Question

Discuss the alpha decay process with example.

Answer 1

When unstable nuclei decay by emitting an α-particle \[\ce{(^4_2He nucleus)}\], 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

\[\ce{^A_ZX -> ^{A-4}_{Z-2}Y + ^4_2He}\]

Here X is called the parent nucleus and Y is called the daughter nucleus.

Example:

Decay of Uranium \[\ce{^238_92U}\] to thorium \[\ce{^234_92Th}\] with the emission of \[\ce{^4_2He}\] nucleus (α-particle)

\[\ce{^238_92U -> ^234_92Th + ^4_2He}\]

As already mentioned, the total mass of the daughter nucleus and \[\ce{^4_2He}\] nucleus is always less than that of the parent nucleus. The difference in mass Q = (∆m_x – m_y – m_α) is released as energy called disintegration energy Q and is given by

Q = (∆m_x – m_y – m_α) 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 the 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.