Plutonium

Plutonium is a silvery actinide metal that quickly tarnishes to a dull grey or olive coating. Its 5f electrons sit on the borderline between bonding and staying localised on each atom, which gives the metal strange physical behaviour. Plutonium has six solid phases at normal pressure, more than any other element, with densities ranging from 15.9 to 19.8 g/cm3. Engineers stabilise the malleable delta phase by alloying it with a little gallium. In solution, plutonium can exist in oxidation states +3 to +7.

Plutonium-239, with a half-life of 24,110 years, fissions readily with neutrons of any energy, which made it the core material of most nuclear weapons and a component of mixed-oxide reactor fuel. It is produced in every uranium reactor when uranium-238 captures neutrons. Plutonium-238, with an 88-year half-life, emits alpha particles but little penetrating radiation, and a pellet of its oxide glows red-hot from its own decay heat. This makes it the standard power source for spacecraft too far from the Sun for solar panels.

Discovery

Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl made plutonium-238 at Berkeley in December 1940 by bombarding uranium with deuterons, and confirmed it chemically in February 1941. It was named after Pluto, then counted as the planet beyond Neptune. Seaborg chose the symbol Pu instead of Pl as a joke, expecting objections that never came. Within four years, industrial reactors at Hanford were producing kilograms for the Manhattan Project.