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.
Uranium-238
The mean lifetime of U is 1.41 seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2 seconds, so 1 mole of U emits 3 alpha particles per second, producing the same number of thorium-234 (Th-234) atoms. In a closed system an equilibrium would be reached, with all amounts except for lead-206 and U in fixed ratios, in slowly decreasing amounts. The amount of Pb-206 will increase accordingly while that of U decreases; all steps in the decay chain have this same rate of 3 decayed particles per second per mole U.