Chapter summary
Transition Elements, at a glance
Connect d-electron configurations to physical trends, variable oxidation states, coloured complexes and catalysis.
A-Level 9476 (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Transition element | A d-block element whose atom has an incomplete d subshell, or which forms a stable cation with an incomplete d subshell. |
| Ligand | A molecule or ion with at least one lone pair of electrons that it donates to a central metal atom or ion to form a dative bond. |
| Complex ion | A central metal atom or ion surrounded by ligands bonded to it by dative bonds. |
| Coordination number | The number of dative bonds formed between the ligands and the central metal ion. |
| Question | Useful reasoning |
|---|---|
| Does it qualify? | Test the atom and its cations for an incomplete d subshell. |
| What is the ion configuration? | Remove 4s electrons before 3d; keep Cr and Cu exceptions in the neutral atoms. |
| Why are trends gentle? | Increasing nuclear charge is partly offset by added 3d shielding. |
| Why several oxidation states? | 3d and 4s energies are close; different numbers of electrons can participate. |
| Why a colour change? | Distinguish ligand exchange, redox and precipitation from the actual species. |
| Why visible colour? | Unequal ligand repulsion splits d levels; a suitable d-d excitation absorbs selected wavelengths. |
| Why catalysis? | A lower-barrier surface or oxidation-state cycle regenerates the catalyst. |
Named redox electron counts: Fe3+/Fe2+: 1; MnO4-/Mn2+: 5; Cr2O72-/Cr3+: 6. The latter two equations require acid. Copper: water blue; excess ammonia deep blue; concentrated chloride favours yellow complex, often green in a mixture. Copper remains +2 in those exchanges.
Quick recall
Define a transition element.