Could someone please explain to me as to why the binding energy per nucleon is lower for atoms of low mass number as compared to the atoms in the middle zone of the binding energy per nucleon versus mass number graph? My conceptual issue lies in the fact that when the mass number is low the coulomb force of repulsion between protons is also low and the internucleon attractive nuclear force should predominate over the coulomb force in these atoms. Hence the positive potential energy acquired when the nucleons are brought together is less than the negative potential energy due to the attractive nuclear forces (which is approx. constant in any atom because nuclear forces are short ranged). This difference between the two should be more for lower mass number. This should result in an even higher binding energy per nucleon than in the atoms in the middle zone of the graph. Kindly clarify / elaborate. Thanks.