The above graph shows the variation of BE per nucleon plotted against the nucleon no.
The difference between the sum of the masses of nucleons (protons and neutrons) in the nucleus and the actual mass of the nucleus is called mass defect which is denoted by 'Δm'.
The packing fraction is the mass defect per nucleon. It is the ratio of mass defect 'Δm' to the mass number 'A'.
The total energy required to liberate all the nucleons from the nucleus (i.e. separating the protons and neutrons of the nucleus infinite distance apart) is called Nuclear Binding Energy. This is the same energy with which the nucleons are held together within the nucleus.
(i) The binding energy of the nucleus is provided by the mass defect which is only a very small fraction of the
nucleus mass.
(ii) The binding energy of the nucleus is very high e.g. it is 2.22 MeV for deuteron nucleus. However, the binding energy of
electrons in an atom is very small. For example, the binding energy of a single electron in hydrogen atom is only 13.6 eV.
In order to compare the stability of the nuclei of different atoms, we determine the binding energy per nucleon of the nucleus.
The binding energy per nucleon (B.E./nucleon) is the average energy required to extract one nucleon from the nucleus to infinite distance. It is given by the total binding energy of a nucleus divided by the mass number of the nucleus.