If a graph of neutron number against proton number is plotted for all known isotopes of the elements it shows a line curving upwards and deviating from the line N=Z.
- For light isotopes N=Z
- Above Z=20 more neutrons are needed to bind the nucleus together.
- Above Z=60 many nuclei are unstable and emit α particles, because the short-range strong nuclear force cannot overcome the electrostatic repulsion between the large number of protons.
- Neutron rich isotopes left of the stability belt are β- emitters.
- Proton rich isotopes right of the stability belt are β+ emitters.