Atomic Number: 92
Group: None
Atomic Weight: 238.02891
Period: 7
CAS Number: 7440-61-1


Noble Gas
Rare Earth Element
Platinum Group Metal
No Stable Isotopes
Solid (Predicted)

Description • Uses/Function

Yellow-colored glass, containing more than 1% uranium oxide and dating back to 79 A.D., has been found near Naples, Italy. Klaproth recognized an unknown elementin pitchblende and attempted to isolate the metal in 1789. The metal apparently was first isolated in 1841 by Peligot, who reduced the anhydrous chloridewith potassium. Uranium is not as rare as it was once thought. It is now considered to be more plentiful than mercury, antimony, silver, or cadmium,and is about as abundant as molybdenum or arsenic. It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane,davidite, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources. Largedeposits of uranium ore occur in Utah, Colorado, New Mexico, Canada, and elsewhere. The U.S.D.O.E. purchases uranium in the form of acceptableU3O8 concentrates. This incentive program has greatly increased the known uranium reserves. Uranium can be made by reducing uranium halides withalkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be producedby electrolysis of KUf5 or Uf4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decompositionof uranium halides on a hot filament. Uranium exhibits three crystallographic modifications as follows. Heating the alpha form to 688 degrees Celsius will convert it to the beta form. Further heating up to 776 degrees Celsius will convert it to the gamma form. Uranium is a heavy, silvery-white metal which is pyrophoric when finely divided. It is a little softer than steel, and is attacked by cold water in a finelydivided state. It is malleable, ductile, and slightly paramagnetic. In air, the metal becomes coated with a layer of oxide. Acids dissolve the metal, butit is unaffected by alkalis. Uranium has twenty three isotopes, one of which is an isomer and all of which are radioactive. Naturally occurring uraniumcontains 99.2745% by weight uranium-238, 0.720% uranium-235, and 0.0055% uranium-234. Studies show that the percentage weight of uranium-235 in natural uranium varies byas much as 0.1%, depending on the source. The U.S.D.O.E. has adopted the value of 0.711 as being their “official” percentage of uranium-235 in natural uranium.Natural uranium is sufficiently radioactive to expose a photographic plate in an hour or so. Much of the internal heat of the earth is thought to beattributable to the presence of uranium and thorium. 238U with a half-life of 4.46 X 10^9 years, has been used to estimate the age of igneous rocks. Theorigin of uranium, the highest member of the naturally occurring elements — except perhaps for traces of neptunium or plutonium — is not clearlyunderstood, although it may be presumed that uranium is a decay product of elements of higher atomic weight, which may have once been presenton earth or elsewhere in the universe. These original elements may have been formed as a result of a primordial “creation,” known as “the big bang,”in a supernova, or in some other stellar processes. Uranium is of great importance as a nuclear fuel. Uranium-238 can be converted into fissionable plutoniumby the following reactions. Neutron absorption by uranium-238 will produce uranium-239. Two successive beta decays yield plutonium-239 via neptunium-239. This nuclear conversion can be brought about in “breeder” reactors where it is possible to produce more new fissionable material than the fissionablematerial used in maintaining the chain reaction. Uranium-235 is of even greater importance, for it is the key to the utilization of uranium. 235U, while occurringin natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made to occurin a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. 235U can be concentrated by gaseousdiffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive. Naturaluranium, slightly enriched with 235U by a small percentage, is used to fuel nuclear power reactors for the generation of electricity. Natural thoriumcan be irradiated with neutrons as follows to produce the important isotope uranium-233. Neutron absorption of thorium-232 yields thorium-233. Two successive beta decays yield uranium-233 via protactinium-233. While thorium itself is not fissionable, uranium-233 is, and in this way may be used as a nuclear fuel. One pound of completely fissioned uranium has the fuelvalue of over 1500 tons of coal. The uses of nuclear fuels to generate electrical power, to make isotopes for peaceful purposes, and to make explosivesare well known. The estimated world-wide production of the 430 nuclear power reactors in operation in January 1994 amounted to about 338,000megawatts. Uranium in the U.S.A. is controlled by the U.S. Nuclear Regulatory Commission. New uses are being found for “depleted” uranium, i.e.,uranium with the percentage of uranium-235 lowered to about 0.2%. It has found use in inertial guidance devices, gyro compasses, counterweights for aircraftcontrol surfaces, as ballast for missile reentry vehicles, and as a shielding material. Uranium metal is used for X-ray targets for production of high-energyX-rays; the nitrate has been used as photographic toner, and the acetate is used in analytical chemistry. Crystals of uranium nitrate aretriboluminescent. Uranium salts have also been used for producing yellow “vaseline” glass and glazes. Uranium and its compounds are highly toxic,both from a chemical and radiological standpoint. Finely divided uranium metal, being pyrophoric, presents a fire hazard. The maximum permissibletotal body burden of natural uranium (based on radiotoxicity) is 0.2 microcuries for soluble compounds. Recently, the natural presence of uranium in manysoils has become of concern to homeowners because of the generation of radon and its daughters (see under Radon). Uranium metal is availablecommercially at a cost of about $200/kg (99.7%) in air-tight glass under argon. 1

• "A pebble bed nuclear reactor is schematically depicted as a funnel-shaped container through which tennis-ball size "pebbles" of fuel are circulated. Each pebble is made of graphite and carbon and contains about 15,000 kernels of fuel. Each kernel has a core of uranium that is coated with a layer each of porous carbon, pyrolytic carbon (which is similar to graphite), silicon carbide, and then a second layer of pyrolytic carbon. These layers are used to moderate the speed of neutrons, helping to control the nuclear reactions of the fuel, and to provide a fireproof seal." 2

Physical Properties

Melting Point:3*  1135 °C = 1408.15 K = 2075 °F
Boiling Point:3* 4131 °C = 4404.15 K = 7467.8 °F
Sublimation Point:3 
Triple Point:3 
Critical Point:3 
Density:4  19.1 g/cm3

* - at 1 atm

Electron Configuration

Electron Configuration:  *[Rn] 7s2 5f4
Block: f
Highest Occupied Energy Level: 7
Valence Electrons: 2

Quantum Numbers:

n = 5
ℓ = 3
m = 0
ms = +½


Electronegativity (Pauling scale):5 1.7
Electropositivity (Pauling scale): 2.3
Work Function:6 3.70 eV = 5.9274E-19 J

Ionization Potential   eV 7  kJ/mol  
Ionization Potential   eV 7  kJ/mol  
Ionization Potential   eV 7  kJ/mol  
1 6.19405    597.6


Specific Heat: 0.116 J/g°C 8 = 27.611 J/mol°C = 0.028 cal/g°C = 6.599 cal/mol°C
Thermal Conductivity: 27.6 (W/m)/K, 27°C 9
Heat of Fusion: 8.52 kJ/mol 10 = 35.8 J/g
Heat of Vaporization: 477 kJ/mol 11 = 2004.0 J/g
State of Matter Enthalpy of Formation (ΔHf°)12 Entropy (S°)12 Gibbs Free Energy (ΔGf°)12
(kcal/mol) (kJ/mol) (cal/K) (J/K) (kcal/mol) (kJ/mol)
(s) 0 0 12.03 50.33352 0 0


Nuclide Mass 13 Half-Life 13 Nuclear Spin 13 Binding Energy
217U 217.02437(9) 26(14) ms [16(+21-6) ms] 1/2-# 1,660.87 MeV
218U 218.02354(3) 6(5) ms 0+ 1,668.94 MeV
219U 219.02492(6) 55(25) μs [42(+34-13) μs] 9/2+# 1,677.01 MeV
220U 220.02472(22)# 60# ns 0+ 1,685.08 MeV
221U 221.02640(11)# 700# ns 9/2+# 1,693.15 MeV
222U 222.02609(11)# 1.4(7) μs [1.0(+10-4) μs] 0+ 1,701.22 MeV
223U 223.02774(8) 21(8) μs [18(+10-5) μs] 7/2+# 1,709.30 MeV
224U 224.027605(27) 940(270) μs 0+ 1,717.37 MeV
225U 225.02939# 61(4) ms (5/2+)# 1,725.44 MeV
226U 226.029339(14) 269(6) ms 0+ 1,733.51 MeV
227U 227.031156(18) 1.1(1) min (3/2+) 1,732.27 MeV
228U 228.031374(16) 9.1(2) min 0+ 1,740.34 MeV
229U 229.033506(6) 58(3) min (3/2+) 1,748.41 MeV
230U 230.033940(5) 20.8 d 0+ 1,756.48 MeV
231U 231.036294(3) 4.2(1) d (5/2)(+#) 1,764.55 MeV
232U 232.0371562(24) 68.9(4) y 0+ 1,772.62 MeV
233U 233.0396352(29) 1.592(2)E5 y 5/2+ 1,780.69 MeV
234U 234.0409521(20) 2.455(6)E5 y 0+ 1,779.45 MeV
235U 235.0439299(20) 7.04(1)E8 y 7/2- 1,787.52 MeV
236U 236.045568(2) 2.342(3)E7 y 0+ 1,795.59 MeV
237U 237.0487302(20) 6.75(1) d 1/2+ 1,803.66 MeV
238U 238.0507882(20) 4.468(3)E9 y 0+ 1,802.42 MeV
239U 239.0542933(21) 23.45(2) min 5/2+ 1,810.49 MeV
240U 240.056592(6) 14.1(1) h 0+ 1,818.56 MeV
241U 241.06033(32)# 5# min 7/2+# 1,817.32 MeV
242U 242.06293(22)# 16.8(5) min 0+ 1,825.39 MeV
Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses. 13


Earth - Source Compounds: oxides 14
Earth - Seawater: 0.0032 mg/L 15
Earth -  Crust:  2.7 mg/kg = 0.00027% 15
Earth -  Total:  14.3 ppb 16
Mercury -  Total:  11.0 ppb 16
Venus -  Total:  15.0 ppb 16
Chondrites - Total: 0.009 (relative to 106 atoms of Si) 17
Human Body - Total: 0.0000001% 18


Safety Information

Material Safety Data Sheet - ACI Alloys, Inc.

For More Information

External Links:


(1) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:33-4:34.
(2) - Langston, Lee S. A Path for Nuclear Power. American Scientist. 2013, 102, 91.
(3) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:132.
(4) - Lide, David R. CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2002; p 4:39-4:96.
(5) - Dean, John A. Lange's Handbook of Chemistry, 11th ed.; McGraw-Hill Book Company: New York, NY, 1973; p 4:8-4:149.
(6) - Speight, James. Lange's Handbook of Chemistry, 16th ed.; McGraw-Hill Professional: Boston, MA, 2004; p 1:132.
(7) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 10:178 - 10:180.
(8) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:133.
(9) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:193, 12:219-220.
(10) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:123-6:137.
(11) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:107-6:122.
(12) - Dean, John A. Lange's Handbook of Chemistry, 12th ed.; McGraw-Hill Book Company: New York, NY, 1979; p 9:4-9:94.
(13) - Atomic Mass Data Center. http://amdc.in2p3.fr/web/nubase_en.html (accessed July 14, 2009).
(14) - Silberberg, Martin S. Chemistry: The Molecular Nature of Matter and Change, 4th ed.; McGraw-Hill Higher Education: Boston, MA, 2006, p 965.
(15) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 14:17.
(16) - Morgan, John W. and Anders, Edward, Proc. Natl. Acad. Sci. USA 77, 6973-6977 (1980)
(17) - Brownlow, Arthur. Geochemistry; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1979, pp 15-16.
(18) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 7:17.