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A nuclear reaction is the process of strong interaction of atomic nucleus with an elementary particle resulting in the formation of new nucleus and one or more new particles. 
The nuclear reaction occurs when the interacting nucleus and the particle approaches each other upto distance of order of 10^-15 m.

An equation representing nuclear reaction can be written as :
                  X + a   ———>   Y + b
where,  X = initial nucleus 
             a = initial interacting particle 
             Y= final nucleus 
             b = final particle 
The above equation can be represented as :
                        X( a, b  ) Y

Nuclear reactions wre usually attended by the liberation or absorption of energy known as reaction energy or dis – integration energy. 
Compound – Nucleus Hypothesis :

Bohr in 1936 proposed Compound – Nucleus Hypothesis for nuclear reactions. According to it many nuclear reaction proceeds in two seperate stages :-
i) The incident particle strikes the initial nucleus ( target nucleus) and these two first combine to form a new nucleus called compound nucleus. The atomic number & atomic mass of compound nucleus is the sum of atomic mass & atomic number of incident particle & target nucleus. 
ii) The compound nucleus immediately breaks up by ejecting a particle or gama rays leaving the final or product nucleus. 
      e.g. The Rutherford’s reaction can be                    written as :

N & He particles combine to form a compound nucleus 9F(18) which after ejecting a proton forms final product as 8O(17) and 1H(1).