OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 15
1 mark · Medium difficulty · Multiple Choice
Identify which of the given transition metal ions contain one or more unpaired electrons.
Practise this questionQuestion
Question text
15 Which ion(s) contain(s) one or more unpaired electrons?
1 Mn3+
2 V3+
3 Cu+
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
15 B 1 1.2
Total 15
How to answer it
Transition Metal Ions & Unpaired Electrons
What this question tests
This question assesses your ability to determine electron configurations of d-block transition metal atoms and their ions, understanding the correct filling and emptying order of 4s and 3d orbitals, and applying Hund's rule to identify unpaired electrons.
Determining Unpaired Electrons in Transition Metal Ions
✅ Correct Answer: B (Only 1 and 2)
Ions Mn³⁺ and V³⁺ both contain unpaired d-electrons, whereas Cu⁺ has a completely full 3d sub-shell with zero unpaired electrons.
💡 Key Knowledge
- Filling/Removal order: Electrons fill the 4s orbital before the 3d orbital, but are removed from the 4s orbital first when forming positive ions.
- Hund's Rule: Orbitals of equal energy (degenerate) fill singly first before pairing up.
- Sub-shell capacities: A fully filled d-sub-shell holds 10 electrons ( d¹⁰ ).
🧠 Exam Technique
Break down each option systematically by writing out the full electron configuration of the neutral atom first, removing electrons from 4s before 3d, and then mapping out the d-orbital box notation to check for pairing.
❌ Common Errors
Students frequently lose marks here by removing electrons from the 3d sub-shell instead of the 4s sub-shell first, or by forgetting that Cu⁺ has a stable 3d¹⁰ configuration.
📐 Step-by-Step Analysis of Each Ion
- Statement 1: Mn³⁺
Neutral Mn (Z = 25): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵
Remove 3 electrons (two from 4s, one from 3d): 3d⁴ .
Four electrons in separate 3d orbitals mean there are 4 unpaired electrons. (Valid) - Statement 2: V³⁺
Neutral V (Z = 23): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³
Remove 3 electrons (two from 4s, one from 3d): 3d² .
Two electrons in separate 3d orbitals mean there are 2 unpaired electrons. (Valid) - Statement 3: Cu⁺
Neutral Cu (Z = 29, note the 4s/3d electron promotion anomaly): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
Remove 1 electron (from the 4s orbital): 3d¹⁰ .
All 10 electrons are paired in the 3d orbitals, resulting in 0 unpaired electrons. (Invalid)
Topics
Module 5: Physical chemistry and transition elements · 5.3 Transition elements
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.