Nuclear fusion
4 marks · HardA student claims that nuclear fission and nuclear fusion are both potential sources of energy.
Explain the difference between fission and fusion in terms of the nuclei involved and the products formed. Then, describe why fusion produces energy only when light nuclei are used.
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Fission involves splitting a heavy nucleus (such as uranium-235) into two or more smaller nuclei and some neutrons, releasing energy. Fusion involves combining two light nuclei to form a heavier nucleus, releasing energy. Fusion only produces energy with light nuclei because, for elements lighter than iron, the binding energy per nucleon increases when they join, releasing energy. Fusion of heavy nuclei does not release energy.
Must show: fission: heavy nucleus splits into smaller nuclei + neutrons, fusion: light nuclei join to form heavier nucleus, energy release due to change in binding energy per nucleon, fusion energy release only for light nuclei (lighter than iron)
Common mistakes: Failing to explain why only light nuclei release energy when fused; Mixing up processes or omitting products
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Marking points
B1 · 1 mark · Describes fission as splitting a heavy nucleus into smaller nuclei plus neutrons.
B2 · 1 mark · Describes fusion as joining light nuclei to form a heavier nucleus.
R1 · 1 mark · States that energy is released because binding energy per nucleon increases
R2 · 1 mark · Explains that fusion of very heavy nuclei does not release energy because binding energy per nucleon does not increase (only light nuclei release energy through fusion).
Worked solution
Fission occurs when a heavy nucleus such as uranium-235 breaks into two or more smaller nuclei and some neutrons, releasing energy. Fusion is the process where two light nuclei, like hydrogen isotopes, combine to form a heavier nucleus, also releasing energy. Fusion produces energy only for light nuclei because joining them increases the binding energy per nucleon—the nucleus formed is more stable—so energy is released. For heavy nuclei, fusion does not release energy because it would decrease the binding energy per nucleon.