OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 9
1 mark · Medium difficulty · Multiple Choice
Identify which element among aluminium, oxygen, chlorine, and phosphorus has atoms with the largest number of unpaired p-electrons.
Practise this questionQuestion
Question text
9 Which element has atoms with the largest number of unpaired p-electrons?
A aluminium
B oxygen
C chlorine
D phosphorus
Your answer [1]
Mark scheme
Show the mark scheme
9 D 1 AO1.2
How to answer it
Determining Unpaired p-Electrons
What this question tests
This question assesses your understanding of electron configurations, specifically sub-shells, orbitals, and Hund's rule (filling degenerate orbitals singly before pairing) within p-block elements.
Multiple Choice Answer Breakdown
✅ Correct Answer: D (phosphorus)
Phosphorus has the outer electronic configuration 3s² 3p³ . Its three p-electrons occupy three separate p-orbitals singly, giving a maximum of 3 unpaired p-electrons.
💡 Key Knowledge: Hund's Rule
- p-sub-shells contain 3 degenerate (equal energy) orbitals: pₓ, pᵧ, and p_z.
- Electrons will fill orbitals singly with parallel spins before pairing up to minimize electron-electron repulsion.
🧠 Exam Technique
Write out the full or outer sub-shell electron configurations for each option rather than guessing. Count the electrons in the p sub-shell and sketch empty/half-filled orbital boxes if needed to visualize pairing.
❌ Common Errors
- Option B (oxygen 2p⁴ ): Students often miscount total electrons or assume a higher atomic number automatically means more unpaired electrons, forgetting that pairing begins after the 3rd electron.
- Confusing total electrons with valence or sub-shell specific electrons.
Topics
Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.