THE STABILITY OF ISOTOPES
3 marks · NormalExplain why adding too many neutrons or too few neutrons to a nucleus can make it unstable.
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Neutrons help to hold the protons together in the nucleus by the strong nuclear force, as protons repel each other electrically. If there are too few neutrons, the repulsion between protons is too strong and the nucleus can fall apart. If there are too many neutrons, the nucleus becomes unstable because the extra neutrons do not increase the strong force enough, and can cause the nucleus to decay.
Also accept: Reference to nuclear force balancing electrical repulsion; Extra neutrons make the nucleus unstable and lead to decay
Common mistakes: Saying protons or neutrons alone make a nucleus stable
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Marking points
M1 · 1 mark · States neutrons help hold protons together against repulsion.
M2 · 1 mark · Explains what happens if there are too few neutrons.
M3 · 1 mark · Explains what happens if there are too many neutrons.
Worked solution
The strong nuclear force between nucleons (protons and neutrons) holds the nucleus together, while electrical repulsion between protons tries to break it apart. If there are too few neutrons, the strong force cannot overcome the proton-proton repulsion, leading to instability. Too many neutrons can also make the nucleus unstable, as the excess neutrons contribute less to the strong force and can disrupt the balance, causing the nucleus to decay.