We know that nucleons in nucleus are bound due to the presence of short-range nuclear force, also known as strong force which is attractive in nature. Nucleons mean number of protons and neutrons. But it’s not meant that nucleus has only one force, there is also another kind of force which exists within the nucleus which is known as repulsive force. Its name comes from proton-proton repulsion. The range of the repulsive force is unlimited, where as the strong force has short range.
The nuclei which contain 210 numbers of nucleons or more are not stable in nature. The reason of it is that in such nuclei the strong force is hardly able to counterbalance the repulsive force because there exists large number of protons that have repulsive force between them and the range of repulsive force is unlimited. As a result Alpha decay occurs. In order to attain more stability such nuclei try to reduce their size.
But here is a question why Alpha decay occurs instead of individual protons or helium nuclei? The answer of it is that Alpha particles have high binding energy. In order to escape from nucleus, a particle must have kinetic energy, and only the mass of Alpha particle is sufficiently smaller than that of its constituent nucleons for such energy to be available.
By computing from the known masses of each particle and the parent and daughter nuclei, the energy is released known as disintegration energy, when various particles are emitted from large nuclei, and that energy is equal to the product of mass difference and square of speed of light. Here the mass difference means the difference in the masses of initial and final nuclei and the mass of particle.
When an Alpha particle emerges, the nucleus recoils with a small amount of kinetic energy, so we can say that the kinetic energy of emitted particle is not exactly equal to the disintegration energy. It can be shown from momentum and energy conservation that the kinetic energy of Alpha particle is related to disintegration energy and mass number, which is known as A, of original nucleus.
There are some cases in which the emission of Alpha particle is energetically possible, but it is not true for all type of nuclei. If Alpha decay occurs in Uranium that has 232 mass number and 92 proton number, energy will be released. If we want to emit a proton or helium nucleus from such nucleus, we must provide energy.