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