Range of the Nuclear Force

While the attractive nuclear force must be stronger than the electric force to hold the protons together in the nucleus, it is not a long range 1/r² force like electricity and gravity. It drops off much more rapidly than 1/r ² , with the result that if two protons are separated by more than a few proton diameters, the electric repulsion becomes stronger than the nuclear attraction. The separation R0 at which the electric repulsion becomes stronger than the nuclear attraction, is about 4 proton diameters. This distance R0 , which we will call the range of the nuclear force, can be determined by looking at the stability of atomic nuclei.
If we start with a small nucleus, and keep adding nucleons, for a while the nucleus becomes more stable if you add the right mix of protons and neutrons. By more stable, we mean more tightly bound. To be explicit, the more stable, the more tightly bound a nucleus, the more energy that is required, per nucleon, to pull the nucleus apart. This stability, this tight binding, is caused by the attractive nuclear force between nucleons.
Iron 56 is the most stable nucleus. It takes more energy per nucleon to take an Iron 56 nucleus apart than any other nucleus. If the nucleus gets bigger than Iron 56, it becomes less stable, less tightly bound. If a nucleus gets too big, bigger than a Lead 208 or Bismuth 209 nucleus, it becomes unstable and decays by itself.
The stability of Iron 56 results from the fact that an Iron 56 nucleus has a diameter about equal to the range of the nuclear force. In an Iron 56 nucleus every nucleon is attracting every other nucleon. If we go to a nucleus larger than Iron 56, then neighboring nucleons still attract each other, but protons on opposite sides of the nucleus now repel each other. This repulsion between distant protons leads to less binding energy per particle, and instability.