Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Plutonium, symbol Pu, radioactive metallic element that is used in nuclear reactors and nuclear weapons. The atomic number of plutonium is 94. The element is one of the transuranium elements in the actinide series of the periodic table.

Isotopes of plutonium were first prepared by the American chemist Glenn T. Seaborg and his associates at the University of California at Berkeley in 1940. Trace amounts of the element have since been found in uranium ores, but plutonium is prepared in relatively large quantities today in nuclear reactors.

Chemically, plutonium is reactive, its properties somewhat resembling those of the rare earth elements. The silvery metal, which becomes slightly yellow through oxidation caused by exposure to air, exists in six varying crystalline forms and has four different oxidation states. The metal gives off heat because of its radioactivity; 15 different isotopes of plutonium, ranging in mass number from 232 to 246, are known. Plutonium melts at about 641° C (about 1186° F), boils at about 3232° C (about 5850° F), and has a specific gravity of 19.84.

The most important isotope, plutonium-239, has a half-life of 24,360 years, and is produced by bombarding uranium-238 with slow neutrons. This forms neptunium-239, which in turn emits a beta particle and forms plutonium-239. Plutonium is the most economically important of the transuranium elements because plutonium-239 readily undergoes fission and can be both used and produced in quantity in nuclear reactors. It is also used in making nuclear weapons. It is an extremely hazardous poison due to its high radioactivity. Plutonium-238 has been used to power equipment on the moon by means of the heat it emits.