Nuclear Binding Energy and Mass Defect
High-Yield Summary
- Mass defect: a nucleus has slightly less mass than the sum of its free protons and neutrons — the 'missing' mass becomes binding energy via E = mc².
- Binding energy = energy released forming the nucleus = energy required to break it back apart into individual nucleons.
- Strong nuclear force holds nucleons together against proton-proton electromagnetic repulsion; acts only at extremely short range (a few femtometers), so it doesn't affect chemistry.
- Weak nuclear force is much weaker but essential for nuclear stability and enables radioactive decay (especially beta decay).
- Binding energy per nucleon peaks near iron-56 — fusion (light nuclei combining) and fission (heavy nuclei splitting) both release energy by moving nuclei toward iron-56.
Mass-Energy Equivalence
E = mc²
- E = Binding energy released/required
- m = Mass defect (mass 'missing' compared to free nucleons)
- c = Speed of light in vacuum
Strong vs. Weak Nuclear Force
| Strong Nuclear Force | Weak Nuclear Force |
|---|---|
| Strongest of the four fundamental forces | Much weaker than the strong force |
| Holds nucleons together, overcoming proton-proton repulsion | Enables nuclear transformations behind radioactive decay (e.g., beta decay) |
| Extremely short range (~femtometers) — doesn't affect chemistry | Also short-range, acts within the nucleus |
Common MCAT Trap
- Both fission and fusion release energy — don't assume only one direction (splitting or combining) is energetically favorable; it depends on which side of iron-56 the starting nuclei are on.
- Higher binding energy per nucleon means more stable, not less — a common sign-confusion trap on this topic.
Quick Recall
Why does fusing two light nuclei (e.g., hydrogen) release energy?
What happens to the 'missing' mass when nucleons bind into a nucleus?