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.
0 of 3 steps revealed.
Photoelectron spectroscopyCourse level: AP Chemistry
Photoelectron Spectroscopy (PES)
What you'll be able to do: Read a PES spectrum by matching subshell features, calibrated electron populations and binding energies.
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 calibrated peak areas or stated schematic intensities to infer electron counts
- Rank binding energies within an atom
- Compare the same subshell across different elements
What the experiment does
A beam of energetic photons ejects electrons from occupied subshells. The electron kinetic energy and the known photon energy give the electron binding energy, E_binding = hν − E_kinetic. A PES spectrum plots signal against binding energy, with distinct features associated with occupied subshells. Check which direction the binding-energy axis runs before assigning inner and outer electrons.
Binding energy is the energy required to eject the electron; the photon supplies that energy plus the outgoing electron's kinetic energy. If a photon has energy 100 eV and the emitted electron has 25 eV of kinetic energy, its binding energy is 75 eV. Use consistent energy units: do not subtract J per photon from kJ per mole. A photon must have enough energy to reach a subshell before electrons from that subshell can appear in a spectrum.
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 area and electron population
The integrated area under a properly calibrated peak relates to the number of electrons in that subshell. Introductory diagrams often draw peaks with heights proportional to electron counts; follow the convention stated on the graph.
For sulfur, [Ne] 3s² 3p⁴, a simplified spectrum shows a 3p-to-3s population ratio of 4:2, or 2:1. A real narrow peak can be taller than a broad peak yet have less total area, so height and area are not interchangeable without that schematic convention.
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.
The one-peak-per-occupied-subshell rule is the simplified atomic model used in introductory problems. Real spectra can have additional splitting and structure, and raw peak areas depend on photoionization response as well as population. Use the calibrated electron counts or schematic convention supplied. Missing a subshell because the photon energy was insufficient is not evidence that the atom has no electrons there.
Comparing elements
The 1s peak of oxygen has a higher binding energy than the 1s peak of carbon. Oxygen has eight protons rather than six, and inner 1s electrons are not significantly shielded by outer-shell electrons.
The other 1s electron does provide some electron–electron shielding, so the electrons are not completely unshielded. In this same-subshell comparison, the greater nuclear charge dominates.
Core-electron binding energies rise steadily with Z, which is what makes PES useful for identifying elements.
Read a spectrum in three passes
First inspect the binding-energy axis: some PES graphs put high binding energy on the left, others on the right. Next identify occupied subshells from distinct peaks and compare their binding energies; inner-shell electrons usually require more energy to remove.
Finally compare each peak's integrated area (or the relative magnitude shown in a simplified classroom spectrum) to estimate how many electrons it represents. A tall, narrow peak need not have more total electrons than a shorter, broad one if the graph is an experimental trace.
Use the stated intensity convention. For a 2:2:6:2:1 subshell pattern, the total is 13 electrons, identifying neutral Al.
Turn a schematic spectrum into a configuration
| Pass | What to read | What it tells you |
|---|---|---|
| 1 | Direction and units of binding energy | Which peaks are tightly bound and which are valence |
| 2 | Ordered subshell features | Candidate occupied subshells |
| 3 | Calibrated relative populations | Superscripts in the configuration |
| 4 | Total electron count and stated charge | Atomic identity if enough information is supplied |
For high-to-low binding-energy populations 2:2:6:2:1, assign 1s² 2s² 2p⁶ 3s² 3p¹. The total is thirteen electrons. If the sample is neutral atomic vapor, that identifies aluminum; if an ion is specified, combine its charge with the electron count before inferring Z. The lowest-binding-energy feature is 3p, not whichever peak happens to be drawn at the left edge.
Peak position and population answer different questions. Moving a 1s peak to higher binding energy says those electrons are more tightly bound; it does not say the 1s subshell now contains more electrons. Compare the same subshell across elements rather than assuming two nearby plotted peaks represent the same orbital.
Area counts electrons; position compares binding energy.
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?
0 of 3 steps revealed.
Worked example 3
A simplified PES spectrum shows five subshell peaks with relative electron counts 2, 2, 6, 2 and 1 from highest to lowest binding energy. Identify the neutral element and the lowest-binding-energy subshell.
Try it first: Add all the electron counts and follow the filling order.
0 of 3 steps revealed.
Worked example 4
Two spectra show 1s peaks for carbon and oxygen. Which 1s peak has greater binding energy, and why?
Try it first: Both peaks describe electrons in the same n = 1 shell.
0 of 3 steps revealed.
Common mistakes
| Mistake | Why it's wrong | Check instead |
|---|---|---|
| Counting one peak per shell instead of per subshell. | 2s and 2p electrons have clearly different binding energies, so they give separate peaks. | Count the terms in the full electron configuration. |
| Reading peak height as binding energy. | Height reports how many electrons were ejected at that energy, not how tightly they were held. | Use the horizontal position for binding energy and the height for electron count. |
| Assuming the valence peak is tallest. | A filled 2p subshell with 6 electrons is taller than a 3s peak with 2, even though 3s is the valence subshell. | Compare superscripts in the configuration to rank heights. |
Scroll horizontally if needed to read all columns.
After the lesson · Question bank
Independent practice
Work through new problems on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
35 topic questions across 3 difficulty levels.
Loading practice questions…
What you should now know
In a simplified atomic PES spectrum, feature positions report binding energies and calibrated relative populations identify occupied subshells and their electron counts. Read the axis direction and intensity convention, total the electrons, and use the stated charge before identifying the element. Experimental peak heights alone need not equal populations.
- Read the direction and units of the binding-energy axis.
- Assign occupied subshells using the stated simplified spectral model.
- Use calibrated areas or explicitly population-scaled schematic heights for electron counts.
- Peak position measures binding energy, not electron population.
- Check electron count and whether the sample is a neutral atom or an ion.
Your next step
Lesson: Not marked completed
Marking a lesson completed records that you finished reading it. Practice establishes mastery separately. Saved on this device.
Try a short set of different questions to check what you can do independently.
Watch and Learn
An optional video to support this lesson.
Loading video resources…
Supplementary learning resources
More resources for this topic →External resources belong to their respective creators. Chem Help is not affiliated with or endorsed by these providers. Links open the original provider; their terms apply.
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
Lesson-specific rights and provenance need review.
The current online Chemistry 2e edition lists CC BY-NC-SA 4.0. The source version used for this lesson and its applicable rights remain unverified.
Recorded use: reference. This describes the stored citation, not verified authorship or reuse permission.
Recorded attribution / license claim (applicability unverified): Access for free at openstax.org
Source rights and review status - NIST Atomic Spectra Database: ionization energiesNational Institute of Standards and TechnologyView source
Lesson-specific rights and provenance need review.
NIST rights depend on the dataset and work category. A generic government-work or public-data label does not establish rights for this lesson's source material.
Recorded use: further reading. This describes the stored citation, not verified authorship or reuse permission.
Recorded attribution / license claim (applicability unverified): Data from the NIST Atomic Spectra Database
Source rights and review status