AQA A-Level Chemistry Paper 3, June 2022: Question 6

Atomic Structure: Unpaired p Electrons

AQA A-Level Chemistry • Physical Chemistry • Paper 1 Multiple Choice

What this question tests

This question assesses your ability to deduce full ground-state electron configurations for atoms across the s, p, and d blocks, and apply Hund's rule to determine the distribution and pairing of electrons within specific subshells (specifically p orbitals).

Question 06 Breakdown

Identifying ground-state atoms with unpaired p electrons

Question: Which atom in the ground state contains at least one unpaired p electron? [1 mark]
Options: A: Na | B: Ne | C: O | D: Sc

✅ Correct Answer: C (Oxygen, O)

Oxygen has an atomic number of 8. Its ground-state electron configuration is:
1s² 2s² 2p⁴

The 2p subshell has three degenerate orbitals. By Hund's rule, electrons fill each orbital singly before pairing:

Orbital 1: [ ↑↓ ] (paired)
Orbital 2: [ ↑ ] (unpaired)
Orbital 3: [ ↑ ] (unpaired)

Oxygen contains two unpaired p electrons, satisfying the condition of having "at least one".

💡 Key Knowledge

Aufbau Principle: Orbitals fill in order of increasing energy: 1s < 2s < 2p < 3s < 3p < 4s < 3d .
Hund's Rule: Orbitals in the same subshell are each occupied singly with parallel spins before any orbital is doubly occupied.
Full Subshells: A filled subshell ( p⁶ , s² , d¹⁰ ) contains zero unpaired electrons.
Only partially filled p subshells ( p¹ to p⁵ ) can have unpaired p electrons.

📐 Step-by-Step Analysis of All Options

Element	Atomic No. (Z)	Full Ground State Configuration	Unpaired Electrons Present?
A: Na	11	1s² 2s² 2p⁶ 3s¹	Has 1 unpaired s electron; all p electrons are paired in a full 2p⁶ subshell. ❌
B: Ne	10	1s² 2s² 2p⁶	Noble gas; all subshells are completely filled. 0 unpaired electrons. ❌
C: O	8	1s² 2s² 2p⁴	The 2p subshell has 4 electrons across 3 orbitals → 2 unpaired p electrons. ✅
D: Sc	21	1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹ 4s²	Has 1 unpaired d electron; both 2p⁶ and 3p⁶ are completely full. ❌

🧠 Exam Technique: Keyword Precision

Underline qualifying words in multiple choice questions:

Notice the question specifically asks for an unpaired p electron, not just any unpaired electron.
Eliminate noble gases immediately ( Ne has fully filled subshells).
Eliminate elements where the partially filled subshell is not a p subshell ( Na is an s-block metal ending in 3s¹ ; Sc ends in 3d¹ ).

❌ Common Errors & Traps

Misreading "p electron": Selecting A (Na) or D (Sc) because they have an odd number of total electrons or an obvious single outer electron ( 3s¹ and 3d¹ ).
Assuming paired means filled: Forgetting that p⁴ still has two unpaired electrons because a full p subshell holds 6 electrons.
Incorrect 4s/3d filling: Writing Scandium's configuration incorrectly or thinking scandium is in the p block.