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

Explanation

After all `""_(2)^(4)He` consists of two protons and two neutrons. For example, if `""_(92)^(228)U` nucleus decays into `""_(90)^(234)Th` by emitting four separate nucleons (two protons and two neutrons), then the disintegration energy Q for this process turns out to be negative. It implies that the total mass of products is greater than that of parent `(""_(92)^(238) U)` nucleus. This kind of process cannot occur in nature because it would violate conservation of energy. In any decay process, the conservation of energy, conservation of linear momentum and conservation of angular momentum must be obeyed.