A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction: 92U238 + 0n1→ 93Np239 + β- (23.5 minutes) → 94Pu239 + β- (2.36 days). The → half-life of 94Pu239 is 2.44 × 104 yr. Plutonium-239 is a → fissile isotope.
The name derives from the planet → Pluto. It was selected because it is the next planet in the solar system beyond the planet → Neptune and the element plutonium is the next element in the → periodic table beyond → neptunium.