Plutonium is a radioactive, metallic, chemical element. It has the symbol Pu and the atomic number 94. It is the element used in most modern nuclear weapons. The most important isotope of plutonium is ²³⁹Pu, with a half-life of 24,110 years. It can be made from natural uranium and is fissile. The most stable isotope is ²⁴⁴Pu, with a half-life of about 80 million years, long enough to be found in extremely small quantities in nature. Nuclear materials workers sometimes call it "plute".

Notable characteristics

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Plutonium is silvery in pure form, but has a yellow tarnish when oxidized. Peculiarly, the metal goes through phases of contraction as its temperature is increased. It is also notable in that it posseses a low-symmetry structure causing it to become progressively more brittle over time.

The heat given off by alpha particle emission makes plutonium warm to the touch in reasonable quantities; larger amounts can boil water. It displays five ionic oxidation states in aqueous solution:

• Pu^III, as Pu³⁺ (blue lavender)
• Pu^IV, as Pu⁴⁺ (yellow brown)
• Pu^VI, as PuO₂²⁺ (pink orange)
• Pu^V, as PuO₂⁺ (thought to be pink; this ion is unstable in solution and will disproportionate into Pu⁴⁺ and PuO₂²⁺; the Pu⁴⁺ will then oxidize the remaining PuO₂⁺ to PuO₂²⁺, being reduced in turn to Pu³⁺. Thus, aqueous solutions of plutonium tend over time towards a mixture of Pu³⁺ and PuO₂²⁺.)
• Pu^VII, as PuO₅^2- (dark red); the heptavalent ion is rare and prepared only under extreme oxidizing conditions.

Note: The color shown by Pu solutions depend on both the oxidation state and the nature of the acid anion, which influences the degree of complexing of the Pu species by the acid anion. Matlack, George: A Plutonium Primer: An Introduction to Plutonium Chemistry and its Radioactivity (LA-UR-02-6594)

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