OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify which of the given electron configurations for chromium, copper, and an iron ion are correct

Practise this question

Question

Multiple choice question 15 asking to identify correct electron configurations among three statements: statement 1 for Cr atom 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1, statement 2 for Cu atom 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^1, and statement 3 for Fe^2+ ion 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1. Four options A, B, C, D give combinations of these statements, followed by a box for the answer and a mark allocation of [1].
Question text

15 Which electron configuration(s) is/are correct?

1 Cr atom: 1s22s22p63s23p63d54s1

2 Cu atom: 1s22s22p63s23p63d104s1

3 Fe2+ ion: 1s22s22p63s23p63d54s1

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table showing question number 15 with the correct answer B and 1 mark allocated.

15 B 1

Total 15

How to answer it

Transition Metal Electron Configurations

What this question tests

This question assesses your ability to write and recognize electron configurations for d-block transition metal atoms and their ions. It specifically tests your knowledge of the stability rules for chromium (Cr) and copper (Cu), and how electrons are removed when forming positive transition metal ions.

Question 15 • Multiple Choice

Analysis & Correct Answer

✅ Correct Option: B (Only 1 and 2)

Statements 1 and 2 show the correct anomalous ground-state configurations for Chromium and Copper. Statement 3 is incorrect because electrons are lost from the 4s orbital before the 3d orbital.

💡 Key Knowledge: Chromium & Copper Anomalies

  • Cr atom (Z=24): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ (half-filled d-subshell stability).
  • Cu atom (Z=29): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ (completely filled d-subshell stability).

❌ Common Errors & Why Statement 3 Fails

Many students incorrectly choose option A because they forget the rule for ionisation of d-block elements. When transition metals form ions, electrons are always removed from the 4s orbital first, before the 3d orbital.

  • Iron atom (Fe, Z=26) configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²
  • To form the Fe²⁺ ion, lose 2 electrons from 4s : 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ (Statement 3 incorrectly left a 4s electron behind).

🧠 Exam Technique

For multiple-choice questions involving multiple statements (1, 2, 3), evaluate each statement independently on your rough paper before looking at the lettered options. Cross out incorrect statements systematically to narrow down your choices.

Mark Scheme Allocation: 1 mark for selecting B.

Topics

Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · 2.2 Electrons, bonding and structure · 5.3 Transition elements

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.