Unstable nuclei attain stability through disintegration. The nuclear stability is related to neutron proton ratio (n/p). For stable nuclei n/p ratio lies close to unity for elements with low atomic numbers (20 or less) but it is more than I for nuclei having higher atomic numbers. Nuclei having n/p ratio either very high or low undergo nuclear transformation. When n/p ratio is higher than required for stability, the nuclei have the tendency to emit B-rays. While when n/p ratio is lower than required for stability, the nuclei either emits $\alpha$-particles or a positron or capture K-electron.