Nucleons, mass defect, and mass

[Jathor]

in high school we are learning about mass defect in nuclear fission and fusion and I know the math: ΔE=Δmc^2

however often the mass changed into energy is much less then the mass of a single nucleon(proton or neutron) and there are no fewer nucleons. for this to happen I would think that you would need to have a part of a nucleon left but we still assume that there mass is a constent.

how dose this work? my physics teacher had no idea.

 

[mathman]

http://en.wikipedia.org/wiki/Nuclear_binding_energy

The above may help.