Transition Elements
Complex ions, colour, catalysis and variable oxidation states.
Before you begin
Complex ions, colour, catalysis and variable oxidation states.
Learning goals
- Transition Elements: Definition and Electron Configurations
- Variable Oxidation States and Redox Systems
- Complex Ions, Ligands, Ligand Exchange
- d-Orbital Splitting and Colour
- Transition-metal Catalysis
Syllabus statements covered
- explain that a transition element is a d block element whose atom has an incomplete d subshell, or which can give rise to cations with an incomplete d subshell
- state the electronic configuration of a first row transition element and its ions (see also 1(h))
- explain why atomic radii and first ionisation energies of the transition elements are relatively invariant
- contrast, qualitatively, the melting point and density of the transition elements with those of calcium as a typical s block element
- describe the tendency of transition elements to have variable oxidation states
- predict from a given electronic configuration, the likely oxidation states of a transition element
- describe and explain the use of Fe3+/Fe2+, MnO4–/Mn2+ and Cr2O72–/Cr3+ as examples of redox systems (see also Section 12)
- predict, using E⦵ values, the likelihood of redox reactions (see also 12(f)(ii))
- define the terms ligand and complex as exemplified by the complexes of copper(II) ions with water, ammonia and chloride ions as ligands (includes the transition metal complexes found in the Qualitative Analysis Notes)
- explain qualitatively that ligand exchange may occur, as exemplified by the formation of the complexes in (i), including the colour changes involved, and CO/O2 exchange in haemoglobin
- describe, using the shape and orientation of the d orbitals, the splitting of degenerate d orbitals into two energy levels in octahedral complexes
- explain, in terms of d orbital splitting and d-d transition, why transition element complexes are usually coloured [knowledge of the relative order of ligand field strength is not required]
- explain how some transition elements and/or their compounds can act as catalysts (see also 8(j))
Lessons
Work through them in order.
- Transition Elements: Definition and Electron ConfigurationsClassify elements, form ions and explain physical trends.
- Variable Oxidation States and Redox SystemsPredict oxidation states, balance the named systems and use standard electrode potential data.
- Complex Ions, Ligands, Ligand ExchangeExplain ligand exchange in copper(II) complexes and in haemoglobin.
- d-Orbital Splitting and ColourConnect octahedral orientation, absorption and observed colour.
- Transition-metal CatalysisDistinguish heterogeneous and homogeneous catalysis, and show the catalyst is regenerated.
Practise and check
Recommended nextTransition Elements topic check
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Topic reference
This topic covers the first-row transition elements: electron configurations, variable oxidation states, complex ions, colour and catalysis. Configurations come first because they supply the oxidation-state and d-electron evidence the later lessons use.
Be comfortable with:
- Orbitals and Electron Configuration: subshell notation, orbital orientation and electron removal.
- Periodic Trends: effective nuclear charge, shielding, atomic radius and first ionisation energy.
- Writing Redox Equations: electron balance in acidic solution.
- Cell Potentials and Spontaneity: choosing reaction direction from standard reduction potentials.
- Dative Bonding and Common Examples: lone-pair donation and coordinate bonds.
Before you start, be able to assign oxidation numbers, tell an observation from an inference, and check an equation for both atoms and charge.
Quick Reference
| Evidence in the question | First move | What to check |
|---|---|---|
| “transition element?” | inspect the atom and any cation supplied | either may qualify if its d subshell is incomplete; d⁰/d¹⁰ does not qualify that species |
| first-row cation | write the atom, then remove 4s before 3d | retain the Cr/Cu atom exceptions |
| trends across the series | balance rising nuclear charge against added 3d shielding | say relatively invariant, not constant |
| oxidation-state prediction | remove accessible 4s then 3d electrons | label it a prediction unless chemistry is supplied |
| acidic permanganate/dichromate | write the memorised reduction half-equation | include H⁺, H₂O and colour evidence |
| two E⦵ values | more positive couple is reduced | positive E⦵_cell is a standard-state prediction |
| copper ligand exchange | write complete bracketed complexes | conserve ligand count, atoms and charge |
| colour explanation | octahedral approach → splitting → absorption | observed colour is complementary |
| catalytic cycle | add the steps | catalyst must cancel and be regenerated |
The 9476 colour model requires octahedral d-orbital orientation, but not memorisation of the relative order of ligand field strengths.
Common Exam Traps
- Defining a transition element as any d-block element, or using an ion-only shortcut that wrongly excludes scandium. Test the atom and any cation supplied.
- Removing 3d electrons before 4s when forming a cation.
- Saying radius or first ionisation energy is constant across the series rather than relatively invariant.
- Explaining high density only with “strong bonding”; density also depends on mass per volume.
- Predicting an oxidation state as guaranteed without considering supplied chemical evidence.
- Using acidic permanganate or dichromate equations without H⁺ and H₂O.
- Treating a positive E⦵_cell as proof of a fast reaction.
- Calling precipitation or redox a ligand exchange without tracking the central metal’s oxidation state and ligands.
- Memorising non-prescribed complex geometries or ligand-strength rankings as though they were required.
- Saying a complex reflects its own colour without describing selective absorption and a d–d transition.
- Claiming a catalyst increases equilibrium yield rather than the rates of both directions.