When unstable nuclei decay by emitting an αparticle (\(^4_2He\) nucleus), it loses two protons and two neutrons. As a result, its atomic number Z decreases by 2, the mass number decreases by 4. We write the alpha decay process symbolically in the following way
Here X is called the parent nucleus and Y is called the daughter nucleus.
Example:
Decay of Uranium \(^{238}_{92} U\) to thorium \(^{238}_{92} Th\) with the emission of \(^4_2 He\) nucleus (α-particle)
As already mentioned, the total mass of the daughter nucleus and \(^4_2He\) nucleus is always less than that of the parent nucleus. The difference in mass Q = (∆mx – my – m\(a\)) is released as energy called disintegration energy Q and is given by Q = (∆mx – my – m\(a\)) C2
Note that for spontaneous decay (natural radioactivity) Q > 0. In alpha decay process, the disintegration energy is certainly positive (Q > 0). In fact, the disintegration energy Q is also the net kinetic energy gained in the decay process or if the parent nucleus is at rest, Q is the total kinetic energy of daughter nucleus and the 2 He nucleus. Suppose Q < 0, then the decay process cannot occur spontaneously and energy must be supplied to induce the decay.
