The residual strong force, also known as the nuclear force, is a very short range (about 1 to 3 fm) force, which acts to hold neutrons and protons together in nuclei. In nuclei, this force acts against the enormous repulsive electromagnetic force of the protons. It acts equally only between pairs of neutrons, pairs of protons, or a neutron and a proton. The term residual is associated with the fact, it is the residuum of the fundamental strong interaction between the quarks that make up the protons and neutrons. The residual strong force acts indirectly through the virtual π and ρ mesons, which transmit the force between nucleons that holds the nucleus together.
The residual strong force acts indirectly through the virtual π and ρ mesons, which transmit the force between nucleons that holds the nucleus together.
As was written, hadrons appear nearly without color charge, and the fundamental strong force is therefore nearly absent between those hadrons except that the cancellation is not quite perfect. The fundamental strong force can leaks out of individual nucleons (as the residual strong force) to influence the adjacent particle. As for the fundamental strong force, the residual force does diminish rapidly with distance, and is thus very short-range. Therefore, neither the residual strong force cannot reach outside the nucleus. This is due to color confinement, which implies that the strong force acts only between pairs of quarks. Simply, color charged particles (such as quarks and gluons) cannot be isolated (below Hagedorn temperature) and therefore in collections of bound quarks (i.e., hadrons), the net color-charge of the quarks essentially cancels out, resulting in a limit of the action of the forces.
Notwithstanding they have identical origin, the residual strong force is much weaker than the fundamental strong force. It is very much like the electromagnetic forces between neutral atoms (van der Waals forces), which form molecules and are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms.
Atomic nuclei consist of protons and neutrons, which attract each other through the nuclear force, while protons repel each other via the electromagnetic force due to their positive charge. These two forces compete, leading to various stability of nuclei. There are only certain combinations of neutrons and protons, which forms stable nuclei. Neutrons stabilize the nucleus, because they attract each other and protons , which helps offset the electrical repulsion between protons. As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus. If there are too many (neutrons also obey the Pauli exclusion principle) or too few neutrons for a given number of protons, the resulting nucleus is not stable and it undergoes radioactive decay. Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, or electron capture. Many other rare types of decay, such as spontaneous fission or neutron emission are known.
See also: Liquid Drop Model