Short on time? This page is the recap. For explanations, a labelled spectrum and worked examples, use the full study guide.
Recap
- Photoelectron spectroscopy (PES) uses high-energy photons to knock single electrons out of gas-phase atoms or ions, from every occupied subshell.
- The binding energy of an electron is the energy needed to remove it. Electrons in the same subshell share one binding energy, so they form one peak.
- Peak height is ideally proportional to the number of electrons in the subshell, so you read relative heights.
- Spectra often plot binding energy increasing to the left on a log scale.
- Separate s and p peaks within a shell are the evidence that shells contain subshells.
Key relationships
| Feature | What it tells you | Coulomb’s law link (F ∝ q₁q₂ / r²) |
|---|---|---|
| Number of peaks | Number of occupied subshells | — |
| Relative peak heights | Superscripts of the configuration; total = number of electrons | — |
| Peak far to high binding energy | Core electrons, close to the nucleus | Small r, almost full nuclear charge |
| Lowest-energy peak | Valence electrons; ≈ first ionization energy | Large r, strong shielding |
| Same subshell in a heavier element | Peak moves to higher binding energy | More protons, larger q |
| Isoelectronic species | Same peaks and heights, shifted positions | More protons → every peak higher |
Units: 1 MJ mol⁻¹ = 1000 kJ mol⁻¹.
Assumptions behind the reading
- Heights are only ideally proportional to electron numbers, so round the ratio to the nearest whole number of electrons.
- The sample is a single element (or ion) in the gas phase, in its ground state.
- Subshells fill in Aufbau order; you will not be asked to write configurations for the elements that break this rule.
Mistakes to avoid
- Tallest = highest energy. Height is electron count, not energy.
- Reading the axis the wrong way. Check which side is high binding energy.
- One peak per electron. It is one peak per subshell.
- Treating peaks as successive ionization energies. Only the lowest peak matches an ionization energy.
- Keeping the valence peak for a cation. Mg²⁺ has no 3s peak.
- Saying “more shells” when comparing the same subshell in two elements. The difference is nuclear charge.
- Ignoring the 2s/2p split when judging a simple shell model.
Quick self-check
- A spectrum has heights 2, 2, 6, 2, 5. Which element? (Chlorine, 17 electrons)
- Which has its 1s peak at higher binding energy, F or Ne? (Ne: one more proton)
- How many peaks does the spectrum of Ca²⁺ have? (Five: 1s, 2s, 2p, 3s, 3p)
- A valence peak is at 0.74 MJ mol⁻¹. What is that in kJ mol⁻¹? (740 kJ mol⁻¹)
Next: practice questions.