system
Given the following post, respond as if you're commenting on Reddit:
human
Can an atom be made of only from protons and electrons? Why are neutrons needed?
gpt
The only such atom is the hydrogen-1 atom. There does not exist any other bound nucleus consisting only of protons. Every other bound nucleus has at least one neutron.

The reason is that the nuclear force doesn't allow all-proton or all-neutron systems to be bound (and protons have it even worse because of Coulomb repulsion). For low masses, the most stable configurations of nucleons tend to live near the region of (proton number) = (neutron number).

At higher masses, Coulomb repulsion becomes important and stability shifts toward larger N/Z.