Transition elements
Explain H2 Chemistry transition-metal oxidation states, complex ions, colour and catalysis, with lessons linking structure to reaction evidence.
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Before you begin
This topic is about the first-row transition elements, scandium to copper, and how an incomplete 3d subshell explains their chemistry. It starts with the definition and electronic configurations, then uses them to explain variable oxidation states and redox, complexes and ligand exchange, colour, and catalysis.
Be comfortable with: orbitals and electronic configuration, dative bonding, cell potentials, catalysis, and assigning oxidation numbers.
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 configurationsDefine transition elements using incomplete d subshells, write atom and ion configurations, and explain trends in their physical properties.
- 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 colourExplain d-orbital splitting in octahedral complexes, how ligands affect the energy gap, and why the observed colour differs from the light absorbed.
- Transition-metal catalysisDistinguish heterogeneous and homogeneous catalysis, and show the catalyst is regenerated.
Practise and check
Or choose
Topic reference
Quick reference
| Question evidence | First move | Essential check |
|---|---|---|
| is it a transition element? | test the atom and its ions | 1–9 d electrons in either; zinc fails both, scandium passes as an atom |
| configuration of an ion | write the atom, then remove 4s before 3d | chromium 3d⁵4s¹ and copper 3d¹⁰4s¹ |
| trend in radius or ionisation energy | added electrons go into the inner 3d subshell | say “relatively invariant”, not “constant” |
| likely oxidation states | remove 4s, then 3d electrons | highest state up to Mn = number of 4s + 3d electrons |
| manganate(VII) or dichromate(VI) | write the acidic half-equation | include H⁺, H₂O and the colour change |
| will a redox reaction happen? | E⦵_cell = E⦵_reduction-E⦵_oxidation | positive means feasible under standard conditions |
| a ligand-exchange equation | write whole complexes in square brackets | balance ligands, atoms and charge; metal’s oxidation state unchanged |
| why is it coloured? | octahedral ligands split the d orbitals | the colour seen is the complement of the light absorbed |
| a catalytic cycle | add the steps | the catalyst must cancel |
Core knowledge to remember
- A transition element is a d-block element whose atom has an incomplete d subshell, or which can form cations with an incomplete d subshell.
- Chromium is [Ar]3d^54s^1 and copper is [Ar]3d¹⁰4s^1. Ions lose their 4s electrons before 3d.
- Atomic radius and first ionisation energy are relatively invariant, because added electrons enter the inner 3d subshell and shield the outer 4s electrons. Melting points and densities are much higher than calcium’s.
- Variable oxidation states arise because the 3d and 4s electrons are close in energy.
- Acidified MnO₄⁻ (purple) is reduced to Mn²⁺ (almost colourless); acidified Cr₂O₇²⁻ (orange) is reduced to Cr³⁺ (green); Fe³⁺ is reduced to Fe²⁺.
- A ligand donates a lone pair to the central metal ion. Copper(II): blue [Cu(H₂O)₆]²⁺, deep blue [Cu(NH₃)₄(H₂O)₂]²⁺ in excess ammonia, yellow [CuCl₄]²⁻ with concentrated chloride. Carbon monoxide replaces oxygen in haemoglobin.
- In an octahedral complex, d_(x²-y²) and d_z² point at the ligands and are raised in energy; d_xy, d_xz and d_yz form the lower set.
- Transition elements catalyse through variable oxidation states (homogeneous) and by adsorbing reactants on their surfaces (heterogeneous).
Common mistakes
- Calling every d-block element a transition element, or excluding scandium by testing only its ion.
- Removing 3d electrons before 4s when forming an ion.
- Saying radius or ionisation energy is constant across the series.
- Leaving H⁺ and H₂O out of acidic half-equations, or giving colour changes the wrong way round.
- Treating a positive E⦵_cell as proof that a reaction is fast.
- Calling the copper(II) hydroxide precipitate a ligand exchange, or adding neutral ligands to the charge of a complex.
- Saying a complex is the colour of the light it absorbs.
- Learning an order of ligand strengths, which is not required.
- Saying a catalyst increases the equilibrium yield.