AQA A-Level Chemistry AS Paper 1, June 2022: Question 17

1 mark · Easy difficulty · Multiple Choice

Identify which of the given isoelectronic ions (S²⁻, Cl⁻, K⁺, Ca²⁺) has the largest ionic radius.

Practise this question

Question

Question 17 asks 'Which of these ions has the largest ionic radius?' with four multiple choice options: A S²⁻, B Cl⁻, C K⁺, and D Ca²⁺, each accompanied by a tick box.
Question text

17 Which of these ions has the largest ionic radius?

[1 mark]

A S2–

B Cl–

C K+

D Ca2+

Mark scheme

Show the mark scheme Mark scheme for question 17 showing correct answer A (AO3) with key S²⁻ worth 1 mark.

17 A (AO3) 1 S2–

How to answer it

Comparing Ionic Radii of Isoelectronic Species

📋 What this question tests

This question assesses your understanding of periodicity, electron configurations, and the factors determining ionic radius across an isoelectronic series (species with identical numbers of electrons and identical shielding, but differing nuclear charges).

Question 17

Multiple Choice: Largest Ionic Radius [1 Mark]

✅ Correct Answer

A: S²⁻

Mark allocation:
• 1 mark for selecting option A (Assessment Objective AO3).

💡 Key Knowledge: The Isoelectronic Principle

  • All four ions are isoelectronic: each has 18 electrons with the argon configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ .
  • Because they have the same number of electrons in the same shells, shielding is identical for all four ions.
  • The radius is determined entirely by the nuclear charge (number of protons).
  • Fewer protons = weaker nuclear pull on the outer electron shell = larger radius.

📐 Step-by-Step Comparative Analysis

Compare the proton-to-electron balance for each option:

Ion Protons (Z) Electrons Nuclear Pull on Electrons Ionic Radius
S²⁻ 16 18 Weakest (16 protons pulling 18 e⁻) Largest
Cl⁻ 17 18 Moderate Large
K⁺ 19 18 Strong Small
Ca²⁺ 20 18 Strongest (20 protons pulling 18 e⁻) Smallest

Order of increasing radius: Ca²⁺ < K⁺ < Cl⁻ < S²⁻

🧠 Exam Technique & Quick Rule

For any isoelectronic series:

  • Most negative ion = largest radius (least protons per electron).
  • Most positive ion = smallest radius (most protons per electron).
  • In multiple-choice questions, spot the isoelectronic relationship immediately by checking the total electrons ( 16+2 = 17+1 = 19-1 = 20-2 = 18 ). The answer for "largest" is always the anion with the highest negative charge.

❌ Common Errors & Pitfalls

  • Confusing mass/atomic number with radius: Assuming Ca²⁺ must be the largest because calcium has the highest atomic number ( Z = 20 ) or is furthest right/down on the periodic table.
  • Mixing up atomic and ionic radii: A neutral calcium atom is larger than a neutral sulfur atom, but losing 2 electrons sheds an entire outer quantum shell and increases effective nuclear pull, making Ca²⁺ much smaller.
  • Overlooking electron gain/loss: Forgetting that anions expand relative to neutral atoms due to increased electron-electron repulsion and lower proton-to-electron ratio.

Topics

Inorganic Chemistry · Physical Chemistry · 3.2.1 Periodicity · 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.