Describe the shape of the graph of binding energy per nucleon against nucleon number.

The curve rises very steeply from hydrogen through the lightest nuclei, reaches a broad maximum at a nucleon number of about 56, and then falls away slowly and steadily towards the heaviest nuclei.

What quantities are plotted on the axes of the binding energy per nucleon curve?

Binding energy per nucleon, usually in MeV per nucleon, is plotted on the y-axis against nucleon number on the x-axis.

Which nucleus lies at the peak of the binding energy per nucleon curve, and what is the approximate value there?

Iron-56 lies at the peak, with a binding energy per nucleon of about 8.8 MeV.

What does the position of the maximum on the curve tell you about nuclear stability?

Nuclei around nucleon number 56 are the most tightly bound and therefore the most stable, and every other nucleus can in principle release energy by moving towards that value.

CCEA A-Level Physics 1210

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Nuclear Energy, Fission and Fusion

The binding energy per nucleon curve

3 things to know
Unit A2 1

Overview

Recall questions on the shape of the curve and what goes on each axis, which nucleus sits at the peak and what that means for stability, why fusion of light nuclei and fission of heavy nuclei both release energy, where each process sits on the curve, and reading an energy release from two points on it.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain, using the curve, why energy is released when a very heavy nucleus splits into two lighter ones.

Key term

Explain, using the curve, why energy is released when two very light nuclei join together.

Key term

Why is the binding energy per nucleon of a very heavy nucleus lower than that of a medium-sized one?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the shape of the graph of binding energy per nucleon against nucleon number.
Model answer
The curve rises very steeply from hydrogen through the lightest nuclei, reaches a broad maximum at a nucleon number of about 56, and then falls away slowly and steadily towards the heaviest nuclei.
2
What quantities are plotted on the axes of the binding energy per nucleon curve?
Model answer
Binding energy per nucleon, usually in MeV per nucleon, is plotted on the y-axis against nucleon number on the x-axis.
3
Which nucleus lies at the peak of the binding energy per nucleon curve, and what is the approximate value there?
Model answer
Iron-56 lies at the peak, with a binding energy per nucleon of about 8.8 MeV.
4
What does the position of the maximum on the curve tell you about nuclear stability?
Model answer
Nuclei around nucleon number 56 are the most tightly bound and therefore the most stable, and every other nucleus can in principle release energy by moving towards that value.
5
Explain, using the curve, why energy is released when a very heavy nucleus splits into two lighter ones.
Model answer
The heavy nucleus sits on the falling part of the curve, well below the peak, while the fragments sit closer to the peak with a higher binding energy per nucleon. The nucleons are therefore more tightly bound after the split, so the total mass falls and the difference is released as energy.
6
Explain, using the curve, why energy is released when two very light nuclei join together.
Model answer
Light nuclei sit on the steeply rising part of the curve, so the combined nucleus has a much higher binding energy per nucleon than either of its parents. The nucleons become far more tightly bound, mass is lost and energy is released.

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