History

(Planet Pluto) Plutonium was the second transuranium element of the actinide series to be discovered. The isotope ²³⁸Pu was produced in 1940 by Seaborg, McMillan, Kennedy, and Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley, California. Plutonium also exists in trace quantities in naturally occurring uranium ores. It is formed in much the same manner as neptunium: by irradiation of natural uranium with the neutrons which are present.

Isotopes

By far of greatest importance is the isotope ²³⁹Pu, with a half-life of 24,100 years, produced in extensive quantities in nuclear reactors from natural uranium: ²³⁸U(n, gamma) --> ²³⁹U--(beta) --> ²³⁹Np--(beta) --> ²³⁹Pu. Fifteen isotopes of plutonium are known.
Plutonium also exhibits four ionic valence states in aqueous solutions: Pu⁺³ (blue lavender), Pu⁺⁴ (yellow brown), PuO⁺ (pink?), and PuO⁺²(pink-orange). The ion PuO⁺ is unstable in aqueous solutions, and it disproportionates into Pu⁺⁴ and PuO⁺². The Pu⁺⁴ thus formed, however, oxidizes the PuO⁺ into PuO⁺², itself being reduced to Pu⁺³, resulting finally in Pu⁺³ and PuO⁺². Plutonium forms binary compounds with oxygen: PuO, PuO₂, and intermediate oxides of variable composition; with the halides: PuF₃, PuF₄, PuCl₃, PuBr₃, PuI₃; with carbon, nitrogen, and silicon: PuC, PuN, PuSi₂. Oxyhalides are also well known: PuOCl, PuOBr, PuOI.

Properties

The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cm3. 
A Periodic Table of the Elements at Los Alamos National Laboratory