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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 (see Periodic Law). The atomic number of uranium is 92. Uranium was discovered in 1789 in the mineral 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 1135°C (about 2075°F), boils at about 4131°C (about 7468°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 has oxidation states of three, four, five, and six. The hexapositive compounds include uranyl trioxide, UO3, and uranyl chloride, UO2Cl2. Uranium tetrachloride, UCl4, and uranium dioxide, UO2, are examples of the tetrapositive, or uranous, compounds. Uranous compounds are usually unstable; they revert to the hexapositive form when excessively exposed to air. Uranyl salts, such as uranyl chloride, may decompose in the presence of strong light and organic matter.
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.
In the classical procedure for extracting uranium, pitchblende is broken up and mixed with sulfuric and nitric acids. Uranium dissolves to form uranyl sulfate, UO2SO4; radium and other metals in the pitchblende ore are precipitated as sulfates. With the addition of sodium hydroxide, uranium is precipitated as sodium diuranate, Na2U2O7 · 6H2O, known also as the yellow oxide of uranium. To obtain uranium from carnotite, the ore is finely ground and treated with a hot solution of caustic soda and potash to dissolve out uranium, radium, and vanadium. After the worthless sandy matrix is washed away, the solution is treated with sulfuric acid and barium chloride. A caustic alkali solution added to the remaining clear liquid precipitates the uranium and radium in concentrated form. These classical methods of extracting uranium from its ores have been replaced in current practice by such procedures as solvent extraction, ion exchange, and volatility methods. For the method of producing the artificial isotope uranium-233, see Thorium.
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