Stable nuclei, measured mass defects, and nuclear-scattering results provide evidence for the strong nuclear force. Together, they show that nucleons experience a powerful, attractive, short-range interaction that overcomes electrostatic repulsion within the nucleus.
The Reasoning
A nucleus containing two or more protons experiences outward electrostatic repulsion because like charges repel. Nevertheless, many such nuclei are stable. Therefore, another attractive interaction must act between nucleons and must be stronger than the electrostatic force at nuclear separations of approximately .
The mass of a nucleus is also less than the total mass of its separate nucleons. This mass defect corresponds to the nuclear binding energy:
Energy must therefore be supplied to separate the nucleus into individual nucleons. This demonstrates that the nucleons form a bound system under an attractive interaction.
| Evidence | What It Shows |
|---|---|
| Stable nuclei containing several protons | An attractive nuclear interaction overcomes proton-proton electrostatic repulsion at short distances. |
| Positive binding energy and measurable mass defect | Work must be done to separate nucleons, so they are strongly bound. |
| Binding energy per nucleon becomes approximately constant for | Each nucleon interacts mainly with nearby nucleons, showing that the strong nuclear force has a short range and reaches saturation. |
| Nuclear radius follows |
A common misconception is that the strong nuclear force acts only between protons. It acts between nucleons, including proton-proton, neutron-neutron, and proton-neutron pairs.
Exam Technique
This is Additional Higher Level (AHL) content in E.3 Radioactive decay. For an IB Explain question, link each observation to an inference: proton repulsion requires a stronger attraction, binding energy proves a bound system, and the binding-energy trend indicates short range and saturation.
