OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 2

1 mark · Easy difficulty · Multiple Choice

Determine the number of p electrons in an atom of chlorine.

Practise this question

Question

Question 2 asks: 'How many p electrons does an atom of chlorine have?' followed by four multiple choice options: A 5, B 7, C 11, and D 17, with an answer box at the bottom worth 1 mark.
Question text

2 How many p electrons does an atom of chlorine have?

A 5

B 7

C 11

D 17

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table row showing question 2 has correct answer C, with 1 mark awarded, and an additional guidance note 'ALLOW 11'.

2 C 1 ALLOW 11

How to answer it

Electron Configurations: Subshells and Orbitals

📋 What this question tests

This question tests your ability to determine the complete electron configuration of a Period 3 atom using subshell notation (s, p, d) and correctly count the total number of electrons occupying a specific subshell type ( p ) across all principal energy levels.

Question 2 • Multiple Choice [1 Mark]

Chlorine p-Electron Count

Identifying total p electrons in a neutral chlorine atom

✅ Correct Answer

C — 11

A neutral chlorine atom has an atomic number of 17. Writing out the full configuration:

1s² 2s² 2p⁶ 3s² 3p⁵

Adding the electrons in all p subshells: 6 (from 2p) + 5 (from 3p) = 11.

📐 Step-by-Step Breakdown

  1. Find atomic number: Chlorine (Cl) is element 17, so a neutral atom has 17 electrons.
  2. Fill subshells in order of increasing energy:
    1s² (2 electrons)
    2s² 2p⁶ (8 electrons)
    3s² 3p⁵ (7 electrons)
  3. Sum only the p subshell electrons:
    Number of p electrons = 6 (in 2p) + 5 (in 3p) = 11.

💡 Key Knowledge

  • An s subshell holds a maximum of 2 electrons (1 orbital).
  • A p subshell holds a maximum of 6 electrons (3 orbitals: px, py, pz).
  • Chlorine is in Group 17 (Group 7) and Period 3, meaning its outermost electrons reside in the 3p subshell ( 3s² 3p⁵ ).

❌ Distractor Breakdown & Traps

  • Trap A (5): Only counted the outer 3p⁵ electrons and forgot the inner 2p⁶ electrons.
  • Trap B (7): Counted the total number of valence electrons ( 3s² 3p⁵ ) rather than the total p electrons.
  • Trap D (17): Stated the total number of electrons in chlorine, ignoring the subshell type entirely.

🧠 Exam Technique & Examiner Commentary

Always write down the full subshell configuration in the margin next to multiple-choice questions like this rather than trying to do it mentally. It takes less than 10 seconds and prevents the classic slip of confusing:

  • Outer shell p electrons (5) vs. Total p electrons (11)
  • Valence electrons (7) vs. Total electrons (17)
Mark Scheme Note: 1 mark for selecting C (the letter C or the number 11 written in the answer box is credited).

Topics

Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.