THE STABILITY OF ISOTOPES

5 marks · Hard

Explain, in detail, why nuclei with either far too few or far too many neutrons compared to protons tend to be unstable. In your answer, refer to the balance of forces in the nucleus and the typical changes that such nuclei might undergo to become more stable.

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Must show: Too many neutrons: weakens nuclear force's effectiveness or introduces instability, leading to beta minus decay, Too few neutrons: electrostatic repulsion overcomes nuclear force, leading to instability and beta plus decay or alpha decay, Stable nuclei require optimal neutron-proton ratio for effective binding, Unstable nuclei decay to adjust their neutron/proton ratio

Common mistakes: confusing neutron number with mass number; saying all heavy nuclei are always unstable without explanation

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Marking points

B1 · 1 mark · Explains that neutrons help offset the repulsive forces between protons

B2 · 1 mark · Describes how too many neutrons can make the nucleus unstable (leading to beta minus decay)

B3 · 1 mark · Describes how too few neutrons can make the nucleus unstable (leading to beta plus or alpha decay)

B4 · 1 mark · States or explains that stable nuclei need a balanced neutron/proton ratio

B5 · 1 mark · Mentions that unstable nuclei will undergo radioactive decay to achieve a more stable ratio

Worked solution

Neutrons are essential for the stability of a nucleus because they provide an attractive force (the strong nuclear force) that helps to hold the positively charged protons together, countering the repulsive electrostatic forces between protons. If a nucleus has too many neutrons, the strong force is less effective at holding the nucleons together and the excess neutrons introduce instability; such nuclei tend to undergo beta minus decay, where a neutron is converted to a proton and a beta particle is emitted, reducing the neutron-proton ratio. If there are too few neutrons, the strong force is insufficient to balance the electrostatic repulsion between protons, leading to instability; nuclei in this condition may undergo beta plus decay or alpha decay to increase their neutron-proton ratio. Stable isotopes have a ratio of neutrons to protons that ensures forces are balanced. Unstable nuclei will spontaneously decay to become more stable.