In alpha decay, why the unstable nucleus emits `""_(2)^(4)"He"` nucleus ? Why it does not emit four separate nucleons?

Explanation

(i) The unstable nucleus is composed of many neutrons (no charge) and protons (possitive charge).
(ii) According to electromagnetic forces, like charges repelect unstable nuclei will loose neutrons and protons when `alpha` - particle is emitted, reducing the size of the nucleus.
(iii) In this decay, the change in B.E. appears as the kinetic energy of the `alpha` - particle and the daughter nucleus.
(iv) Therefore this energy must be shared between these two particles and therefore the particle and daughter nucleus must have equal and opposite momenta, the emitted `alpha` - particle and recoiling nucleus will each have a well defined energy after the decay.
(v) Because of its smaller mass, most of the kinetic energy goes to the `alpha` - particle.