Query: Uranium has an atomic number.

Relevant passages:
Uranium <=SEP=> 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 six naturally known isotopes , uranium-233 to uranium-238 , varying between 69 years and 4.5 billion years ) . The most common isotopes in natural uranium are uranium-238 ( which has 146 neutrons and accounts for over 99 % ) and uranium-235 ( which has 143 neutrons ) . Uranium has the highest atomic weight of the primordially occurring elements . Its density is about 70 % higher than that of lead , and 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 % ) . 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 , making them useful in dating the age of the Earth .   Many contemporary uses of uranium exploit its unique nuclear properties . Uranium-235 is the only naturally occurring fissile isotope , which makes it widely used in nuclear power plants and nuclear weapons . However , because of the tiny amounts found in nature , uranium needs to undergo enrichment so that enough uranium-235 is present . 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 . 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 lemon yellow to green colors . Uranium glass fluoresces green in ultraviolet light . 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 . See Nuclear proliferation .