The residual strong force acts indirectly through the virtual π and ρ mesons, which transmit the force between nucleons that holds the nucleus together.
In physics, the atomic nucleus is the central part of an atom. In comparison to an atom, it is much more smaller and contains most of the mass of the atom. Atomic nuclei are not elementary particles, they are themselves composed of more fundamental particles. The particles that comprise atomic nuclei are termed nucleons. A nucleon is either a proton or a neutron. Neutrons and protons, i.e. nucleons, are bound together in the atomic nucleus, where they account for 99.9 percent of the atom’s mass. Research in high-energy particle physics in the 20th century revealed that neither the neutron nor the proton is not the smallest building block of matter.
See also: What is Proton
See also: What is Neutron
Protons and neutrons have also their structure. Inside the protons and neutrons, we find true elementary particles called quarks. The proton and neutron are both baryons and both fermions. The proton carries a positive net charge and the neutron carries a zero net charge. Thus, they can be viewed as two states of the same nucleon. In order to distinguish between these nucleons, modern understanding defines isospin as a quantum number related to the strong force. The observations have showed that the strong interaction does not distinguish between these nucleon. The strength of the strong interaction between any pair of nucleons is the same, independent of whether they are interacting as neutrons or as protons. Instead of regarding protons and neutrons as totally different species, as far as strong interactions are concerned, they are regarded as being different isospin states of the same underlying nucleon particle.
The proton has isotopic spin ½ as the neutron has isotopic spin ½, but in case of proton the spin is pointing upwards and the spin of neutron is pointing downwards.
Within the nucleus, protons and neutrons are bound together through the residual strong force, a fundamental interaction that governs the behaviour of the quarks that make up the individual protons and neutrons. The residual strong force, also known as the nuclear force, acts to hold neutrons and protons together in nuclei. In nuclei, this force acts against the enormous repulsive electromagnetic force of the protons. 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 strong interaction is very complicated interaction, because it significantly varies with distance. At distances comparable to the diameter of a proton, the strong force is approximately 100 times as strong as electromagnetic force. At smaller distances, however, the strong force between quarks becomes weaker, and the quarks begin to behave like independent particles. In particle physics, this effect is known as asymptotic freedom.