Why Is Nuclear Binding Energy Proportional to the Mass Defect?

[semc]

lets say the nucleus of the nuclide is M, why is binding energy of the nucleus directly proportional to (mass of proton and neutron)-M ? shldnt it be M-(mass of proton and neutron) ?

 

[Astronuc]

Binding energy is the difference between the total mass (before) and the total mass (after) the combination.

If one takes a nucleus of mass M+1 and compares it to the initial nuclear of mass M and the proton or neutron mass, one will find the total mass before is greater than the mass after. The difference in mass is the binding energy, and in many cases the energy is transformed into a gamma ray.

http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/nucbin.html