Draw a plot of binding energy per nucleon against mass number. Use it to explain the release of energy in nuclear fission and fusion. Why is Ebn nearly constant for medium nuclei?

1. 1 · Define the quantity. Binding energy per nucleon Ebn = Eb/A, a measure of how tightly a nucleus is bound.
2. 2 · Draw the curve. [Graph: Ebn (MeV) against A from 0 to 240; steep rise, peaks for ⁴He, ¹²C, ¹⁶O, maximum 8.75 MeV at A = 56, fall to 7.6 MeV at A = 238; fusion and fission marked]
3. 3 · Give the key values.
   • About 8.75 MeV at A = 56
   • About 8 MeV for 30 < A < 170
   • About 7.6 MeV at A = 238
   • Low for light nuclei
4. 4 · Explain fission. A heavy nucleus (Ebn ≈ 7.6 MeV) splits into two medium nuclei (≈ 8.5 MeV); binding energy rises by about 0.9 MeV per nucleon, about 200 MeV in all.
5. 5 · Explain fusion. Two light nuclei join into one with a much higher Ebn, so energy is released.
6. 6 · Explain the flat middle. The nuclear force is short range and saturates, so each nucleon adds about the same binding.
7. 7 · Real life. Iron, at the summit, is the end point of energy release in stars.
8. 8 · Close in one line. So, nuclei release energy by moving towards A = 56, by fission from above or fusion from below.

Scoring tip: Label both axes with units and write the three values 8.75, 8 and 7.6 MeV on the curve. Mark fission and fusion on the correct sides.