High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament. 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. 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. The origin of uranium, the highest member of the naturally occurring elements - except perhaps for traces of neptunium or plutonium, is not clearly understood.