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Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-white metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all its isotopes are unstable (with half-lives of the 6 naturally known isotopes, uranium-233 to uranium-238, varying between 69 years and 4.5 billion years). The most common isotopes of uranium are uranium-238 (which has 146 neutrons and accounts for almost 99.3% of the uranium found in nature) and uranium-235 (which has 143 neutrons, accounting for 0.7% of the element found naturally). Uranium has the second highest atomic weight of the primordially occurring elements, lighter only than plutonium. Its density is about 70% higher than that of lead, but slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite. In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%).〔(【引用サイトリンク】url = http://www.globalsecurity.org/wmd/intro/u-isotopes.htm )〕 Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years,〔(【引用サイトリンク】title=WWW Table of Radioactive Isotopes )〕 making them useful in dating the age of the Earth. Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is fissionable by fast neutrons, and is ''fertile'', meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology. While uranium-238 has a small probability for spontaneous fission or even induced fission with fast neutrons, uranium-235 and to a lesser degree uranium-233 have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction. This generates the heat in nuclear power reactors, and produces the fissile material for nuclear weapons. Depleted uranium (238U) is used in kinetic energy penetrators and armor plating.〔.〕 Uranium is used as a colorant in uranium glass producing orange-red to lemon yellow hues. It was also used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal and its radioactive properties were discovered in 1896 by Henri Becquerel. Research by Otto Hahn, Lise Meitner, Enrico Fermi and others, such as J. Robert Oppenheimer starting in 1934 led to its use as a fuel in the nuclear power industry and in ''Little Boy'', the first nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used uranium metal and uranium-derived plutonium-239. The security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 is an ongoing concern for public health and safety.〔("U.S. to pump money into nuke stockpile, increase security," ) ''RIA Novosti'' 18 February 2010〕 See Nuclear proliferation. ==Characteristics== When refined, uranium is a silvery white, weakly radioactive metal. It has a Mohs hardness of 6, sufficient to scratch glass and approximately equal to that of titanium, rhodium, manganese and niobium. It is malleable, ductile, slightly paramagnetic, strongly electropositive and a poor electrical conductor.〔 Uranium metal has a very high density of 19.1 g/cm3,〔(【引用サイトリンク】title=Uranium )〕 denser than lead (11.3 g/cm3),〔(【引用サイトリンク】title=Lead )〕 but slightly less dense than tungsten and gold (19.3 g/cm3).〔(【引用サイトリンク】title=Tungsten )〕〔(【引用サイトリンク】title=Gold )〕 Uranium metal reacts with almost all non-metal elements (with an exception of the noble gases) and their compounds, with reactivity increasing with temperature. Hydrochloric and nitric acids dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element very slowly.〔 When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.〔 Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry. Uranium-235 was the first isotope that was found to be fissile. Other naturally occurring isotopes are fissionable, but not fissile. On bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy and more neutrons. If too many of these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs that results in a burst of heat or (in special circumstances) an explosion. In a nuclear reactor, such a chain reaction is slowed and controlled by a neutron poison, absorbing some of the free neutrons. Such neutron absorbent materials are often part of reactor control rods (see nuclear reactor physics for a description of this process of reactor control). As little as 15 lb (7 kg) of uranium-235 can be used to make an atomic bomb. The first nuclear bomb used in war, ''Little Boy'', relied on uranium fission, while the very first nuclear explosive (''The gadget'') and the bomb that destroyed Nagasaki (''Fat Man'') were plutonium bombs. Uranium metal has three allotropic forms: * α (orthorhombic) stable up to 660 °C * β (tetragonal) stable from 660 °C to 760 °C * γ (body-centered cubic) from 760 °C to melting point—this is the most malleable and ductile state. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Uranium」の詳細全文を読む スポンサード リンク
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