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Author: Robert Husted, Mollie Boorman

Atomic Number 92
Atomic Symbol U
CAS ID No. 7440-61-1
Atomic Weight 238.0300 amu
Electron Configuration [Rn] 5f3 6d1 7s2
Melting Point 1132.0 C
Boiling Point 4134 C
Density 18.900 g/cm3

(Planet Uranus, named after the Greek Ouranos, the son and husband of Gaia) 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 element in pitchblende and attempted to isolate the metal in 1789.

The metal apparently was first isolated in 1841 by Peligot, who reduced the anhydrous chloride with potassium.


Uranium, not as rare as once thought, 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, and tobernite. It is also found in phosphate rock, lignite, monazite sands, and can be recovered commercially from these sources.

The United States Department of Energy purchases uranium in the form of acceptable U3O8 concentrates. This incentive program has greatly increased the known uranium reserves.

Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.


Uranium exhibits three crystallographic modifications as follows:

alpha --(688C)--> beta --(776C)--> gamma.

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 finely divided state. It is malleable, ductile, and slightly paramagnetic.

In air, the metal becomes coated with a layer of oxide. Acids dissolve the metal, but it is unaffected by alkalis.


Uranium has sixteen isotopes, all of which are radioactive. Naturally occurring uranium nominally contains 99.28305 by weight 238U, 0.7110% 235U, and 0.0054% 234U. Studies show that the percentage weight of 235U in natural uranium varies by as much as 0.1%, depending on the source. The US DOE has adopted the value of 0.711 as being their official percentage of 235U 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 be attributable to the presence of uranium and thorium.

Uranuim-238 with a half-life of 4.51 x 109 years, has been used to estimate the age of igneous rocks. The origin of uranium, the highest member of the naturally occurring elements - except perhaps for traces of neptunium or plutonium, is not clearly understood. However it may be presumed that uranium is a decay product of elements with higher atomic weight, which may have once been present on 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 plutonium by the following reactions: 238U(n, gamma) --> 239U --(beta)--> 239Np --(beta)--> 239Pu. This nuclear conversion can be brought about in breeder reactors where it is possible to produce more new fissionable material than the fissionable material used in maintaining the chain reaction.

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 235U, while occurring in 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 in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone.

Uranium-235 can be concentrated by gaseous diffusion and other physical processes, if desired, and used directly as a nuclear fuel, instead of natural uranium, or used as an explosive.

Natural uranium, slightly enriched with 235U by a small percentage, is used to fuel nuclear power reactors to generate electricity. Natural thorium can be irradiated with neutrons as follows to produce the important isotope 233U: 232Th(n, gamma)--> 233Th --(beta)--> 233Pa --(beta)--> 233U. While thorium itself is not fissionable, 233U is, and in this way may be used as a nuclear fuel. One pound of completely fissioned uranium has the fuel value of over 1500 tons of coal.

The uses of nuclear fuels to generate electrical power, to make isotopes for peaceful purposes, and to make explosives are well known. The estimated world-wide capacity of the 429 nuclear power reactors in operation in January 1990 amounted to about 311,000 megawatts.

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  235U lowered to about 0.2%.

Uranium is used in inertial guidance devices, in gyro compasses, as counterweights for aircraft control surfaces, as ballast for missile reentry vehicles, and as a shielding material. Uranium metal is used for X-ray targets for production of high-energy X-rays; the nitrate was once used as a photographic toner, and the acetate was once used in analytical chemistry.

Crystals of uranium nitrate are triboluminescent. 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.

Working with uranium requires the knowledge of the maximum allowable concentrations that may be inhaled or ingested.

Recently, the natural presence of uranium in many soils has become of concern to homeowners because of the generation of radon and its daughters.

Last Update: 2011-02-16