AQA A-Level Physics/Nuclear Instability

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Nuclear stability is linked to its number of nucleons (neutrons and protons). The nucleons interact with each other through the strong nuclear force, this must overcome the electrostatic repulsion between the protons to make the nucleus stable. When the electrostatic repulsion exceeds the strong force the nucleus is now unstable and will then change its nucleons causing radioactive decay. When the graph of number of neutrons (N) against protons (Z) is drawn, and all the stable istopes are plotted on it, there is quite a clear correlation between N and Z.

Initially stable isotopes are found on the line N=Z but soon the number of neutrons needed to keep the nucleus stable becomes larger than the number of protons and the stable isotopes curve away from N=Z. This is because as the nuclear radius increases a greater strong force is required to keep it stable but adding more protons will only increase electrostatic repulsion and so the number of neutrons becomes more than that of protons.

Radioactive decay occurs because a nuclei is unstable. It emits alpha particles, beta particles or gamma rays to become more stable.


Z is the proton number (sometimes called atomic number) it is the number or protons in the nucleus.

A is the nucleon number (some times called mass number) it is the total number of protons AND neutrons in the nucleus.

N is the number of neutrons = A – Z

For light isotopes (up to Z=20) the stable nuclei have equal numbers of protons and neutrons for heavier nuclei to be stable they need to have more neutrons than protons.

  • Nuclei with too many protons become more stable by emitting a beta+ (b+) particle when a proton converts into a neutron
  • Nuclei with too few protons are to the left of the stability line and become more stable by emitting a beta- (b-)particle when a neutron converts into a proton