Radioactive nuclei spontaneously emit α , β and γ -radiations to achieve stability, α -particles are helium nuclei, β -particles are fast moving electrons and γ -radiations are electromagnetic waves of high energy. During radioactive decay the total charge and nucleon number are conserved. Rutherford and Soddy suggested that the radioactivity is a nuclear phenomenon and is unaffected by physical conditions. According to them rate of decay of radioactive nuclei at any time is directly proportional to the number of undecayed nuclei present at that moment i.e. = – λ N, where λ is a decay constant.

(i) The particle X emitted in the reaction → + X is –