Question

why is there repulsion at large nucleon seperation?

Answer 1

this is because the strong nuclear force is dominant but this force only acts a very short range while the electromagnetic repulsive forces between protons are long range and always act on all protons. in heavier nuclei, how ever, the distance between many of the nucleons are to great an the cohesion by strong interaction is no longer as intence. on the other hand, the electromagnetic repulsion forces are getting stronger as there are more protons and these are long-range. thus starting with iron, the electrical potential barrier that must be broken in order to add one more proton to the nucleus suparface the energetic benefit that the strong interaction gives when joining it with the rest of the nucleons. this also explains the gentle slope of the energy obtained by the fission since it is really given by the excess over the cohesion by strong interaction while the fusion energy is the opposite, the energy is provided by the strong interaction it far exceeds repulsive force especially on lighter atoms like hydrogen or helium with hardly any positive charges.