AQA A-Level Chemistry Paper 3, June 2025: Question 21
1 mark · Easy difficulty · Multiple Choice
Identify which electron configuration corresponds to an element with a maximum oxidation state of +5 in its compounds.
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Question text
21 Which electron configuration represents an element that can have a
maximum oxidation state of +5 in its compounds?
[1 mark]
A 1s2 2s2 2p1
B 1s2 2s2 2p6 3s2 3p5
C 1s2 2s2 2p6 3s2 3p6 4s2 3d3
D 1s2 2s2 2p6 3s2 3p6 4s2 3d5
Mark scheme
Show the mark scheme
21 C 1 (AO1) 1s2 2s2 2p6 3s2 3p6 4s2 3d3
How to answer it
Electron Configurations & Maximum Oxidation States
- Transition Metal Chemistry: Identifying elements from their full electron configurations and linking outer sub-level electrons to variable oxidation states.
- Maximum Oxidation State Rules: Understanding that for first-row transition elements up to manganese, the highest oxidation state corresponds to the loss/involvement of all 4s and unpaired 3d electrons.
- Main Group vs. Transition Elements: Recognising the difference between s-, p-, and d-block valence shells.
Question 21
Multiple Choice Analysis • [1 Mark]
✅ Correct Answer
Option C: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³
📐 Element Identification & Max States
| Option | Element | Outer Electrons | Max Oxidation State |
|---|---|---|---|
| A | Boron (B) | 2s² 2p¹ | +3 |
| B | Chlorine (Cl) | 3s² 3p⁵ | +7 (e.g., ClO₄⁻) |
| C | Vanadium (V) | 4s² 3d³ | +5 (e.g., VO₂⁺) |
| D | Manganese (Mn) | 4s² 3d⁵ | +7 (e.g., MnO₄⁻) |
💡 Key Knowledge
- For early 3d transition metals (Sc to Mn), the maximum oxidation state equals the sum of the 4s electrons and the 3d electrons:
Max Oxidation State = Number of 4s electrons + Number of 3d electrons - For Vanadium ( 4s² 3d³ ): 2 + 3 = +5. Well-known species include the dioxovanadium(V) ion, VO₂⁺ , and vanadium(V) oxide, V₂O₅ .
- Chlorine is in Group 7 (17) and has 7 valence electrons, so its maximum oxidation state is +7 (not +5, even though +5 exists in chlorate(V) ClO₃⁻ ).
🧠 Exam Technique
- Notice the word "maximum": The question does not ask which element can form a +5 oxidation state, but which has a maximum of +5. Chlorine forms +5 in ClO₃⁻ , but its maximum is +7!
- Count valence electrons: Add together the electrons beyond the noble gas core (Ar core: 18 electrons).
• Option C has 23 electrons: 23 − 18 = 5 outer electrons → maximum state of +5.
❌ Common Errors
- Selecting Option B (Chlorine): Students often remember compounds like KClO₃ where chlorine is in the +5 oxidation state and jump to this option, overlooking that chlorine can reach +7 in perchlorates ( ClO₄⁻ ).
- Confusing Option C and D: Miscounting d-electrons under timed conditions. Manganese ( 4s² 3d⁵ ) has 7 valence electrons, giving a maximum oxidation state of +7 in MnO₄⁻ .
- Only counting 3d electrons: Forgetting that 4s electrons are of lower energy when empty/lost first, but both 4s and 3d electrons participate in bonding for early transition metals.
Topics
Physical Chemistry · Inorganic Chemistry · 3.1.1 Atomic Structure · 3.2.5 Transition Metals · 3.1.7 Oxidation, Reduction and Redox Equations
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.