This means that it is barely radioactive, less so than many other isotopes in rocks and sand. U-238 decays very slowly, its half-life being about the same as the age of the Earth (4500 million years). Meanwhile, like all radioactive isotopes, they decay. It is therefore said to be 'fissile' and we use the expression 'nuclear fission'. The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy.
Natural uranium as found in the Earth's crust is a mixture largely of two isotopes: uranium-238 (U-238), accounting for 99.3% and uranium-235 (U-235) about 0.7%. These isotopes differ from each other in the number of uncharged particles (neutrons) in the nucleus. Like other elements, uranium occurs in several slightly differing forms known as 'isotopes'. On a scale arranged according to the increasing mass of their nuclei, uranium is one of the heaviest of all the naturally-occurring elements (hydrogen is the lightest). The high density of uranium means that it also finds uses in the keels of yachts and as counterweights for aircraft control surfaces, as well as for radiation shielding.While it is not common in the solar system, today its slow radioactive decay provides the main source of heat inside the Earth, causing convection and continental drift. Uranium was apparently formed in supernovae about 6.6 billion years ago.It was named after the planet Uranus, which had been discovered eight years earlier. Uranium was discovered in 1789 by Martin Klaproth, a German chemist, in the mineral called pitchblende.Uranium occurs in seawater, and can be recovered from the oceans. Uranium occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth's crust as tin, tungsten and molybdenum.Uranium is a heavy metal which has been used as an abundant source of concentrated energy for over 60 years.
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