What are mass defect and binding energy? Draw the binding energy per nucleon curve and explain how it shows that energy is released in fission and fusion.

1. 1 · Define mass defect. Δm = [Zmₚ + (A − Z)mₙ] − M: the nucleus is lighter than its free nucleons.
2. 2 · Define binding energy. BE = Δm c², the energy needed to split the nucleus into free nucleons; 1 u ≈ 931 MeV.
3. 3 · Define B. Binding energy per nucleon, B = BE/A, measures stability.
4. 4 · Draw the curve. [Graph: B (MeV) against A; rises steeply from 1.1 MeV (²H), small peaks at ⁴He, ¹²C, ¹⁶O, maximum about 8.8 MeV near A = 56, falls to 7.6 MeV at A = 238]
5. 5 · Explain fission. A heavy nucleus (B ≈ 7.6 MeV) splits into middle nuclei (B ≈ 8.5 MeV); the extra binding, about 200 MeV, is released.
6. 6 · Explain fusion. Light nuclei with low B join into a heavier nucleus with higher B, releasing energy.
7. 7 · Real life. Power stations split uranium; the Sun joins hydrogen nuclei.
8. 8 · Close in one line. So, both processes move towards iron, the most stable region.

Scoring tip: Label the axes, the peak near A = 56 and the uranium end. Then write fission on the right and fusion on the left.