plutonium plo͞otōˈnēəm [key], radioactive chemical element; symbol Pu; at. no. 94; mass no. of most stable isotope 244; m.p. 641℃; b.p. 3,232℃; sp. gr. 19.84 at 20℃; valence +3, +4, +5, or +6. Plutonium is a silver-gray radioactive metal that has six allotropic forms (see allotropy). It is a member of the actinide series in Group 3 of the periodic table.

Plutonium is chemically reactive. It tarnishes in air, taking on a yellow cast when oxidized. It dissolves in hydrochloric, hydriodic, and perchloric acids and reacts with the halogens, carbon, nitrogen, and silicon. Pure plutonium metal may be prepared by reduction of the trifluoride, PuF3, with calcium metal. Sixteen isotopes of plutonium are known. The most stable is plutonium-244 (half-life about 82 million years). By far the most important is plutonium-239 (half-life about 24,000 years), a nuclear fission fuel.

Plutonium-239 is produced in large quantities in nuclear reactors from uranium-238, an abundant but nonfissionable isotope. Uranium-238 absorbs neutrons emitted by the fission of uranium-235; uranium-239 is formed, which emits a beta particle and decays to neptunium-239; the neptunium-239 emits another beta particle, becoming plutonium-239. Once begun, the reaction proceeds spontaneously until the uranium fuel rods in the reactor are converted to a certain uranium-plutonium mixture. The rods are dissolved in acid and the plutonium separated by chemical means, especially by solvent extraction.

Plutonium is important for its use in nuclear weapons and nuclear reactors. Plutonium-238 has been used to power scientific equipment in lunar exploration and implanted heart pacemakers (see pacemaker, artificial). Plutonium is an extremely dangerous poison; it collects in the bones and interferes with the production of white blood cells.

Plutonium is found naturally in very small quantities in association with uranium ores. However, it was discovered in 1940 at the Univ. of California at Berkeley by Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy, and Arthur C. Wahl; using a cyclotron to bombard uranium oxide with deuterons, they produced plutonium-238 (half-life about 87 years). Plutonium, the second transuranium element, was named for Pluto, then regarded as the second planet beyond Uranus.

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