Graph of average binding energy per nucleon against nucleon number.

The idea

The graph plots average binding energy per nucleon, in MeV, on the vertical axis against nucleon number A on the horizontal axis. Average binding energy per nucleon equals the total binding energy of the nucleus divided by A, so it measures how tightly each nucleon is held. The curve rises steeply from hydrogen and helium, peaks near iron-56 at roughly 8.8 MeV per nucleon, then falls slowly towards uranium. A higher value means a more stable nucleus, because more energy must be supplied per nucleon to separate the nucleus into free nucleons. The mass defect relates to binding energy through E = mc², so the graph is a stability map of the nuclides.

What earns the marks

• Average binding energy per nucleon is total binding energy divided by nucleon number A, with units of MeV per nucleon.
• The curve rises sharply for light nuclei, reaches a broad maximum around A = 56 near iron, then decreases gradually for heavy nuclei.
• A larger value of average binding energy per nucleon indicates a more stable nucleus because more energy is needed per nucleon to separate it.
• Binding energy arises from the mass defect, calculated using E = mc², where m is the difference between the mass of separate nucleons and the nuclear mass.
• The graph is a smooth curve through nuclide data, not a straight line, and the peak region is broad rather than a single sharp point.

Common mistakes and how to put them right

• Treating the vertical axis as total binding energy rather than binding energy per nucleon.

  Put it right by dividing total binding energy by A before plotting or reading values.

• Assuming the most stable nucleus has the largest nucleon number.

  Put it right by identifying the peak near iron-56 as the region of greatest stability.

• Confusing binding energy with the energy released in a reaction.

  Put it right by using differences in binding energy per nucleon between initial and final nuclei to find energy release.

Exam tips

• Label both axes with quantity and unit, for example average binding energy per nucleon / MeV and nucleon number A.
• Quote the peak position approximately, for example around A = 56, and its value near 8.8 MeV per nucleon, rather than claiming an exact universal figure.
• When explaining stability, refer to energy required per nucleon to separate the nucleus, not to the total energy alone.