Worked example 1
In the PES of gaseous sulfur (Z = 16), what is the ratio of the 3p peak to the 3s peak?
Try it first: Write the configuration and read the two superscripts.
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Periodic TrendsPhotoelectron spectroscopyContent level: Challenge 25 min
Photoelectron Spectroscopy (PES)
What you'll be able to do: Read a PES spectrum: match peaks to subshells, use peak height for electron counts and binding energy for shell depth.
Best after: Electron Configurations and Orbital Filling, Shielding and Effective Nuclear Charge
Introduction
Photoelectron spectroscopy is the direct experimental evidence for the shell and subshell model you have been using. Once you can read a spectrum, electron configuration stops being an assertion and becomes a measurement.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Match PES peaks to subshells in an electron configuration
- Use relative peak heights to determine electron counts
- Rank binding energies within an atom
- Compare the same subshell across different elements
Lesson
What the experiment does
High-energy photons knock electrons out of every occupied subshell at once. The instrument measures the energy needed to free each electron, called the binding energy, and plots how many electrons came off at each energy. The result is a bar-like spectrum with one peak per subshell.
Peak position: binding energy
Electrons closer to the nucleus are held more tightly, so core electrons appear at very high binding energy and valence electrons at low binding energy. For magnesium the peaks in order of decreasing binding energy are 1s, then 2s, then 2p, then 3s.
Many PES axes are drawn with binding energy increasing to the left, so read the axis before ranking peaks.
Peak height: how many electrons
The height, or area, of a peak is proportional to the number of electrons in that subshell. For sulfur, [Ne] 3s² 3p⁴, the 3p peak is twice the height of the 3s peak, a 2:1 ratio, because 4 electrons versus 2. A filled p subshell against a filled s subshell gives 3:1.
S: 1s² 2s² 2p⁶ 3s² 3p⁴
Counting peaks
The number of peaks equals the number of occupied subshells, not shells. Neon (1s² 2s² 2p⁶) has three peaks, with 1s at the highest binding energy. Magnesium (1s² 2s² 2p⁶ 3s²) has four peaks.
Comparing elements
The 1s peak of oxygen sits at a much higher binding energy than the 1s peak of carbon, because oxygen has 8 protons pulling on its 1s electrons versus 6 for carbon, with essentially no shielding between them. Same subshell, more protons, deeper binding.
Core-electron binding energies rise steadily with Z, which is what makes PES useful for identifying elements.
Key ideas
Definition
Binding energy
The energy required to eject a specific electron from an atom, measured directly in PES.
Rule
One peak per subshell
The number of peaks equals the number of occupied subshells.
Rule
Height is population
Relative peak height or area gives the ratio of electrons in each subshell.
Rule
Depth and charge
Binding energy increases closer to the nucleus and increases with nuclear charge for the same subshell.
Worked examples
Worked example 2
How many peaks appear in the PES of neon, and which has the highest binding energy?
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Common mistakes
Counting one peak per shell instead of per subshell.
Why it's wrong: 2s and 2p electrons have clearly different binding energies, so they give separate peaks.
Check instead: Count the terms in the full electron configuration.
Reading peak height as binding energy.
Why it's wrong: Height reports how many electrons were ejected at that energy, not how tightly they were held.
Check instead: Use the horizontal position for binding energy and the height for electron count.
Assuming the valence peak is tallest.
Why it's wrong: A filled 2p subshell with 6 electrons is taller than a 3s peak with 2, even though 3s is the valence subshell.
Check instead: Compare superscripts in the configuration to rank heights.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
In a PES spectrum each peak is one subshell, peak height is proportional to the number of electrons in it, and binding energy increases for electrons closer to the nucleus and for higher nuclear charge.
- One peak per occupied subshell
- Peak position gives binding energy, height gives electron count
- Core electrons bind far more tightly than valence electrons
- The same subshell binds more tightly in an element with more protons
Sources and further reading
- Chemistry 2e, Section 6.4: Electronic Structure of Atoms (Electron Configurations)Flowers, Theopold, Langley, Robinson · OpenStax, Rice University · Chapter 6.4View source
Access for free at openstax.org License
- NIST Atomic Spectra Database: ionization energiesNational Institute of Standards and TechnologyView source
Data from the NIST Atomic Spectra Database