E.3.15 (HL)—Neutron-proton ratio

Light stable nuclei

For small proton numbers, stable nuclei tend to have similar numbers of neutrons and protons, so $N\approx Z$.

Heavy stable nuclei

As $Z$ increases, proton–proton electromagnetic repulsion grows. Stable heavy nuclei therefore need extra neutrons to add strong-force binding without adding proton repulsion, so $N>Z$.

Read the stability band

The line of stable nuclides bends above $N=Z$ at larger $Z$. Nuclei on either side can decay toward the band, often through beta decay.

Common trap

Do not say every stable nucleus has more neutrons than protons. The approximation $N\approx Z$ is useful for light nuclei.