Plutonium is a man-made heavy metal.
It was discovered and named for the planet Pluto in 1941 by Dr. Glenn T. Seaborg and other scientists at the University of California at Berkeley. Plutonium is a solid, silvery-gray metal, the atoms of which can split when bombarded with neutrons. This splitting of atoms can cause a chain reaction that releases tremendous amounts of energy, making plutonium ideal for nuclear weapons. After the breakup of the Soviet Union, the U.S. and Russia signed a strategic arms reduction treaty that led the U.S to shrink its arsenal from 13,000 nuclear weapons to 7,000. That number could be reduced to 3,500 if Russia ratifies a second strategic arms reduction treaty, which the U.S. has already ratified.