For a partial look into the strong nuclear force we can examine how much energy is needed to break apart a nucleus.

The binding energy is not only the energy needed to break apart an existing nucleus into separate nucleons, it is also the energy which is released when a nucleus is formed from separate nucleons. This is the result of the mass which "disappears", generally called the mass defect (Dm). Inserted into Einstein's mass-energy equation, the mass defect gives the binding energy of the nucleus.

Comparing the binding energy per nucleon for known isotopes reveals a steep increase in stability in the light elements (He-4 is exceptionally high) until Fe-56. Then there is a gradual decline in binding energy as nuclei become heavier.

To an extent, the conclusions drawn from a comparison of binding energies contradict ordinary experience. Uranium--which has no non-radioactive isotopes at all--has a relatively high binding energy. Why does it fall apart?