AQA A-Level Chemistry Paper 3, 2024: Question 6

1 mark · Easy difficulty · Multiple Choice

Identify which row contains two species with different numbers of electrons.

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Question

AQA A-Level Chemistry Paper 3, 2024: Question 6
Question text

06 Which row contains two species with different numbers of electrons?

[1 mark]

A NH3 and HF

B CO 2– and NO –

C H O+ and HF +

D CH and NH –

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2024: Question 6

Question Marking Guidance Mark Comments

06 C 1 (AO2) H O+ and HF +

How to answer it

Counting electrons in molecules and ions

What this question tests

Core skill

  • Calculate total number of electrons in a species from atomic numbers.
  • Adjust electron count correctly for ionic charge (add for −, subtract for +).
  • Spot “isoelectronic” pairs vs pairs with different electron totals.

Exam focus

  • Work quickly but systematically: sum atomic electrons, then apply charge at the end.
  • Don’t confuse electrons with valence electrons or oxidation states.
Question 06 • 1 mark (AO2)

Which row contains two species with different numbers of electrons?

Mark scheme answer: C (H₃O⁺ and HF₂⁺).

✅ Correct answer (what to write/choose)

C — H₃O⁺ and HF₂⁺ have different total numbers of electrons.

  • H₃O⁺: 10 electrons
  • HF₂⁺: 18 electrons

💡 Key knowledge

  • Atomic numbers (electrons in neutral atoms): H = 1, C = 6, N = 7, O = 8, F = 9.
  • Total electrons in a species = sum of atomic numbers ± charge adjustment.
  • Charge rule: +1 means subtract 1 electron; −1 means add 1 electron; −2 means add 2, etc.

📐 Calculations (fast, step-by-step)

Method: sum neutral-atom electrons → then apply charge once at the end.

Part (06) Option C: H₃O⁺ vs HF₂⁺

  1. H₃O⁺: H₃ gives 3×1 = 3; O gives 8 → total (neutral) = 3 + 8 = 11
  2. Apply +1 charge: 11 − 1 = 10 electrons
  3. HF₂⁺: H gives 1; F₂ gives 2×9 = 18 → total (neutral) = 1 + 18 = 19
  4. Apply +1 charge: 19 − 1 = 18 electrons
  5. 10 ≠ 18, so they have different numbers of electrons → Option C.

🧠 Exam technique (how to guarantee the mark)

  • Do not “eyeball” it. The options are designed so several pairs are isoelectronic.
  • Use a quick table or mental sums. Example pattern: “sum atomic numbers, then ± charge”.
  • Check you applied the sign correctly: + means fewer electrons, − means more.

❌ Common errors (why students lose this mark)

  • Forgetting the charge adjustment (treating ions as neutral molecules).
  • Adding electrons for a positive ion (it’s the opposite: you subtract).
  • Counting valence electrons instead of total electrons. The question asks total electrons in the whole species.
  • Mixing up “number of electrons” with “number of electron pairs” or bond pairs.

Extra check: why the other options are NOT correct

Option A: NH₃ and HF

  • NH₃: N (7) + 3×H (3) = 10 electrons
  • HF: H (1) + F (9) = 10 electrons
  • Same → not the row you want.

Option B: CO₃²⁻ and NO₃⁻

  • CO₃²⁻: C (6) + 3×O (24) = 30; then −2 means +2 e⁻ → 32
  • NO₃⁻: N (7) + 3×O (24) = 31; then −1 means +1 e⁻ → 32
  • Same → not correct.

Option D: CH₄ and NH₂⁻

  • CH₄: C (6) + 4×H (4) = 10 electrons
  • NH₂⁻: N (7) + 2×H (2) = 9; then −1 means +1 e⁻ → 10
  • Same → not correct.

Examiner insight (what distinguishes top responses)

  • This is a 1-mark AO2 multiple-choice item: full marks come from accurate application of electron counting, not extended explanation.
  • High-scoring students typically spot that three options form isoelectronic pairs (A, B, D), leaving one option as the “odd one out”.
  • Most mistakes come from ion charge direction (mixing up add/subtract) and from not including the charge at all.

Topics

Physical Chemistry · 3.1.1 Atomic Structure

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.