OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 6

1 mark · Medium difficulty · Multiple Choice

Identify which element has the largest third ionisation energy from a choice of four options.

Practise this question

Question

Multiple choice question 6 asking which element has the largest third ionisation energy, with options A (Li), B (F), C (Ne), and D (Na), alongside a box for the answer and a mark allocation of [1].
Question text

6 Which element has the largest third ionisation energy?

A Li

B F

C Ne

D Na

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 6 indicating the correct answer is A, with an allowance for writing Li, worth 1 mark and classified under AO1.2.

6 A 1 AO1.2 ALLOW Li

How to answer it

Comparing Third Ionisation Energies

What this question tests

This question assesses your understanding of successive ionisation energies, electronic configurations, and how removing an electron from a shell closer to the nucleus (a new principal quantum level) dramatically increases the energy required due to significantly less shielding and much greater electrostatic attraction.

Question 6

Exam Breakdown

✅ Correct Answer

A (Li)

Lithium has the electron configuration 1s² 2s¹ . Its first two electrons are removed from the 2s sub-shell, but the third electron must be removed from the inner 1s shell, which is much closer to the nucleus with virtually no shielding.

💡 Key Knowledge

  • Successive Ionisation Energies: Each subsequent electron removal requires more energy due to increasing positive charge relative to remaining electrons.
  • Shell Change: Removing an electron from a new, inner principal quantum shell causes a massive "leap" in ionisation energy.
  • Atomic Number Trends: Do not just pick the element with the highest atomic number (like Ne or Na)—always look at where the electron is being removed from!

🧠 Exam Technique

Write out the full electronic configurations for the relevant electron removal stage:

  • Li: Removes 3rd electron from 1s¹ (Inner shell!)
  • F: Removes 3rd electron from 2p sub-shell.
  • Ne: Removes 3rd electron from 2p sub-shell.
  • Na: Removes 3rd electron from 2p sub-shell.

❌ Common Errors

The "Biggest Atom / Highest Z" Trap: Many students automatically select Fluorine (F) or Neon (Ne) because they have high nuclear charges. However, for all of those elements (F, Ne, Na), the 3rd electron is still being removed from the outer 2nd shell. Only Lithium requires breaking into the 1s core shell for its 3rd electron.

Mark Scheme Note: 1 mark awarded for selecting A (or writing "Li"). Tested under AO1.2 (Demonstrating knowledge and understanding of scientific ideas).

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.