When a radioactive nucleus emits 1α -particle, the mass number decreases by 4 units and atomic number decreases by 2 units. When a radioactive nucleus emits 1 positron the atomic number decreases by 1 unit but mass number remains constant. ∴ Mass number of final nucleus =A−12 Atomic number of final nucleus =Z−8 ∴ Number of neutrons, Nn=(A−12)−(Z−8)=A−Z−4 Number of protons, Np=Z−8 ∴ Required ratio =