The binding energy per nucleon graph shows iron-56 as the most stable nucleus

Nuclear Energy & Binding Energy — AQA A-Level Physics

Key Definition

Binding energy per nucleon — The total binding energy of a nucleus divided by the number of nucleons. A higher value means a more stable nucleus.

Iron-56 ($A = 56) has$ the highest binding energyThe capacity to do work. Measured in joules (J). per nucleonThe binding energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability. (~8.8 MeV) and is the most stable nucleus.
Helium-4, carbon-12 and oxygen-16 are anomalously stable — they have higher binding energies per nucleon than their neighbours.

Key features of the graph

At low A: binding energy per nucleonThe binding energyThe energy required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability. rises steeply. Light nuclei are relatively weakly bound.
At high A: binding energy per nucleonThe binding energyThe energy required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability. decreases gradually. The heaviest nuclei are less stable.
The steep rise at low A means fusion of light nuclei releases a large amount of energy per nucleon.
The gradual decrease at high A means fission of heavy nuclei also releases energy, but less per nucleon than fusion.

Examiner Tips and Tricks

When drawing the binding energy per nucleon graph: start at A = 1 (hydrogen), not $A = 0$.
Mark the He-4 anomaly with a cross.
Label the peak at Fe-56 with coordinates (~56, ~8.8 MeV).
Show fusion region (left) and fission region (right).