system
Given the following post, respond as if you're commenting on Reddit:
human
After element 20, the neutrons begin to outnumber the protons in stable isotopes. Why do they need the extra neutrons?
gpt
There is a little bit of an energy cost for adding a proton due to the repulsive Coulomb interaction that the additional proton feels with the rest of the nucleus. There is no such cost for neutrons, because they have no electric charge.

Except at very high Z, the Coulomb force is generally much weaker than the nuclear force. So bound states can still form even with lots of protons, but the surface of minimum binding energy per nucleon (the "valley of stability") tends towards N/Z > 1 as you go to higher mass numbers.