---
title: "Transition Metals: Variable Oxidation State, Colour and Catalysis"
description: "A partly filled d sub-shell is what makes a transition metal a transition metal. It explains several oxidation states, coloured compounds, and why so many of these metals catalyse reactions."
canonical: https://lightmysky.com/learn/science/transition-metals-variable-oxidation-state-colour-and-catalysis-mt_UIVC6d7wak
source: https://lightmysky.com/learn/science/transition-metals-variable-oxidation-state-colour-and-catalysis-mt_UIVC6d7wak.md
retrieved: 2026-09-12
---

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# Transition Metals: Variable Oxidation State, Colour and Catalysis

A partly filled d sub-shell is what makes a transition metal a transition metal. It explains several oxidation states, coloured compounds, and why so many of these metals catalyse reactions.

Subject: Science · Area: Chemistry · Ages 17 to 18
Page: https://lightmysky.com/learn/science/transition-metals-variable-oxidation-state-colour-and-catalysis-mt_UIVC6d7wak

## Ready when they can

- Defines a transition element by its d sub-shell and explains why scandium and zinc do not qualify
- Gives the common oxidation states of a named transition metal with a compound of each
- Explains colour as a d sub-shell split by ligands, with part of the visible spectrum absorbed
- Names a heterogeneous and a homogeneous catalyst and explains the part the oxidation states play

## Lesson: What makes a transition metal

A transition metal has a partly filled d sub-shell, or easily forms an ion with one. Scandium only reaches Sc3+ with an empty d sub-shell, and zinc sits at Zn2+ with a full one, so neither qualifies even though both sit in the block.

**Example.** Iron shows the flexible side: Fe2+ in FeCl2 and Fe3+ in FeCl3. Reaching for different electron counts is exactly what the roomy d sub-shell allows, and you can name a compound for each state.

Ligands split the d sub-shell into new levels, and electrons jumping the gap swallow part of the visible light. Copper compounds look coloured, while zinc compounds stay white, since a full sub-shell has no gap to jump.

**Tip.** Many of these metals catalyse because their oxidation states flip back and forth mid-route. Nitric oxide gas catalyses ozone loss the same way gases meet, with nitrogen going +2 to +4 and back, while a car converter works with gases streaming past a catalyst that stays put. A catalyst in a different phase from the reactants is heterogeneous, like the converter surface with exhaust gases. One mixed in the same phase is homogeneous, like NO gas with ozone.

**Recap.** Part-filled d shells bring many states, split levels bring colour, and flipping states brings catalysis.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [The Rate-Determining Step and Reaction Mechanisms](https://lightmysky.com/learn/science/the-rate-determining-step-and-reaction-mechanisms-mt_PM1Vo2CkVD)
- [Orbitals, Sub-Shells and Electron Configuration](https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ)
- [Redox Titrations with Manganate(VII) and Thiosulfate](https://lightmysky.com/learn/science/redox-titrations-with-manganate-vii-and-thiosulfate-mt_tKZTSfo0Ra)

## Opens up

- [The Beer-Lambert Law and Quantitative Absorption](https://lightmysky.com/learn/science/the-beer-lambert-law-and-quantitative-absorption-mt_JGdJxTKxav)
- [Complex Ions, Ligands and Their Shapes](https://lightmysky.com/learn/science/complex-ions-ligands-and-their-shapes-mt_xxWYYqgZp_)
- [Crystal Field Theory and the Splitting of d Orbitals](https://lightmysky.com/learn/science/crystal-field-theory-and-the-splitting-of-d-orbitals-mt_YfIBGXj8su)