(i) Define 'activity' of a radioactive material and write its S.I. unit.
(ii) Plot a graph showing variation of activity of a given radioactive sample with time.
(iii) The sequence of stepwise decay of a radioactive nucleus is
`D overset(alpha) (to) D_(1) overset(beta-) (to) D_(2)`
If the atomic number and mass number of `D_(2)` are 71 and 176 respectively, what are their corresponding values for D ?

Explanation

(i) The total decay rate (of a sample) at the given instant i.e. the number of radio-nuclides disintegrating per unit time is called the activity of that sample.
The SI unit for activity is becquerel (Bq).
(ii) Graph:

(iii) `72 and 180`.