AQA A-Level Chemistry AS Paper 1, November 2021: Question 23

1 mark · Medium difficulty · Multiple Choice

Identify which isoelectronic ion has the largest radius among F-, Mg2+, Na+, and O2-.

Practise this question

Question

Multiple choice question 23 asking 'Which ion has the largest radius?' worth 1 mark. The options are A: F-, B: Mg2+, C: Na+, and D: O2-, each with an empty box next to it for selection.
Question text

23 Which ion has the largest radius?

[1 mark]

A F–

B Mg2+

C Na+

D O2–

Mark scheme

Show the mark scheme Mark scheme indicating question 23 is answered by option D, corresponding to O2-.

23 D 1 O2–

How to answer it

Comparing Ionic Radii of Isoelectronic Species

What this question tests

This question assesses your understanding of periodic trends, specifically how ionic radius varies across isoelectronic species (ions that have the exact same number of electrons). You must apply your knowledge of nuclear charge (proton number) to determine the attractive force exerted on the electron cloud.

Question 2.3 [1 Mark]

Which ion has the largest radius?

Options: A: F⁻ | B: Mg²⁺ | C: Na⁺ | D: O²⁻

✅ Correct Answer

D: O²⁻

Awarded 1 mark for selecting D.

💡 Key Knowledge

  • All four ions ( F⁻ , Mg²⁺ , Na⁺ , O²⁻ ) are isoelectronic—they all possess 10 electrons (the same electron configuration as Neon: 1s² 2s² 2p⁶).
  • When comparing isoelectronic ions, size is determined entirely by the nuclear charge (number of protons).
  • Oxygen has the lowest atomic number (8 protons) among the choices, meaning it has the weakest attraction between the nucleus and the 10 electrons, resulting in the largest ionic radius.

🧠 Exam Technique

Whenever you see a multiple-choice question comparing ions or atoms with different charges, immediately check if they are isoelectronic. Count the electrons for each option, then look at the proton numbers to deduce the pull of the nucleus.

❌ Common Errors

  • Assuming neutral atomic trends apply directly: Students often look at standard atomic radius trends across a period and get confused by positive and negative ions.
  • Forgetting proton numbers: Confusing higher negative charge with smaller radius, or missing that O²⁻ has fewer protons than F⁻ (9 protons) and therefore holds its electrons less tightly.

Topics

Inorganic Chemistry · 3.2.1 Periodicity

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.