Why is there a mass defect in the nucleus? I think about mass defect in this way - A comparatively unstable nucleus( i.e. low binding energy) converts to a stabel nucleus( high binding energy) Intially internal energy was U1 and then it became U2. So when it converts then some energy should be released which should have a value U1 - U2 Eg:- two protons far apart have some potential energy, when they get closer potential energy decreases so the lost energy is seen as kinetic energy right? Similarly when a neutron converts to another why the change in internal energy is seen as a mass defect and not kinetic energy or something else?