg, Edwin McMillan, and Arthur Wahl. During World War II, he led the CM Division at the Manhattan Project's Los Alamos Laboratory, where he oversaw research onto the chemistry and metallurgy of uranium and plutonium. After the war, he was recruited as a professor at Washington University in St. Louis, where he is credited with transforming a university primarily concerned with undergraduate teaching into one that also boasts strong graduate and research programs. He died of cancer of the stomach at the age of 40.

Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen, silicon, and hydrogen. When exposed to moist air, it forms oxides and hydrides that can expand the sample up to 70% in volume, which in turn flake off as a powder that is pyrophoric. It is radioactive and can accumulate in bones, which makes the handling of plutonium dangerous.