Question

Explain why:
[(i)] Lighter nuclei fuse together to form a heavy nucleus.
[(ii)] A heavy nucleus splits into two lighter nuclei.
[(iii)] Energy is released in both phenomenon of nuclear fission and nuclear fusion.

Hint:

Remember: Binding energy per nucleon curve peaks at iron. Fusion releases energy for light nuclei; fission releases energy for heavy nuclei.

Answer 1

Step 1: Binding energy per nucleon curve.
- The binding energy per nucleon ($B.E./A$) is a measure of nuclear stability.
- It increases sharply for light nuclei, reaches a maximum around iron ($^{56}Fe$), and then decreases gradually for heavy nuclei.

Step 2: Fusion of light nuclei.
- Light nuclei (H, He, Li) lie on the left side of the curve.
- When two light nuclei fuse, the product nucleus has a higher $B.E./A$.
- Example: $^2H + ^2H \to ^4He + \text{energy}$
- The increase in binding energy per nucleon means the product is more stable. The difference in energy is released as kinetic energy and radiation.
Step 3: Fission of heavy nuclei.
- Very heavy nuclei (U, Th) lie on the right side of the curve where $B.E./A$ decreases with $A$.
- Splitting them into two medium nuclei (Ba, Kr) increases the $B.E./A$.
- Example: $^{235}U + n \to ^{144}Ba + ^{89}Kr + 3n + \text{energy}$
- Since products are more stable, the mass defect appears as released energy.
Step 4: Why energy is released in both.
- In both fusion and fission, the system moves toward greater stability (higher binding energy per nucleon).
- Energy released is given by Einstein’s relation: \[ \Delta E = \Delta m \, c^2 \] - In fusion, $\Delta m$ is positive because a heavy nucleus formed is more stable.
- In fission, $\Delta m$ is positive because medium nuclei are more stable than very heavy ones.
Step 5: Conclusion.
(i) Light nuclei fuse to reach higher $B.E./A$ and become stable.
(ii) Heavy nuclei split for the same reason.
(iii) Both processes release energy because the final nuclei lie closer to the peak (iron region) of the binding energy curve.