PES peak positions and sizes answer different questions
You will be able to: Read binding energy and subshell electron counts from an idealized PES plot.
How can a spectrum reveal an electron configuration?
Removing a tightly held core electron takes more energy than removing an outer electron. A photoelectron spectrum sorts these removal energies, helping us infer how electrons are arranged.
A useful starting point: Balance nuclear charge, distance and shielding →
Words and symbols before equations
- Binding energy
- Energy needed to remove an electron; here plotted in arbitrary energy units for a schematic spectrum.
- Photoelectron
- An electron ejected after absorbing sufficient photon energy.
- Idealized peak size
- In the AP model, proportional to subshell electron count when comparison conditions are controlled.
What this picture assumes
Idealized PES: fixed schematic energies 80, 15 and 5 arbitrary units isolate occupancy. They are not real across-element energies. Equal-width peaks have heights proportional to occupancy; real intensities depend on conditions. Energy increases to the right.
Read the picture in three steps
- Identify the chemical species and the quantities each label or axis represents. Read the units and any scale assumptions before comparing values.
- 1s² 2s² 2p⁴; 8 electrons. Peak counts 2:2:4; binding energies 80, 15, 5 arbitrary units.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
Read the binding-energy axis direction first. This model increases to the right; many classroom PES plots reverse that direction. A high-binding-energy peak corresponds to a more tightly held subshell regardless of which side it appears on.
The peak locations distinguish subshell removal energies; relative peak sizes indicate occupancies in the idealized model. For 1s²2s²2p⁴, three peaks have relative sizes 2:2:4.
Do not confuse this with isotope mass spectrometry. PES describes electron removal energies, not isotope masses or abundances. Real signal intensities also depend on experimental conditions; our equal-width schematic peaks are not measured data.
A worked example, step by step
A neutral atom’s idealized PES has three peaks, ordered highest to lowest binding energy, with electron counts 2, 2 and 5. Identify its configuration and electron total.
- The highest-energy peak is assigned to 1s².
- The next is 2s²; the lowest is 2p⁵.
- Total electrons = 2+2+5=9, consistent with neutral fluorine.
- The largest peak holds more electrons, not necessarily the most tightly held ones.
Read the axis direction; peak size and peak position encode different information.
Does the tallest peak always have greatest binding energy?
Compare with an explanation
No. Peak size represents occupancy; position represents binding energy.
Predict. Change one thing. Explain.
Change the 2p occupancy while holding the schematic energies fixed. Predict which peak grows and verify the total electron count. This isolates counting, not real across-element energy shifts.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
1s² 2s² 2p⁴; 8 electrons. Peak counts 2:2:4; binding energies 80, 15, 5 arbitrary units.
Idealized PES: fixed schematic energies 80, 15 and 5 arbitrary units isolate occupancy. They are not real across-element energies. Equal-width peaks have heights proportional to occupancy; real intensities depend on conditions. Energy increases to the right.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use particle counts, mass or charge balance, electron structure, or nuclear attraction to justify your prediction. Separate an observation from an explanation.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionAn idealized PES has peaks corresponding to 1s, 2s and 2p, with relative occupancies 2, 2 and 6. Write the configuration, identify the neutral element, and explain what must be checked before deciding which plotted peak is most tightly bound.
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Compare with the answer and four-point rubric
- 1 point: The configuration is 1s²2s²2p⁶.
- 1 point: There are 10 electrons: neutral neon.
- 1 point: The 1s peak corresponds to the greatest binding energy.
- 1 point: Check axis labels and increasing direction; highest energy is not always drawn on the same side.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What does a PES horizontal coordinate mean?
Electron binding energy, with its axis direction stated.
RECALL 2What does idealized peak size mean?
Number of electrons in the corresponding subshell.
RECALL 3How is PES different from isotope mass spectra?
Electron removal energy versus isotope mass-to-charge and abundance.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
PES peak positions and sizes answer different questions
- Peak position → binding energy.
- Idealized relative peak size → subshell occupancy.
- Sum of occupancies → electron count.
Remember: Read the axis direction; peak size and peak position encode different information.
Conditions: Idealized PES: fixed schematic energies 80, 15 and 5 arbitrary units isolate occupancy. They are not real across-element energies. Equal-width peaks have heights proportional to occupancy; real intensities depend on conditions. Energy increases to the right.
Refresh Kid · AP Chemistry Unit 1 · Objectives 1.6.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 1.6, objectives 1.6.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 1: Atomic Structure and Properties, Topics 1.1–1.8. The topic mapping identifies a framework area; focused lesson titles are our own teaching sequence. Molecular-formula scaling is an application of empirical composition. Models explicitly distinguish atom counts, molecule counts, mass fractions and electron structure. Spectra marked schematic are not measured data. Mass spectra here use single-element, singly charged monatomic ions. Configurations avoid Aufbau exceptions and individual quantum-number assignments. Qualitative attraction and size indices are not exact atomic predictions. The optional NaCl-type spatial block supplements complete charge-balance explanations. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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