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.


Uranium, symbol U, chemically reactive radioactive metallic element that is the main fuel used in nuclear reactors. Uranium is a member of the actinide series in the periodic table. The atomic number of uranium is 92.

Uranium was discovered in 1789 in pitchblende by the German chemist Martin Heinrich Klaproth, who named it after the planet Uranus. It was first isolated in the metallic state in 1841. The radioactive properties of uranium were first demonstrated in 1896 when the French physicist Antoine Henri Becquerel produced, by the action of the fluorescent salt potassium uranyl sulfate, an image on a photographic plate covered with a light-absorbing substance. The investigations of radioactivity that followed Becquerel's experiment led to the discovery of radium and to new concepts of atomic organization. See Atom; Nuclear Energy.


Uranium melts at about 1132° C (about 2070° F), boils at about 3818° C (about 6904° F), and has a specific gravity of 19.05 at 25° C (77° F); the atomic weight of the element is 238.03. Uranium has three crystalline forms, of which the one that forms at about 770° C (about 1418° F) is malleable and ductile. Uranium is soluble in hydrochloric and nitric acids, and it is insoluble in alkalies. Uranium displaces hydrogen from mineral acids and from the salt solutions of such metals as mercury, silver, copper, tin, platinum, and gold. When finally divided, it burns readily in air at 150° to 175° C (302° to 347° F). At 1000° C (1832° F), uranium combines with nitrogen to form a yellow nitride.

Uranium never occurs naturally in the free state but is found as an oxide or complex salt in minerals such as pitchblende and carnotite. It has an average concentration in the crust of the earth of about 2 parts per 1 million, and, among the elements, ranks about 48th in natural abundance in crustal rocks. Pure uranium consists of more than 99 percent of the isotope uranium-238, less than 1 percent of the fissile isotope uranium-235, and a trace of uranium-234, formed by radioactive decay of uranium-238. Among the artificially produced isotopes of uranium are uranium-233, uranium-237, and uranium-239. Isotopes ranging from mass number 222 to 242 are known.


After the discovery of nuclear fission, uranium became a strategic metal, and its uses were at first restricted mainly to the production of nuclear weapons. In 1954 the United States government relaxed controls to permit leasing of uranium enriched in the isotope uranium-235 to various private and foreign agencies for the development of nuclear power.