Uranium
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth. Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of U atom, and also 49% by U (since the latter is formed from the former) and about 2.0% of them by the U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of U was probably much lower than this.
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.