Assertion (A) and Reason (R) type question based on the Binding Energy per Nucleon curve.
Step 1 (Governing Principle): [0.5 Mark]The stability of a nucleus is determined by its Binding Energy per Nucleon ($BE/A$). According to the $BE/A$ vs. Mass Number ($A$) curve:
- For light nuclei ($A < 30$), $BE/A$ increases with increasing $A$.
- For heavy nuclei ($A > 170$), $BE/A$ decreases with increasing $A$.
- Energy is released in a nuclear reaction if the total binding energy of the products is greater than that of the reactants (i.e., the system moves toward a more stable state).
Assertion (A): True. In fission, a heavy nucleus splits into lighter nuclei with higher $BE/A$. In fusion, light nuclei combine to form a heavier nucleus with higher $BE/A$. In both cases, the final state is more stable, releasing energy.
Reason (R): False. The statement claims $BE/A$ increases with $Z$ for heavy nuclei, which is incorrect. In reality, for heavy nuclei, $BE/A$ decreases as $A$ (and $Z$) increases due to the increased Coulombic repulsion between protons.
Step 3 (Final Concluded Answer): [0.25 Mark]Conclusion: Assertion (A) is True, but Reason (R) is False.
Think of binding energy like a "stability score"—the higher the score, the happier the nucleus. Heavy nuclei are "unhappy" and want to split to get a better score, while tiny nuclei are "unhappy" and want to join together to get a better score; both processes release energy because they are moving toward a more stable, "happier" state.