Natural uranium consists primarily of isotope 238U (99.28%), therefore the atomic mass of uranium element is close to the atomic mass of 238U isotope (238.03u). Natural uranium also consists of two other isotopes: 235U (0.71%) and 234U (0.0054%). The abundance of isotopes in the nature is caused by difference in the half-lifes. All three naturally-occurring isotopes of uranium (238U, 235U and 234U) are unstable.
Uranium in the Environment
All three naturally-occurring isotopes of uranium (238U, 235U and 234U) have very long half-life (e.g. 4.47×109 years for 238U). Because of this very long half-life uranium is weakly radioactive and contributes to low levels of natural background radiation in the environment. These isotopes are alpha radioactive (emitting alpha particle), but they can also rarely undergo a spontaneous fission. All naturally-occurring isotopes belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.54×109 years). Uranium has the second highest atomic mass of these primordial nuclides, lighter only than plutonium. Moreover the decay heat of uranium and its decay products (e.g. radon, radium etc.) contributes to heating of Earth’s core. Together with thorium and potassium-40 in the Earth’s mantle is thought that these elements are the main source of heat that keeps the Earth’s core liquid.