The mean life of a radioactive element is equal to the ratio of the sum of life-time of the entire active atom to the total numbers of active atom. It can be expressed as, τ=‌
1
λ
Here, τ is the mean life and λ is the decay constant. The half-life of radioactive nuclei is given by, T1∕2‌‌=‌
0.693
λ
‌‌=0.693τ Here, 0.693<1, so the average life time is more than the half life of atom.