rā`nēəm), radioactive metallic chemical element; symbol U; at. no. 92; at. wt. 238.0289; m.p. 1,132°C;; b.p. 3,818°C;; sp. gr. 19.1 at 25°C;; valence +3, +4, +5, or +6.
Properties
Uranium is a hard, dense, malleable, ductile, silver-white, radioactive metal of the actinide series actinide series, a series of radioactive metallic elements in Group 3 of the periodic table . Members of the series are often called actinides, although actinium (at. no. 89) is not always considered a member of the series.
..... Click the link for more information. in Group 3 of the periodic table periodic table, chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley . In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the
..... Click the link for more information. . Uranium has three distinct forms (see allotropy allotropy (əlŏ`trəpē) [Gr.,=other form].
..... Click the link for more information. ); the orthorhombic crystalline structure occurs at room temperature. It is a highly reactive metal and reacts with almost all the nonmetallic elements and their compounds, especially at elevated temperatures. It dissolves readily in nitric and hydrochloric acids but resists attack by alkalies. It forms solid solutions and intermetallic compounds with many of the metals. Metallic uranium tarnishes in air and when finely divided ignites spontaneously.
Isotopes and Radioactive Decay
Naturally occurring uranium is a mixture of three isotopes isotope (ī`sətōp)
..... Click the link for more information. . The most abundant (greater than 99%) and most stable is uranium-238 (half-life half-life, measure of the average lifetime of a radioactive substance (see radioactivity ) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
..... Click the link for more information. 4.5×109 years); also present are uranium-235 (half-life 7×108 years) and uranium-234 (half-life 2.5×105 years). There are 16 other known isotopes. Uranium-238 is the parent substance of the 18-member radioactive decay series known as the uranium series (see radioactivity radioactivity, spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation . The energy produced by radioactivity has important military and industrial applications.
..... Click the link for more information. ). Some relatively long-lived members of this series include uranium-234, thorium-230, and radium-226; the final stable member of the series is lead-206. Uranium-235, also called actinouranium, is the parent substance of the so-called actinium series, a 15-member radioactive decay series ending in stable lead-207; protactinium-231 and actinium-227 are the relatively stable members of this series. Because the rate of decay in these series is constant, it is possible to estimate the age of uranium samples (e.g., minerals) from the relative amounts of parent substance and final product (see dating dating, the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
..... Click the link for more information. ).
Natural Occurrence and Processing
Uranium is widely distributed in its ores but is not found uncombined in nature. It is a fairly abundant element in the earth's crust, being about 40 times as abundant as silver. Several hundred uranium-containing minerals have been found but only a few are commercially significant. The most important is pitchblende pitchblende (pĭch`blĕnd'), dark, lustrous, heavy mineral, a source of radium and uranium.
..... Click the link for more information. , mined in the Congo River basin and NW Canada. Coffinite (a uranium silicate) and carnotite (a potassium uranate-vanadate) are important minerals found in Colorado and Utah. Ores with as little as 0.1% uranium are mined and processed. Most ores are processed by chemical methods including leaching and solvent extraction. The uranium is obtained as pure uranyl nitrate, UO2(NO3)2·6H2O, which is typically decomposed to the trioxide, UO3, by heating and reduced to the dioxide, UO2, with hydrogen. The dioxide is chemically and physically stable at high temperatures, and is the form most often used as nuclear reactor fuel. The dioxide may be converted to the tetrafluoride, UF4, by treatment with hydrogen fluoride gas, HF. The pure metal is obtained by electrolysis or chemical reduction of the tetrafluoride, or by chemical reduction of the dioxide.
Discovery and Uses
The discovery of uranium is commonly credited to Martin H. Klaproth Klaproth, Martin Heinrich (mär`tēn hīn`rĭkh kläp`rōt), 1743–1817, German chemist.
..... Click the link for more information. , who in 1789, while experimenting with pitchblende, concluded that it contained a new element, which he named after the planet Uranus, discovered only eight years earlier. However, the substance that Klaproth identified was not pure uranium but an oxide. Eugene M. Péligot isolated the element in 1841. Antoine H. Becquerel Antoine César Becquerel, 1788–1878, was a pioneer in electrochemical science. He was professor of physics at the Muséum d'Histoire naturelle from 1838 until his death.
..... Click the link for more information. discovered its radioactivity in 1896. Before the discovery of nuclear fission by Otto Hahn Hahn, Otto (ô`tō hän), 1879–1968, German chemist and physicist.
..... Click the link for more information. and Fritz Strassmann in 1939, the principal use of uranium (chiefly as the oxides) was in pigments, ceramic glazes, and a yellow-green fluorescent glass and as a source of radium for medical purposes. It has also been added to steels to increase their strength and toughness. However, because of the high toxicity (both chemical and radiological) of uranium and its compounds, and because of their importance as nuclear fuel, these earlier uses have been largely curtailed.
Uranium gained importance with the development of practical uses of nuclear energy nuclear energy, the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity . In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E
..... Click the link for more information. . Uranium-235 is the only naturally occurring nuclear fission fuel, but this isotope is only about 1 part in 140 of natural uranium; the balance is mostly uranium-238. Because the supply of uranium-235 is limited, the use of fast breeder reactors that convert nonfissionable uranium-238 to fissionable plutonium-239 is becoming increasingly important (see nuclear reactor nuclear reactor, device for producing controlled release of nuclear energy . Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste
..... Click the link for more information. ). Uranium-235 can be separated from uranium-238 by a diffusion process using the gaseous hexafluoride, UF6 ; the compound of the lighter isotope diffuses faster.
uranium
Chemical element of the actinide series (with many transition element properties), chemical symbol U, atomic number 92. A dense, hard, silvery white metal that tarnishes in air, it is isolated from such ores as pitchblende. Until the discovery of the first transuranium element in 1940, uranium was believed to be the heaviest element. Radioactivity was discovered in uranium by A.-H. Becquerel. All its isotopes are radioactive; several have half-lives long enough to permit determination of the age of the Earth by uranium-thorium-lead dating and uranium-234–uranium-238 dating. Nuclear fission was discovered in 1938 in uranium bombarded with neutrons, and the self-sustaining nuclear chain reaction, the atomic bomb, and the generation of nuclear power followed. Uranium has various valences in compounds, some of which have been used as colours in ceramic glazes, in lightbulb filaments, in photography, and as dyes and mordants.
uranium
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