N Against Z Graph

Here is a graph of the number of neutrons against the number of protons in a nucleus. It shows stable and unstable nuclei.

Stable nuclei/isotopes are found on the black line/dots.

The shaded areas above and below the line of stability represent radioactive isotopes.

Why doesn’t it follow N=Z?

Protons repel each other with the electromagnetic force but the strong nuclear force is stronger at small distances and keeps them together in the nucleus. We can see the line of stability follows N=Z at low values.

As the nucleus gets bigger there are more protons, when they become a certain distance apart they no longer experience the strong nuclear force that keeps them together, only the electromagnetic which pushes them apart. To keep the nucleus together we need more neutrons which feel no electromagnetic repulsion only the attraction of the strong nuclear force.

Points to remember

Follows N=Z around Z=20, then curves to go through Z=80 N=120

β^- emitters above the line, β⁺ emitters below the line and α at the top