residual strong (nuclear) force

An atomic nucleus is a tight clump of protons and neutrons. But protons all carry positive electric charge, and like charges repel fiercely at such close quarters — so what stops the nucleus from blowing itself apart? The answer is the residual strong force: a powerful but short-range attraction between protons and neutrons that overwhelms their electric repulsion and binds them together.

Here is the subtle part: this is not quite the same as the fundamental strong force. The true strong force, described by QCD, acts directly between quarks via gluons, deep inside each proton and neutron. Protons and neutrons are colorless overall, so they feel no net strong force from a distance. But up very close, a little of that internal strong force leaks out — much as two electrically neutral atoms can still attract through faint residual electric forces and form molecules. This leftover, leaked-out part is the residual strong force, and it is mostly transmitted not by gluons directly but by pions, the lightest particles made of quarks.

This residual force is what nuclear physics is built on. It explains why nuclei hold together, why there is a most-stable size of nucleus (around iron), and why both fusion (joining light nuclei) and fission (splitting heavy ones) release energy. It is enormously strong over its tiny range — a few times 10^-15 meters — but falls off so fast beyond that range that distant nuclei barely feel it at all, which is why nuclei do not simply clump into ever-larger lumps.