Why does fusion of light nuclei release energy per nucleon more efficiently than fission of heavy nuclei?

• A. Binding energy per nucleon increases toward iron, so light-nucleus fusion moves toward higher binding energy per nucleon
• B. Binding energy per nucleon decreases toward iron during fusion
• C. Fission of heavy nuclei increases binding energy per nucleon, releasing energy
• D. Fusion of light nuclei requires more mass to be converted into energy than fission of heavy nuclei

Answer: (A)

When a nucleus forms, energy is released if the resulting nucleus is more tightly bound per nucleon than the initial pieces. This binding-energy-per-nucleon measure peaks around iron, meaning nuclei on the light side have the most to gain by moving toward that peak.

Fusion of light nuclei builds up from very low binding energy per nucleon toward higher values, so the fused nucleus ends up more tightly bound per nucleon. That gain in binding energy shows up as released energy, and because the increase from light elements toward iron is substantial, the energy released per nucleon is large.

By contrast, heavy nuclei sit near the peak of the curve. Splitting them (fission) can also move toward higher binding energy per nucleon, but the improvement per nucleon is smaller because the curve is flatter near the top. This is why fusion of light nuclei releases energy more efficiently on a per-nucleon basis than fission of heavy nuclei.