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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. 
From eqn. (ii) we have :

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