OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 7

1 mark · Medium difficulty · Multiple Choice

Identify the row showing elements in order of increasing first ionisation energy among Ca, Si, P, and N.

Practise this question

Question

Multiple-choice question 7 asking which row shows elements in order of increasing first ionisation energy. Options are A: Ca < Si < P < N, B: N < P < Si < Ca, C: Ca < N < P < Si, D: C < Si < P < Ca. Includes an answer box and is worth 1 mark.
Question text

7 Which row shows elements in order of increasing first ionisation energy?

A Ca < Si < P < N

B N < P < Si < Ca

C Ca < N < P <Si

D C < Si < P < Ca

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing the correct answer for question 7 is A, with 1 mark allocated.

7 A 1 AO1.1

How to answer it

Comparing First Ionisation Energies

What this question tests

This question assesses your understanding of periodic trends in first ionisation energy across periods and down groups, including the ability to combine these trends into an overall ascending sequence.

Question 7

Exam Breakdown

✅ Correct Answer: A

The correct sequence in order of increasing first ionisation energy is: Ca < Si < P < N

💡 Key Knowledge

  • Down a group: First ionisation energy decreases (e.g., Calcium is much lower than elements in Periods 2 and 3 due to increased shielding and atomic radius).
  • Across a period: Generally increases due to increasing nuclear charge and decreasing atomic radius.
  • Sub-shell dips: Group 15 (P, N) has a higher first ionisation energy than Group 16, but also note anomalies between groups (though here we compare Si (Group 14) and P (Group 15)).

🧠 Exam Technique

Tackle multiple-choice questions on ionisation energy by identifying extreme values first. Calcium (Ca) is in Period 4, meaning its outer electrons are much further from the nucleus with more shielding, giving it the lowest value by far. This immediately narrows your options down to A or C.

❌ Common Errors

Students frequently misremember the relative positions of Period 2 and Period 3 elements or forget that descending a group drastically lowers ionisation energy, leading them to incorrectly select options like B or D.

Mark Allocation: 1 mark available for identifying option A (AO1.1 Knowledge and Understanding).

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, Periodic table, elements and physical chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.