Uranium,  92U
Two hands in brown gloves holding a blotched gray disk with a number 2068 hand-written on it
Pronunciationm/ (AY-nee-əm)
Appearancesilvery gray metallic; corrodes to a spalling black oxide coat in air
Standard atomic weight Ar, std(U)238.02891(3)[1]
Uranium in the periodic table
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon


Atomic number (Z)92
Groupgroup n/a
Periodperiod 7
Element category  Actinide
Electron configuration[Rn] 5f3 6d1 7s2
Electrons per shell
2, 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)1101001 k10 k100 k
at T (K)232525642859323437274402
Atomic properties
Oxidation states+1, +2, +3,[2] +4, +5, +6 (a weakly basic 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
Color lines in a spectral range
Spectral lines of uranium
Other properties
Natural occurrenceprimordial
Crystal structureorthorhombic
Orthorhombic crystal structure for uranium
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
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
232Usyn68.9 ySF
233Utrace1.592×105 ySF
234U0.005%2.455×105 ySF
235U0.720%7.04×108 ySF
236Utrace2.342×107 ySF
238U99.274%4.468×109 yα234Th
| references

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]

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 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.[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 and their fissile material following the breakup of the Soviet Union in 1991 is an ongoing concern for public health and safety.[8] See Nuclear proliferation.


A diagram showing a chain transformation of uranium-235 to uranium-236 to barium-141 and krypton-92
A neutron-induced nuclear fission event involving uranium-235

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.[9][10] Uranium metal has a very high density of 19.1 g/cm3,[11] denser than lead (11.3 g/cm3),[12] but slightly less dense than tungsten and gold (19.3 g/cm3).[13][14]

Uranium metal reacts with almost all non-metal elements (with the exception of the noble gases) and their compounds, with reactivity increasing with temperature.[15] Hydrochloric and nitric acids dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element very slowly.[9] When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.[10] 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.[16] The first nuclear bomb used in war, Little Boy, relied on uranium fission, but the very first nuclear explosive (the Gadget used at Trinity) and the bomb that destroyed Nagasaki (Fat Man) were both plutonium bombs.

Uranium metal has three allotropic forms:[17]

  • α (orthorhombic) stable up to 668 °C. Orthorhombic, space group No. 63, Cmcm, lattice parameters a = 285.4 pm, b = 587 pm, c = 495.5 pm.[18]
  • β (tetragonal) stable from 668 °C to 775 °C. Tetragonal, space group P42/mnm, P42nm, or P4n2, lattice parameters a = 565.6 pm, b = c = 1075.9 pm.[18]
  • γ (body-centered cubic) from 775 °C to melting point—this is the most malleable and ductile state. Body-centered cubic, lattice parameter a = 352.4 pm.[18]