Nuclear fission and fusion

What just happened?

The curve shows binding energy per nucleon — the higher, the more tightly bound the nucleus. The peak is at iron (A≈56). Energy is always released when the product nuclei lie closer to the peak than the reactants.

Two ways, one principle:

Fusion — merging light nuclei (A < 56): the product moves right, toward the peak. Fission — splitting heavy nuclei (A > 56): the fragments move left, also toward the peak.

The difference in binding energy shows up as a mass defect $\Delta m$ — this "missing" mass turns into energy. The number of nucleons is always conserved; only how tightly they are bound changes.

What powers stars, and what powers reactors?

In the Sun, fusion takes place: hydrogen merges into helium, releasing enormous energy per nucleon. In nuclear power stations we use fission of heavy uranium — it is easier to initiate and control than fusion, which requires millions of degrees.

Why this matters for the exam:

Look at the curve and ask: is the product closer to iron? If so — energy is released. Light → fusion, heavy → fission. You compute the gain from the difference in binding energy and $E=\Delta m\,c^2$.

Remember: both processes push nuclei toward the peak of the curve (Fe, A≈56). Light ones merge, heavy ones split — and the released energy is the mass defect times $c^2$.