Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey 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 isotopes of uranium are unstable; the half-lives of its naturally occurring isotopes range between 159,200 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% of uranium on Earth) 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.[4]

Uranium, 92U
Pronunciation/jʊˈrniəm/ (yuu-RAY-nee-əm)
Appearancesilvery gray metallic; corrodes to a spalling black oxide coat in air
Standard atomic weight Ar°(U)
  • 238.02891±0.00003
  • 238.03±0.01 (abridged)[1]
Uranium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Atomic number (Z)92
Groupgroup n/a
Periodperiod 7
Block  f-block
Electron configuration[Rn] 5f3 6d1 7s2
Electrons per shell2, 8, 18, 32, 21, 9, 2
Physical properties
Phase at STPsolid
Melting point1405.3 K (1132.2 °C, 2070 °F)
Boiling point4404 K (4131 °C, 7468 °F)
Density (near r.t.)19.1 g/cm3
when liquid (at m.p.)17.3 g/cm3
Heat of fusion9.14 kJ/mol
Heat of vaporization417.1 kJ/mol
Molar heat capacity27.665 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2325 2564 2859 3234 3727 4402
Atomic properties
Oxidation states−1,[2] +1, +2, +3,[3] +4, +5, +6 (an amphoteric oxide)
ElectronegativityPauling scale: 1.38
Ionization energies
  • 1st: 597.6 kJ/mol
  • 2nd: 1420 kJ/mol
Atomic radiusempirical: 156 pm
Covalent radius196±7 pm
Van der Waals radius186 pm
Spectral lines of uranium
Other properties
Natural occurrenceprimordial
Crystal structure orthorhombic
Speed of sound thin rod3155 m/s (at 20 °C)
Thermal expansion13.9 µm/(m⋅K) (at 25 °C)
Thermal conductivity27.5 W/(m⋅K)
Electrical resistivity0.280 µΩ⋅m (at 0 °C)
Magnetic orderingparamagnetic
Young's modulus208 GPa
Shear modulus111 GPa
Bulk modulus100 GPa
Poisson ratio0.23
Vickers hardness1960–2500 MPa
Brinell hardness2350–3850 MPa
CAS Number7440-61-1
Namingafter planet Uranus, itself named after Greek god of the sky Uranus
DiscoveryMartin Heinrich Klaproth (1789)
First isolationEugène-Melchior Péligot (1841)
Main isotopes of uranium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
232U syn 68.9 y SF
α 228Th
233U trace 1.592×105 y SF
α 229Th
234U 0.005% 2.455×105 y SF
α 230Th
235U 0.720% 7.04×108 y SF
α 231Th
236U trace 2.342×107 y SF
α 232Th
238U 99.274% 4.468×109 y α 234Th
ββ 238Pu
 Category: Uranium
| references

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%).[5] 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,[6] 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 studied for future industrial use in nuclear technology.[citation needed] 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.[7] 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 recently discovered 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 is closely monitored. Since around 2000, plutonium obtained by dismantling Cold War-era bombs is used as fuel for nuclear reactors. The development and deployment of these nuclear reactors continue on a global base as they are powerful sources of CO2-free energy.

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