3.8.1.4
The N-Z graph shows which nuclei are stable and predicts their decay mode
Nuclear Structure & Radiation — AQA A-Level Physics
Key Definition
Nuclear stability graph — A plot of neutron number (N) against proton number (Z). Stable nuclei form a band called the line of stability. Nuclei away from this band are unstable.
- For light nuclei (Z < 20): stable nuclei follow $N = Z (equal neutrons$ and protons).
- For heavy nuclei (Z > 20): the line curves above $N = Z. More neutrons are needed$ to overcome the electrostatic repulsion between protons.
- Extra neutrons add strong nuclear force binding without adding electrostatic repulsion.
- No stable nuclei exist beyond $Z = 82 (lead)$.
Predicting decay mode from N-Z position
- Left of the stability line (neutron-rich): β⁻ emission. A neutron converts to a proton.
- Right of the stability line (proton-rich): β⁺ emission or electron capture. A proton converts to a neutron.
- Below the stability line with Z > 82 (too many nucleons): alpha emission. The nucleus ejects ⁴₂He.
- Excited state after decay: gamma emission. No change in N or Z, just energyThe capacity to do work. Measured in joules (J). released.
Examiner Tips and Tricks
- Remember: beta-minus emitters are neutron-rich (left of stability).
- Beta-plus emitters are proton-rich (right of stability).
- Draw the stability graph from memory — it comes up frequently.