Plutonium

Plutonium (chemical symbol Pu) is a chemical element with atomic number 94. In nature it has been detected in trace quantities, but only after it had been prepared in the laboratory by Glen Seaborg, Joseph W. Kennedy, and Arthur C. Wahl in 1940. All isotopes of the element are radioactive, they are α-emitters, except for  Pu-241, which is a β⁻-emitter. The fissionable isotope Pu-239 is used in nuclear weapons—the atomic bomb Fat Man that detonated over Nagasaki on August 9, 1945 had a Pu-239 core, as had the device (Trinity) tested in Jornada del Muerto (New Mexico) a few weeks earlier (on July 16).

Solid state

Solid plutonium exhibits six different allotropes (crystal structures), labeled α, β, γ, δ, δ', and ε. They exist at increasing temperatures. The allotropes show fairly large volume changes upon phase transitions, their densities ("specific weights") vary from 16.00 g/cm³ to 19.86 g/cm³. The α crystalline form exists at room temperatures. It is not a very good conductor of electricity, it has the highest electrical resistivity of any pure metallic element (1.46×10⁻⁶ Ω· m). Just as water, but unlike many other materials, plutonium becomes denser when it melts (at 639.4 °C, normal pressure).