Mass becomes binding energy.
A bound nucleus weighs less than the same number of isolated protons and neutrons. That missing mass is not lost; it is the binding energy of the nuclear state.
The binding-energy curve explains two routes toward tighter binding.
Light nuclei can release energy by fusing toward more tightly bound nuclei. Very heavy nuclei can release energy by splitting toward intermediate masses. The peak sits in the iron–nickel region, not at the heaviest elements.
Nuclear stability lives in a band, not on N = Z forever.
For light nuclei, stable proton and neutron counts are often similar. As Z grows, Coulomb repulsion favors progressively more neutrons. Shell effects and detailed nuclear structure then decide which individual nuclides are actually stable.
Fusion can occur below the classical Coulomb barrier.
Two positively charged nuclei repel electrically. Quantum mechanics replaces a hard classical prohibition with a small but nonzero tunnelling amplitude. Nuclear reaction physics then determines what happens after the nuclei get close enough.