In a nuclear reaction, not only the atomic number and mass number is conserved but also total energy remains conserved.

Let us consider the nuclear reaction :

**X + a ———> Y + b ……..(i)**

Let Mx, Ma, My & Mb be the rest masses of X, a, Y and b respectively & Ea, Ey and Eb be the kinetic energy of a, Y and b respectively.

The target nucleus X is initially at rest, so it has no kinetic energy.

By the conservation of total energy, the equation (i) can be written as :

Let Q be the energy released or absorbed during the nuclear reaction. It is the difference between the kinetic energy of the product of reaction & that of initial interacting particle. i.e.

Reactions for which Q is positive called exothermic reaction & reactions for which Q is negative called endothermic reactions.