E.3 · IB Physics
N–Z chart and decay modes
Each square is a nuclide with Z protons and N neutrons. The stable nuclides form a narrow band that starts on the line N = Z and bends upwards, because heavy nuclei need extra neutrons. Move the cursor off the band to predict how the nuclide decays, see each decay as an arrow, or step through the uranium-238 decay series (simplified to its main branch).
Predict first
Start in Explore mode with the defaults (Z = 6, N = 8, carbon-14). Press Decay ▸ once. Which nuclide does the cursor move to?
Carbon-14 has too many neutrons, so it decays by β⁻: a neutron becomes a proton, so Z rises by 1 and N falls by 1 while A stays 14. That gives nitrogen-14.
Check it in the model: Press Decay ▸ once and read the Nuclide and Daughter readouts, then compare the Z and N sliders (7 and 7).
stable β⁻ (too many neutrons) β⁺ or electron capture α
In Explore mode you can also click or drag on the chart.
α: Z − 2, N − 2 (A − 4)
β⁻: Z + 1, N − 1 (A unchanged)
β⁺ or EC: Z − 1, N + 1 (A unchanged)
γ: no change in Z or N
Try this
- In Explore mode, choose carbon (Z = 6) and raise N from 6 to 10. Which nuclides are stable, and which way does each unstable one decay?
- Choose the Carbon-14 preset and press Decay ▸. Check that A is unchanged and that the arrow in the close-up is diagonal. Then try Fluorine-18 and compare the arrow direction.
- Run the U-238 series to the end, counting the α and β⁻ steps. Check that (238 − 206) ÷ 4 gives the number of α decays.
Common error: drawing a β⁻ arrow straight up or along the Z axis only. In β decay both N and Z change by one, so on an N–Z chart the arrow is diagonal. Gamma emission changes neither N nor Z, so it does not move the nuclide on the chart.