The element with atomic number 94 named after Pluto is plutonium.
Plutonium belongs to the actinide series and has the chemical symbol Pu. It was first produced at the University of California, Berkeley, between December 1940 and February 1941 by Glenn T. Seaborg, Edwin McMillan, Emilio Segrè, Joseph W. Kennedy and Arthur Wahl. They bombarded uranium-238 with deuterons, creating neptunium-238, which then underwent beta decay to form plutonium-238.
The name follows a planetary sequence. Uranium was named after Uranus, and neptunium after Neptune. When element 94 was identified, Pluto was still widely regarded as the ninth planet, so “plutonium” was a natural continuation. The discovery was kept secret during World War II and was not publicly documented until 1948.
Plutonium is radioactive and has several important isotopes. Plutonium-239 can sustain a nuclear chain reaction and has been used in nuclear weapons and reactor fuel, while plutonium-238 powers some deep-space spacecraft through radioisotope thermoelectric generators. A common mix-up is confusing its name with neptunium: neptunium is atomic number 93, one position earlier.