A nucleus of mass $M+\Delta m$ is at rest and decays into two daughter nuclei of equal mass $M/2$ each. Speed of light is $c$. If the binding energy per nucleon for the parent nucleus is $E_1$ and that for the daughter nuclei is $E_2$, then:

For a nucleus to decay spontaneously, the products must be more stable than the parent nucleus. Stability is characterized by a higher binding energy per nucleon. Thus, the binding energy per nucleon of the daughter nuclei ($E_2$) must be greater than that of the parent nucleus ($E_1$).