Uranium displays three crystallographic modifications as follows: alpha -- (688ºC) --> beta -- (776ºC) --> gamma. Uranium is a heavy, silvery-white metal which is pyrophoric when thinly divided.
It is a slight softer than steel, and is attacked by cold water in a thinly divided state. Uranium is malleable, ductile, and slightly paramagnetic.
In air, the metal becomes coated with a layer of oxide and acids dissolve the metal, but it is unaffected by alkalis.
Uranium has sixteen isotopes, which all are radioactive. Naturally taking place uranium nominally has 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 0.1%, depending upon the source. The US DOE has adopted the value of 0.711 since their official percentage of 235U in natural uranium. Natural uranium is adequately radioactive to expose a photographic plate in an hour.
More 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 about 4.51 x 109 years has been utilized to estimate the age of igneous rocks. The source 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 assumed that uranium is a decay product of elements with higher atomic weight, which in turn have been present on earth or elsewhere in the universe. These original elements may be formed as a result of a primordial creation, known as the big bang, in a supernova or in some other stellar processes.
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