AQA A-Level Chemistry AS Paper 1, June 2022: Question 10
1 mark · Easy difficulty · Multiple Choice
Identify which atom in its ground state contains only two unpaired electrons.
Practise this questionQuestion
Question text
10 An atom has all its electrons in their lowest energy levels.
Which atom contains only two unpaired electrons?
[1 mark]
A Helium
B Beryllium
C Oxygen
D Iron
Mark scheme
Show the mark scheme
10 C (AO1) 1 Oxygen
How to answer it
Determining Unpaired Electrons in Ground-State Atoms
This question evaluates fundamental knowledge of atomic structure and electronic configurations (AQA Chemistry Specification 3.1.1):
- Writing ground-state sub-shell electron configurations for s, p, and d block elements.
- Applying Hund's Rule (electrons occupy orbitals singly before pairing up) and the Pauli Exclusion Principle.
- Distinguishing between total outer shell electrons, paired electrons, and unpaired electrons.
Identifying the Atom with Exactly Two Unpaired Electrons
AQA Chemistry Paper 1 • Section B • AO1 Recall & Application
✅ Correct Answer: C (Oxygen)
An isolated ground-state oxygen atom contains exactly 2 unpaired electrons in its outer 2p sub-shell.
🧠 Exam Technique: Systematic Orbital Audit
When an exam question asks for unpaired electrons:
- Find the atomic number (Z) on the Periodic Table provided.
- Write out the full sub-shell configuration (e.g., 1s² 2s² 2p⁴).
- Identify the highest energy, partially filled sub-shell. Fully filled sub-shells always have 0 unpaired electrons!
- Draw orbital boxes for that partially filled sub-shell to see unpaired spins clearly.
📐 Step-by-Step Analysis of Each Option
| Option | Atomic Number (Z) | Full Electronic Configuration | Orbital Diagram of Incomplete Sub-shell | Unpaired Electrons |
|---|---|---|---|---|
| A Helium | 2 | 1s² | 1s orbital is completely full: [ ↑↓ ] | 0 |
| B Beryllium | 4 | 1s² 2s² | Both 1s and 2s are completely full: [ ↑↓ ] | 0 |
| C Oxygen | 8 | 1s² 2s² 2p⁴ | Three 2p orbitals: [ ↑↓ ][ ↑ ][ ↑ ] (Hund's Rule) | 2 (Correct!) |
| D Iron | 26 | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² | Five 3d orbitals: [ ↑↓ ][ ↑ ][ ↑ ][ ↑ ][ ↑ ] | 4 |
💡 Key Knowledge: Hund's Rule of Maximum Multiplicity
- Ground State: "Lowest energy levels" means no electrons have been promoted to higher orbitals.
- p Sub-shell: Contains 3 degenerate orbitals (px, py, pz). In oxygen's 2p⁴ , electrons fill each orbital singly before pairing up:
• Orbital 1: paired (2 electrons)
• Orbital 2: unpaired (1 electron)
• Orbital 3: unpaired (1 electron)
Total unpaired = 2. - d Sub-shell: Contains 5 degenerate orbitals. In iron's 3d⁶ , one orbital is paired and four are singly occupied, giving 4 unpaired electrons.
❌ Common Student Pitfalls
- Confusing Beryllium with unpaired electrons: Beryllium has two valence electrons ( 2s² ), but they are paired with opposite spins in the 2s orbital, leaving 0 unpaired.
- Confusing unpaired electrons with vacancies: Oxygen needs 2 electrons to complete its octet, which coincidentally matches the number of unpaired electrons (2). Don't rely on this coincidence—iron needs 4 to half-fill or 4 to fill up, but configuration rules differ!
- Forgetting 3d in Iron: Students often mistakenly write 3d⁶ as three paired orbitals (3 × 2 = 6), missing Hund's rule entirely and thinking Iron has 0 unpaired electrons.
Topics
Physical Chemistry · 3.1.1 Atomic Structure
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.