Why do atoms larger than iron release energy when they undergo fission?

For this problem we need to consider the strong nuclear force (SNF), and the electromagnetic force (EMF). Recall that the SNF attracts very strongly over very small distances (≈10^-15m), and the EMF is weaker but acts over larger distances. Protons and neutrons (nucleons) both experience the SNF and will attract each other, but only the proton experiences the EMF since the neutron is uncharged.Now let’s build up a nucleus with a few protons and neutrons. Usually we would expect the like charges on the protons to repel each other causing the nucleus to break apart. However, since the nucleons are all very close to each other, the attraction from the SNF can overcome this repulsion and keep the nucleons tightly bounded. Hence without the SNF, there would be no nuclei and we could not exist!Now let’s throw some more nucleons into the mix. As we add them, we will introduce more protons and therefore increase the forces of repulsion within the nucleus. We are also making the nucleus larger and hence the average distance between nucleons is increasing. This is a problem for the short-sighted SNF, which will not attract the nucleons on the outside of the nucleus as strongly.When the nuclei become larger than iron’s, the electromagnetic repulsion can start to overcome the attraction of the SNF. This can cause parts of the nuclei to shoot off, releasing energy. Due to conservation of energy, the potential energy within the smaller nucleus left behind must decrease.Hence when atoms larger than iron undergo fission, i.e. their nucleus breaks into smaller parts, energy is dispersed into the surroundings.