Photoelectron Spectroscopy
Use ideal photoelectron peak populations and binding energies as evidence for occupied subshells.
AP Chemistry Topic 1.6 · CED printed p. 36 / PDF p. 43. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Electron configurations
- Effective nuclear attraction
PES tells you how tightly electrons are held
Photoelectron spectroscopy supplies photons energetic enough to eject electrons. In a simplified picture, the photon’s energy is divided between removing the electron and its outgoing kinetic energy: binding energy = photon energy − electron kinetic energy. An electron that was held more tightly leaves with less kinetic energy for the same incoming photon energy. PES binding energy therefore reveals electronic subshell structure, not isotope masses.
Read the axis direction before anything else. Some PES plots put high binding energies on the left, so the core-electron peak appears on the left. Other graphs reverse that. “Farther right” is not a chemical explanation. Refer to larger or smaller binding energy. In the AP idealized representation, each occupied subshell contributes a peak, and relative peak height ideally follows the electron number in that subshell. Real signal intensities have instrumental and subshell sensitivity effects; use the stated ideal proportionality in exam models.
Use position and population together
A 1s22s22p6 atom has three occupied subshells with peak-height ratio 2:2:6. The tightly held 1s core electrons have the largest binding energy; 2p generally binds less tightly than 2s in the same multielectron atom. Add the relative electron populations to identify the electron count, then use a neutral-atom or ion assumption to identify the species. A 10-electron configuration alone could be Ne, Na+ or Mg2+; the nuclear attraction distinguishes them.
Compare the same subshell across species. With similar shell structure and shielding, greater nuclear charge holds those electrons more strongly and shifts the peak to greater binding energy. Compare different subshells using distance and shielding as well. A high-binding-energy peak is not taller because it contains more energy; its height and horizontal position answer different questions.
Let's work through it
Work it through
An idealized neutral-atom PES has four peaks, ordered from greatest to least binding energy, with electron populations 2, 2, 6 and 1. Identify the atom and explain the low-binding-energy peak.
- Associate the four populations with 1s, 2s, 2p and 3s: 1s22s22p63s1.
- Add the populations: 11 electrons. Neutrality means 11 protons, identifying Na.
- The 3s electron is in a more distant shell and shielded by the core. It has the least binding energy and a smaller ideal peak because only one electron occupies that subshell.
- For Mg, the comparable 3s peak represents two electrons and is shifted to greater binding energy because the nuclear charge is greater with a similar core.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
A PES peak is three times taller than another in the stated ideal model. What does that mean?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
A PES peak is three times taller than another in the stated ideal model. What does that mean?
Worked answer and reason
It represents three times as many electrons in that subshell, not three times the binding energy.
Height is population; horizontal position is removal energy.
Retrieval 2
Ne and Na+ each have 10 electrons. Which generally has higher corresponding binding energies?
Worked answer and reason
Na+, because its nucleus has 11 protons rather than 10 with similar shell structure.
For an isoelectronic comparison, increased nuclear attraction shifts binding energies upward.
Catch the tempting mistake
- The spectrum with more peaks necessarily has more electrons.
- Peaks count occupied subshells; their relative populations give electron counts.
- The rightmost peak always belongs to valence electrons.
- Axis orientation varies; find the lowest binding energy rather than memorizing a side.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 1 problem 12 · ap application · data
- Unit 1 problem 13 · ap synthesis · multiple
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For the teacher
- Have students label the axis and subshell population before naming the atom.
- Give two isoelectronic possibilities so students have to use nuclear attraction, not just electron totals.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 1.6.A
- Explain the relationship between the photoelectron spectrum of an atom or ion and: i. The ground-state electron configuration of the species. ii. The interactions between the electrons and the nucleus.
- 1.6.A.1
- The energies of the electrons in a given shell can be measured experimentally with photoelectron spectroscopy (PES). The position of each peak in the PES spectrum is related to the energy required to remove an electron from the corresponding subshell, and the relative height of each peak is (ideally) proportional to the number of electrons in that subshell.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §6.4, Electronic Structure of Atoms — Pauli and Hund rules, ground-state and transition-metal ion configuration conventions.
- OpenStax Chemistry 2e §6.5, Periodic Variations — Effective nuclear charge, shielding, ionization-energy exceptions and electron-affinity limits.
Glossary and deeper reading · 6 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Electron configuration
An electron configuration describes how an atom's or ion's electrons occupy shells and subshells. A ground-state configuration places electrons in the lowest-energy arrangement consistent with the electron-filling rules. For example, neutral helium has two electrons in its 1s subshell.
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Photon
A photon is a quantum of electromagnetic radiation. Its energy increases with frequency and decreases as wavelength increases. For example, a blue-light photon carries more energy than a red-light photon.
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Electron subshell
A subshell is a set of atomic orbitals with the same principal and angular-momentum quantum numbers. The s, p, d, and f subshells contain one, three, five, and seven orbitals, respectively. For example, a p subshell can accommodate six electrons because each of its three orbitals can hold two.
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Core electron
Core electrons occupy inner shells and are usually less directly involved in bonding than valence electrons. They help shield outer electrons from the nucleus's full positive charge. For example, sodium's ten inner electrons form its neon-like core beneath its outer electron.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Electron shielding
Electron shielding is the reduction in nuclear attraction experienced by an electron because other electrons also repel it. Inner electrons generally shield an outer electron more effectively than electrons in the same shell. For example, adding an occupied inner shell helps explain why atomic size usually increases down a periodic-table group.