Having explored mass defect and binding energy, we now look at how these nuclear phenomena relate to the fundamental forces of nature.

Protons within a nucleus are positively charged, so they experience mutual Coulomb repulsion (electrostatic force). Because electrostatic force follows an inverse-square law, protons packed together at nuclear distances (around 10^-15 m, or 1 femtometer) exert massive repulsive forces on each other—on the order of dozens of newtons per proton pair.

For atomic nuclei to remain stable, there must be an attractive interaction that overcomes this electrostatic repulsion: the strong nuclear force.

Key Characteristics of the Strong Nuclear Force

• Charge-Independent: It acts equally between proton-proton (p-p), neutron-neutron (n-n), and proton-neutron (p-n) pairs.
• Extremely Short Range: It operates effectively only over short subatomic distances (≈ 1 to 3 fm). Beyond roughly 3 fm, its influence drops to zero.
• Distance-Dependent Behavior:
  • At distances < 0.7 fm: The force becomes strongly repulsive, preventing nucleons from collapsing into one another.
  • At distances ≈ 0.7 to 1.5 fm: The force is strongly attractive, binding nucleons together tightly.
  • At distances > 2.5 fm: The force rapidly decays to zero, allowing Coulomb repulsion to dominate in larger nuclei.

Because neutrons contribute to the strong attractive force without adding to electrostatic repulsion, they act as a "nuclear glue." This explains why heavier elements require a higher neutron-to-proton (N/Z) ratio to maintain nuclear stability.