---
title: "Colour of vanadium +5 vs +3 compounds"
subject: "Engineering Chemistry"
chapter: 5
key: "semester_2/engineering_chemistry:v5-v3-colour"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/v5-v3-colour
site: BCT Engineering
---

# Colour of vanadium +5 vs +3 compounds

Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications

- Asked in: 2071 Bhadra, 2.5 marks
- Video: [Why Does Vanadium Turn Four Colours? Variable Oxidation States | A-Level Chemistry](https://www.youtube.com/watch?v=rt4iojTbbCk) — Pass my GCSE science
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/v5-v3-colour
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5

## Question

Give reasons: The components formed by symbol 'V' element in +5 oxidation state are colourless but those formed in +3 oxidation state are colourful.

## Short answer

V⁵⁺ is 3d⁰ with no d electrons, so no d–d transition can occur and it appears colourless. V³⁺ is 3d²; the two d electrons can undergo d–d transitions by absorbing visible light, giving coloured compounds.

## Answer

Colour in transition metal compounds is due to d–d transitions, which need at least one d electron.

- V⁵⁺ has the configuration [Ar] 3d⁰. It has no d electron, so no d–d transition can occur and no visible light is absorbed. Such compounds show no colour from d–d transitions (any pale colour, as in V₂O₅, comes from charge transfer).
- V³⁺ has the configuration 3d². The two d electrons can be excited from t₂g to e_g by absorbing visible light, so V(III) compounds are coloured (for example [V(H₂O)₆]³⁺ is green).

Hence V(III) compounds are coloured while d⁰ V(V) compounds are colourless.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is needed for a d-d transition colour?
   A. A completely empty d subshell
   B. A completely filled d subshell
   C. At least one d electron
   D. Only s electrons
   Answer: C. With no d electron there is nothing to excite. (Key point: Condition for d-d colour)

2. Why are V(V) compounds free of d-d colour?
   A. V5+ is 3d2 with paired electrons
   B. V5+ is 3d10
   C. V5+ has too many unpaired electrons
   D. V5+ is 3d0, with no d electron
   Answer: D. d0 ions cannot undergo d-d transitions. (Key point: V5+ colourless)

3. Why is [V(H2O)6]3+ coloured (green)?
   A. V3+ is 3d2 and the d electrons are excited t2g to e_g by visible light
   B. V3+ is 3d0
   C. V3+ is 3d10
   D. V3+ absorbs only UV light by s-p transitions
   Answer: A. The two d electrons absorb visible light in a d-d transition. (Key point: V3+ coloured)

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