All radioactive elements which emit `beta-` rays have the same half `-` life periods.

Which is the radioactive element in group 1 element? What is its half-life period?

The half - line period a radioactive element X is same as the mean life time of another radioactive element Y . Initially both of them have the same number of atoms. Then:

The half - line period a radioactive element X is same as the mean life time of another radioactive element Y . Initially both of them have the same number of atoms. Then: