(So named (Pluto) in 1942 by Glenn T. Seaborg, Arthur C. Wahl, and Joseph W. Kennedy, U.S. physicists who isolated it in 1940, because it comes next to neptunium, as Pluto comes next to Neptune) A radioactive, metallic chemical element, one of the actinides, found in trace quantities in native uranium ores and produced by bombarding uranium with deuterons.

Symbol: Pu
Atomic number: 94
Atomic weight: 244 (isotope with the longest known half-life)
Density (at room temperature and pressure): 19.84 g/cc
Melting point: 640°C
Boiling point: 3,232°C
Main valence: +4
Ground state electron configuration: [Rn]5f67s2

See also: plutonium-239