Plutonium << ploo TOH nee uhm >> is a radioactive metallic element. Almost all plutonium is produced artificially. Only an extremely small amount of it occurs naturally. Plutonium has a variety of scientific and industrial uses.
Plutonium’s chemical symbol is Pu, and its atomic number (number of protons) is 94. Plutonium melts at 640 °C and boils at 3460 °C. At 20 °C, plutonium has a density of 19.86 grams per cubic centimeter (see Density).
Chemists classify plutomium in the actinide group. For information on the position of plutonium on the periodic table, see the article Periodic table.
Plutonium is highly poisonous because it rapidly gives off radiation in the form of high-energy alpha particles (see Alpha particle). These particles may cause cancer or other serious health problems. In addition, plutonium is extremely explosive. It must be kept in quantities smaller than a critical mass, the amount at which it would explode spontaneously.
Scientists have discovered 18 isotopes of plutonium, forms with the same number of protons but different numbers of neutrons. The most important of the isotopes has an atomic mass number (total number of protons and neutrons) of 239. Its symbol is Pu-239. That isotope readily undergoes fission when struck by a neutron. In the fission process, the nucleus of an atom is split into two nearly equal parts, and energy is released. Pu-239 is a source of energy in nuclear reactors. It is also used in nuclear weapons. Scientists produce Pu-239 by bombarding uranium 238 with neutrons. The same process forms Pu-239 as a waste product in nuclear reactors that use uranium as the basic fuel. 
Pu-239 has a half-life of 24,100 years—that is, due to radioactive decay, only half the atoms in a sample of Pu-239 would still be atoms of that isotope after 24,100 years. Because the half-life of Pu-239 is so long, the disposal of waste Pu-239 has become a serious problem.
Plutonium has various other applications. For example, Pu-238 powers heart pacemakers, certain instruments on spacecraft, and some other devices. Pu-242 and Pu-244 are useful in studying chemicals and metals.
Plutonium was discovered in 1940 by four American scientists—Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy, and Arthur C. Wahl. They produced Pu-238 by bombarding uranium 238 with deuterons, the nuclei in atoms of deuterium, an isotope of hydrogen. In 1971, scientists discovered the most stable isotope of plutonium, Pu-244, in nature. Pu-244 has a half-life of 80 million years.
See also Nuclear energy; Transuranium element.
