--- 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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 element forms at least one stable ion with a partly filled d sub-shell. Scandium forms only Sc3+, which has no d electrons. Zinc forms only Zn2+, whose d sub-shell is full with 10 electrons. Neither has an ion with a partly filled d sub-shell, so neither counts as a transition element, even though both sit in the d block and the scandium atom itself is 3d1. When these atoms form ions, the 4s electrons are lost before the 3d ones. *(drawing: Only a partly filled d sub-shell passes the test.)* **Example.** Iron shows two common oxidation states: +2 in iron(II) chloride, FeCl2, and +3 in iron(III) chloride, FeCl3. The 4s and 3d electrons are close in energy, so iron can lose different numbers of them. Each state keeps a partly filled d sub-shell: Fe2+ is 3d6 and Fe3+ is 3d5. In a complex, ligands (molecules or ions bonded to the metal by a lone pair) split the d sub-shell into two sets of levels. A d electron can absorb visible light and jump to the upper set, and we see the colours left over. Cu2+ is 3d9, so the jump is possible and copper(II) compounds are coloured. In Zn2+ every d orbital is full, so no electron can move up, and zinc compounds are white. Different ligands give a different split, so the colour changes when the ligands change. Many transition metals catalyse reactions because they can change oxidation state and change back. A heterogeneous catalyst is in a different phase from the reactants, like solid iron with nitrogen and hydrogen gases in the Haber process. In the Contact process, solid vanadium(V) oxide goes from +5 to +4 and back. A homogeneous catalyst is in the same phase: Fe2+ ions catalyse peroxodisulfate reacting with iodide ions in solution, going to Fe3+ and back to Fe2+. **Recap.** A partly filled d sub-shell gives several oxidation states, split d levels give colour, and changing oxidation state and back lets these metals catalyse. *Original chemistry lesson and assessment text written for os-taxonomy.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [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) ## Optional connections - [The Rate-Determining Step and Reaction Mechanisms](https://lightmysky.com/learn/science/the-rate-determining-step-and-reaction-mechanisms-mt_PM1Vo2CkVD) ## 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)