Meson theory of nuclear forces
Neutron is found to play a leading role in binding the nuclear particles. It has been established that neutron-proton attractions are stronger than the proton-proton or neutron-neutron attractions. This is evident by the fact that the deuteron, 1H2 having one proton and one neutron, is quite stable while no particle having either two neutrons or two protons is known.
Yukawa in 1935, put forward a postulate that neutrons and protons are held together by very rapid exchange of nuclear particles called pi mesons (cf the formation of a covalent bond by sharing of electrons). The nuclear forces called into play by this rapid exchange of pi mesons between nucleons are also called exchange forces.
The binding forces between unlike nucleons (p and n) are explained by the oscillation of a charged pi meson (π+ ог π-).
(a) \(p_1 + n_2 \rightleftharpoons n_1 + \pi ^+ + n_2 \rightleftharpoons n_1 + p_2\)
(b) \(p_1 + n_2 \rightleftharpoons p_1 + \pi ^- + p_2 \rightleftharpoons n_1 + p_2\)
Binding forces between like particles (p-p or n-n) result from the exchange of neutral mesons (π) as represented below
(c) \(p_1 \rightleftharpoons p_2 + \pi ^0\) or \(p_1 + \pi^0 \rightleftharpoons p_2\)
(d) \(n_1 \rightleftharpoons n_2 + \pi ^0\) or \(n_1 + \pi^0 \rightleftharpoons n_2\)
