OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 5

1 mark · Medium difficulty · Multiple Choice

Identify the correct statement about elements in the d block of Period 4 regarding electron configurations and ion properties.

Practise this question

Question

Multiple choice question 5 asking which statement about elements in the d block of Period 4 of the periodic table is correct. Option A states Cr atoms have the electron configuration 1s2 2s2 2p6 3s2 3p6 3d5 4s1. Option B states Cu+ ions contain an incomplete 3d sub-shell. Option C states Fe2+ ions contain 3 unpaired electrons. Option D states Sc forms ions with different oxidation states. An answer box is provided at the bottom.
Question text

5 Which statement about elements in the d block of Period 4 of the periodic table is correct?

A Cr atoms have the electron configuration: 1s22s22p63s23p63d54s1.

B Cu+ ions contain an incomplete 3d sub-shell.

C Fe2+ ions contain 3 unpaired electrons.

D Sc forms ions with different oxidation states.

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table indicates the correct answer for question 5 is option A.

5 A 1 1.1

How to answer it

Transition Elements: d-Block Electron Configurations

What this question tests

This question assesses your understanding of Period 4 d-block transition metal chemistry, specifically focusing on anomalous electron configurations (chromium), ionisation and electron removal order (4s before 3d), Hund's rule and pairing in d-orbitals, and the unique oxidation states characteristic of early transition metals like scandium.

Question 5 Analysis

Correct Answer: A

✅ Why Option A is Correct

Chromium (Cr) has an atomic number of 24. Rather than following the expected filling order of 3d⁴4s² , it exhibits an electronic configuration anomaly where an electron promotes from the 4s orbital to the 3d orbital to achieve a stable half-filled d sub-shell ( 3d⁵4s¹ ).

💡 Key Knowledge: Anomalous Configurations

  • Chromium (Cr): [Ar] 3d⁵4s¹ (half-filled sub-shell stability)
  • Copper (Cu): [Ar] 3d¹⁰4s¹ (fully-filled sub-shell stability)
  • Always fill and empty the 4s orbital before the 3d orbital when dealing with transition metal ions.

🧠 Exam Technique

Evaluate each statement systematically rather than guessing. Eliminate incorrect options by writing out full configurations or ion electron structures on your rough paper before selecting your final answer.

Mark Scheme Allocation: 1 mark awarded for selecting A. (Specification reference: 1.1 Transition Elements).
Distractor Breakdown

Why the Other Options are Incorrect

❌ Why B is Incorrect (Cu⁺ ions)

Neutral copper has the configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ . When forming a Cu⁺ ion, the single 4s electron is lost first, leaving 3d¹⁰ . This means the 3d sub-shell is completely full, not incomplete.

❌ Why C is Incorrect (Fe²⁺ ions)

Neutral iron (Fe) is [Ar] 3d⁶ 4s² . Fe²⁺ loses two electrons from the 4s orbital to become [Ar] 3d⁶ . In a 3d⁶ configuration across five degenerate orbitals, electrons pair up in the first orbital, leaving 4 single (unpaired) electrons—not 3.

❌ Why D is Incorrect (Scandium)

Scandium (Sc) is a d-block element, but it is not a transition element because it only forms one stable ion ( Sc³⁺ with a 3d⁰ configuration). It does not exhibit variable oxidation states.

Topics

Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · 5.3 Transition elements · 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 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.