Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Natural abundance
As an example, uranium has three naturally occurring isotopes: U, U and U. Their respective natural mole-fraction abundances are 99.2739–99.2752%, 0.7198–0.7202%, and 0.0050–0.0059%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,274 U atoms, approximately 720 U atoms, and very few (most likely 5 or 6) U atoms. This is because U is much more stable than U or U, as the half-life of each isotope reveals: 4.468 × 10 years for U compared with 7.038 × 10 years for U and 245,500 years for U.