Conservation of mass is the key to this topic. Any loss of mass is in the form of energy E=mc2. Given a reaction A+B=C+D+ mass defect , the mass defect accounts for energy released .
Similarly if 6 proton +6 neutron =A + mass defect, the Energy released is called Binding Energy of A . If BE divide by 12 (nucleons) than you have BE per nucleon.The higher the BE per nucleon the greater the stability of the nucleus.
Use BE to calculate energy released:
A + B = C becomes (A+ BE1)+ (B +BE2) = (C+ BE3)as the RHS and LHS have the same nucleon to start with. Since BE3 is greater than (BE1+BE2) , hence net energy released (BE3-BE1-BE2)
Alternatively if BE3 is greater means that C has a smaller mass than A+B. Written as A+B = C + mass defect. Hence energy is released.
Consider a reaction :
A + B = C + 3 neutrons
Method 1: A+ B = C +3n + mass defect where mass defect = mass on LHs - mass on Rhs
Method 2: (mass defect) c2 = BE (of C) - BE (of A+B)
Notice that method 2 omit the neutrons as there is no BE. Method 1 include that mass of neutrons in the mass calculation on RHS