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Chemical Elements

Uranium

actinide series, carnotite, pitchblende, planet Uranus, isotopes of uranium

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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. 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.

Occurence

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.

Extraction

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.

Contributors

Seaborg, Glenn T., Ph.D.

Professor of Chemistry, University of California, Berkley. Former Chair, U.S. Atomic Energy Comission. Recipient, Nobel Prize in Chemistry (1951). Author of "Transuranium Elements".



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