The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the recently discovered planet Uranus. Depleted uranium ( 238U) is used in kinetic energy penetrators and armor plating. This generates the heat in nuclear power reactors, and produces the fissile material for nuclear weapons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction.
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.
Another fissile isotope, uranium-233, can be produced from natural thorium and is studied for future industrial use in nuclear technology. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor.
However, because of the extreme lowness of concentrations of uranium-235 in naturally occurring uranium (which is, overwhelmingly, mostly uranium-238), uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-235 is the only naturally occurring fissile isotope, which makes it widely used in nuclear power plants and nuclear weapons. Many contemporary uses of uranium exploit its unique nuclear properties. 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. Its density is about 70% higher than that of lead, and slightly lower than that of gold or tungsten. Uranium has the highest atomic weight of the primordially occurring elements. The most common isotopes in natural uranium are uranium-238 (which has 146 neutrons and accounts for over 99% of uranium on Earth) and uranium-235 (which has 143 neutrons).
The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. Uranium radioactively decays by emitting an alpha particle. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. It is a silvery-grey metal in the actinide series of the periodic table. Uranium is a chemical element with symbol U and atomic number 92.
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