The strong nuclear force binds protons and neutrons into nuclei and is one of the four fundamental forces. It is attractive at intermediate range (~1–2 fm) but repulsive at very short range (hard core at ~0.5 fm). The force is charge-independent (nearly identical for pp, nn, and pn pairs) and spin-dependent. The strong force is much stronger than the electromagnetic repulsion between protons, allowing nuclei to form.
Compare nuclear binding energies for light nuclei and infer the range and strength of the force. Understand why the Coulomb repulsion eventually dominates for heavy nuclei, limiting nuclear size.
The strong force is not experienced equally by all nucleons at all separations (it drops sharply beyond ~2 fm). Neutrons are not held in place by the Coulomb force; they are bound by the strong force alone.
From your study of nuclear structure you know that nuclei consist of protons and neutrons (nucleons) packed into a volume of radius ~1–5 femtometers (1 fm = 10⁻¹⁵ m). The puzzle is immediate: protons carry positive charge and repel each other via the Coulomb force. Two protons separated by 1 fm experience an electrostatic repulsion of about 230 N — an enormous force on nuclear scales. For a nucleus to be stable, something must overcome this repulsion. That something is the strong nuclear force, also called the strong force or hadronic force.
The strong force has several properties that distinguish it sharply from gravity and electromagnetism. First, it is short-ranged: it drops to essentially zero beyond about 2 fm, falling off much faster than the 1/r² Coulomb force. Two nucleons at 5 fm apart barely feel each other; at 1 fm they are strongly bound. This short range explains why nuclear properties (like binding energy per nucleon) saturate — each nucleon only interacts with its immediate neighbors, not with the entire nucleus. Second, it is charge-independent: the force between two protons (pp), two neutrons (nn), and a proton-neutron pair (pn) is nearly identical when in the same spin state. This is called isospin symmetry and it means neutrons and protons behave almost interchangeably from the perspective of the strong force. Third, it is spin-dependent: a proton-neutron pair with spins aligned (triplet state, spin-1) is bound (the deuteron), while the same pair with spins anti-aligned (singlet state) is not.
At very short separations (below ~0.5 fm), the strong force becomes repulsive — a "hard core" that prevents nucleons from collapsing into each other. The potential well is attractive at 1–2 fm and repulsive inside 0.5 fm, giving something like a Lennard-Jones potential in form (though not in origin). This structure means each nucleon sits in a potential energy minimum, like a ball in a bowl, and nuclear matter has a characteristic equilibrium density of about 0.17 nucleons per fm³.
The competition between the strong force and Coulomb repulsion determines nuclear stability. For light nuclei, the strong force easily wins and all nucleons contribute to binding. As nuclei grow larger, the strong force saturates (each nucleon only feels its neighbors) but the Coulomb repulsion accumulates with every proton added (every proton repels every other proton across the entire nucleus). This is why neutron-to-proton ratio increases for heavy nuclei — extra neutrons provide additional strong-force binding without adding Coulomb repulsion. Beyond a certain size (around Z = 83, bismuth), no stable configuration exists: the Coulomb repulsion eventually wins, and all heavier elements undergo radioactive decay. This competition, encoded in the semi-empirical mass formula, quantitatively explains the entire landscape of nuclear stability.