OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify which sequence shows elements in order of increasing first ionisation energy.

Practise this question

Question

Multiple choice question 8 asking which sequence has elements in order of increasing first ionisation energy. Options are A: Na < Mg < Al, B: Mg < Al < Si, C: Al < Si < P, and D: Si < P < S, with an answer box at the bottom and [1] mark allocation.
Question text

8 Which sequence has elements in order of increasing first ionisation energy?

A Na < Mg < Al

B Mg < Al < Si

C Al < Si < P

D Si < P < S

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme shows the correct answer for question 8 is C, worth 1 mark, classified under AO1.2.

8 C 1 AO1.2

How to answer it

First Ionisation Energy Trends across Period 3

OCR AS Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your understanding of the periodic trends in first ionisation energy across Period 3 elements. Specifically, it tests your knowledge of the general increase in nuclear charge across a period, as well as the crucial anomalies (dips) that occur due to subshell configurations (specifically between Group 2 and Group 3, and between Group 5 and Group 6).

Question 8 Analysis

Identifying the Correct Sequence

✅ Correct Answer: C

Sequence Al < Si < P correctly follows the order of increasing first ionisation energy.

  • Aluminium (Al) has a lower ionisation energy than expected due to shielding by the 3s subshell (removal from 3p).
  • Silicon (Si) has a higher nuclear charge than Al, leading to a higher ionisation energy.
  • Phosphorus (P) continues the upward trend across the period.

💡 Key Knowledge

  • General Trend: First ionisation energy generally increases across a period due to decreasing atomic radius and increasing nuclear charge with constant shielding.
  • The Group 2/3 Dip (Mg to Al): Al drops because its outer electron is in a higher-energy 3p orbital, which is shielded by the 3s² electrons.
  • The Group 5/6 Dip (P to S): S drops due to spin-pairing repulsion in one of its 3p orbitals.

🧠 Exam Technique

  • Check every option: Do not just look at the first two elements. Evaluate the entire inequality sequence given in each option.
  • Sketch the trend: Quickly sketch the Period 3 ionisation energy zig-zag graph on your rough paper if you are unsure of the dips.

❌ Common Errors

  • Assuming ionisation energy strictly increases linearly across the whole period without any drops.
  • Failing to spot the drop between magnesium and aluminium (option A falsely claims Mg < Al).
  • Confusing group trends with period trends.
Mark Scheme Allocation: 1 mark awarded for selecting option C (AO1.2 - Demonstrate knowledge and understanding of scientific trends).

Topics

Module 3: Periodic table and energy · Module 2: Foundations in chemistry · 3.1 The periodic table · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.