Practice a College Board-style short-answer question on Atomic Structure & Properties. Write your response, then reveal the model answer to see exactly what earns each point.
Short Answer Question · Unit 1 · Photoelectron Spectroscopy & Periodic Trends
The photoelectron spectrum of a neutral atom of an unknown element shows peaks at the following relative binding energies and relative peak areas (number of electrons).
Relative binding energy
Relative number of electrons
Highest
2
High
2
Medium
6
Low
2
Lowest
3
A
Identify the total number of electrons in this atom, and write its ground-state electron configuration.
✓ Model answer (earns the point)
The atom has 15 electrons total (2 + 2 + 6 + 2 + 3 = 15), so the electron configuration is 1s² 2s² 2p⁶ 3s² 3p³, matching phosphorus.
Why it scores: Correctly sums all peak electron counts to 15 AND writes a configuration that fills subshells in the correct order with the correct electron counts in each.
B
Explain why the peak with 6 electrons has a binding energy between the peak with 2 electrons just above it and the peak with 2 electrons just below it, in terms of electron shell and shielding.
✓ Model answer (earns the point)
The 6-electron peak corresponds to the 2p subshell, while the higher-binding-energy 2-electron peak is 2s and the lower-binding-energy 2-electron peak is 3s. Electrons in 2p are farther from the nucleus and more shielded than 2s electrons in the same principal energy level, so they're held less tightly (lower binding energy) than 2s — but they're still closer to the nucleus and less shielded than 3s electrons in the next principal energy level, so they're held more tightly than 3s.
Why it scores: Names the specific subshells (2s, 2p, 3s) AND explains the binding-energy ordering using distance from the nucleus and shielding — not just restating "it's in the middle."
C
Predict whether this element has a larger or smaller atomic radius than the element directly to its left on the periodic table, and justify your answer using effective nuclear charge.
✓ Model answer (earns the point)
This element (phosphorus) has a smaller atomic radius than the element to its left (silicon). Moving one element to the right within the same period adds one more proton to the nucleus without adding a new principal energy level or any significant additional shielding. This increases the effective nuclear charge felt by the valence electrons, pulling them closer to the nucleus and shrinking the atom.
Why it scores: Makes a clear, correct prediction (smaller) AND explains it using Zeff — specifically that protons increase while shielding stays roughly constant within a period. A response that just says "it's a trend" without the Zeff mechanism would not earn full credit.
How to score points on SAQs
Be specific. "The peak is in the middle" doesn't score. "The 2p subshell has a binding energy between 2s and 3s because of shell and shielding differences" does.
Name subshells and quantities. Graders look for exact subshell labels (1s, 2p, 3s) and electron counts, not vague descriptions.
Always connect cause to effect. Don't just state the trend — explain the underlying mechanism (effective nuclear charge, shielding, principal energy level).
Answer the actual question. If it asks "identify," give a name or number. If it asks "explain" or "justify," give the answer PLUS the reasoning that supports it.
Keep it tight. 1–3 sentences per part is plenty. Long answers don't score higher; they just waste exam time.