9701 Chemistry · Topic 26 · A Level
Transition Elements Cheat Sheet — A Level Chemistry 9701
Transition metal chemistry is where colour, shape and redox come together, and where a well-learned explanation of d-orbital splitting is worth several marks. These sheets cover the definition of a transition element, variable oxidation states, complex ion shapes and coordination numbers, ligand exchange with stability constants, why complexes are coloured, and the role of transition metals in catalysis.
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What’s on this cheat sheet
Transition Elements — Properties
01 · Definition
A transition element is a d‑block element that forms one or more stable ions with a partially filled d sub‑shell.
Sc and Zn are in the d block but are not transition elements: Sc³⁺ is 3d⁰ and Zn²⁺ is 3d¹⁰. Their compounds are white and they show only one oxidation state.
02 · Electron configurations
4s fills before 3d, so Fe is [Ar] 3d⁶ 4s². Exceptions: Cr = [Ar] 3d⁵ 4s¹ and Cu = [Ar] 3d¹⁰ 4s¹ — a half‑filled or full d sub‑shell is more stable.
When ions form, the 4s electrons leave first: Fe²⁺ = 3d⁶, Fe³⁺ = 3d⁵, Cu²⁺ = 3d⁹.
03 · The four characteristic properties
1 · Variable oxidation states — 4s and 3d are close in energy, so different numbers of electrons can be lost.
2 · Coloured compounds — from d–d transitions in a split d sub‑shell.
3 · Catalytic activity — variable oxidation states allow electron transfer, and surfaces adsorb reactants.
4 · Complex formation — small, highly charged ions with vacant orbitals accept lone pairs.
04 · Common oxidation states
| Species | O.N. | Colour |
|---|---|---|
| MnO₄⁻ | +7 | purple |
| Cr₂O₇²⁻ | +6 | orange |
| Cr³⁺ | +3 | green |
| Fe³⁺ | +3 | yellow‑brown |
| Fe²⁺ | +2 | pale green |
| Mn²⁺ | +2 | very pale pink |
| Cu²⁺ | +2 | blue |
05 · Why the compounds are coloured
Ligands split the five d orbitals into two energy levels. An electron absorbs a photon of visible light and is promoted across the gap — a d–d transition.
The colour seen is the complement of the light absorbed. The size of the gap, and therefore the colour, depends on the metal, its oxidation state, the ligand and the shape of the complex.
No colour when the d sub‑shell is empty (Sc³⁺) or full (Zn²⁺, Cu⁺): there is no vacancy to be promoted into.
06 · Complexes and ligands
A ligand is a species with a lone pair that forms a dative covalent bond to the central metal ion. Coordination number = number of such bonds.
| Type | Examples |
|---|---|
| monodentate | H₂O, NH₃, Cl⁻, CN⁻, OH⁻ |
| bidentate | ethanedioate, 1,2‑diaminoethane |
| polydentate | EDTA⁴⁻ (six bonds) |
07 · Shapes of complexes
| C.N. | Shape | Example |
|---|---|---|
| 6 | octahedral, 90° | [Fe(H₂O)₆]²⁺ |
| 4 | tetrahedral, 109.5° | [CuCl₄]²⁻ |
| 4 | square planar | cisplatin |
| 2 | linear, 180° | [Ag(NH₃)₂]⁺ |
Large ligands such as Cl⁻ favour a coordination number of 4; small ones such as H₂O and NH₃ give 6.
08 · Naming and charge
[Cu(H₂O)₆]²⁺ — Cu is +2, water neutral
[CuCl₄]²⁻ — +2 + 4(−1) = −2
[Fe(CN)₆]³⁻ — +3 + 6(−1) = −3
Write the formula in square brackets with the charge outside.
09 · Stereoisomerism in complexes
Cis–trans in square planar and octahedral complexes: cisplatin is the cis isomer of [Pt(NH₃)₂Cl₂] and is the anticancer drug; the trans isomer is inactive.
Optical isomerism when an octahedral complex holds three bidentate ligands — the two forms are non‑superimposable mirror images.
10 · Physical properties
Transition metals are hard, dense and have high melting points: both 4s and 3d electrons are delocalised, giving strong metallic bonding.
Across the series the atomic radius changes very little — the added 3d electrons shield the extra nuclear charge almost completely.
11 · Worked example — configuration and colour
Zn²⁺ = [Ar] 3d¹⁰ — the d sub‑shell is full, so no d–d transition is possible and no visible light is absorbed.
Cu²⁺ = [Ar] 3d⁹ — a vacancy exists, so an electron is promoted across the split d orbitals, absorbing red light and leaving the solution blue.
12 · Why the properties arise
The 3d and 4s sub‑shells are very close in energy. That single fact explains variable oxidation states, since removing one more electron costs little extra.
The ions are also small with a high charge density and empty low‑energy orbitals, which is why they attract lone pairs and form complexes so readily — and why those complexes, having a partly filled d sub‑shell, are coloured.
Definitions to quote
Transition element — a d‑block element that forms at least one stable ion with a partially filled d sub‑shell.
Ligand — a molecule or ion with a lone pair that forms a dative covalent bond to a central metal ion.
Complex — a central metal ion surrounded by ligands bonded datively.
Marks lost here
— Calling Sc and Zn transition elements.
— Writing Fe³⁺ as 3d³4s²; the 4s electrons go first.
— Saying a complex is coloured “because it is a transition metal” instead of explaining d–d transitions and the split d sub‑shell.
Ligand Exchange, Redox and Stability
13 · Ligand exchange
One ligand replaces another. The colour usually changes because the new ligand splits the d orbitals by a different amount; the shape may change too if the new ligand is larger.
pale blue → yellow, octahedral → tetrahedral
[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
pale blue → deep blue, shape unchanged
14 · Stability constants
The equilibrium constant for forming a complex from the aqueous ion. A larger Kstab means a more stable complex, so that ligand displaces one with a smaller value.
Order of stability for most ions: CN⁻ > NH₃ > H₂O > Cl⁻. Polydentate ligands such as EDTA give very large values.
15 · Worked example — Kstab
Kstab[Cu(NH₃)₄(H₂O)₂]²⁺ = 1.2 × 10¹³
Kstab[CuCl₄]²⁻ = 4.2 × 10⁵
The ammonia complex has the far larger Kstab, so ammonia displaces both water and chloride and the deep blue colour appears.
16 · Reactions with NaOH and NH₃
| Ion | With OH⁻ | In excess NH₃ |
|---|---|---|
| Cu²⁺ | pale blue ppt | deep blue solution |
| Fe²⁺ | green ppt, browns in air | insoluble |
| Fe³⁺ | red‑brown ppt | insoluble |
| Cr³⁺ | green ppt, soluble in excess | violet solution |
The precipitate is the neutral hydroxide, e.g. Cu²⁺ + 2OH⁻ → Cu(OH)₂.
17 · Redox chemistry of manganese
purple → colourless in acid
In alkali the reduction stops at MnO₂ (brown solid), so acid conditions matter. Use dilute sulfuric acid, never HCl or HNO₃.
18 · Redox chemistry of iron and copper
Fe²⁺ is readily oxidised to Fe³⁺ (E° = +0.77 V) — by MnO₄⁻, Cr₂O₇²⁻, Cl₂ or Br₂, and slowly by air.
Cu²⁺ + I⁻: 2Cu²⁺ + 4I⁻ → 2CuI + I₂. The iodine is then titrated with thiosulfate, 2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻, using starch near the end point.
19 · Worked example — iron titration
n(MnO₄⁻) = 1.86 × 10⁻⁴ mol
n(Fe²⁺) in 25 cm³ = 5 × 1.86 × 10⁻⁴ = 9.30 × 10⁻⁴
in 250 cm³ = 9.30 × 10⁻³ mol
m(Fe) = 9.30 × 10⁻³ × 55.8 = 0.519 g
% Fe = 0.519 ÷ 1.20 × 100 = 43.2 %
20 · Effect of ligand on E°
Changing the ligand changes the stability of the two oxidation states, and so changes E°.
[Fe(H₂O)₆]³⁺/²⁺ is +0.77 V, but with cyanide [Fe(CN)₆]³⁻/⁴⁻ is only +0.36 V: CN⁻ stabilises the +3 state, making it harder to reduce. Adding a ligand that stabilises the higher state always lowers E°.
21 · Catalysis by transition metals
Heterogeneous: Fe in the Haber process, V₂O₅ in the Contact process, Ni in hydrogenation, Pt/Rh in catalytic converters. Reactants adsorb onto active sites, weakening their bonds.
Homogeneous: Fe³⁺ or Fe²⁺ catalysing S₂O₈²⁻ + 2I⁻; Mn²⁺ autocatalysing the manganate(VII)–ethanedioate reaction. Both work by cycling between oxidation states.
22 · Complexes in life and medicine
Haemoglobin — Fe²⁺ held in a porphyrin ring; oxygen binds reversibly at the sixth site. Carbon monoxide binds far more strongly and irreversibly, which is why it is toxic.
Cisplatin — square planar Pt(II) complex that binds DNA and prevents replication.
23 · Chromium chemistry
orange → green in acid
Cr₂O₇²⁻ + H₂O ⇌ 2CrO₄²⁻ + 2H⁺
orange in acid ⇌ yellow in alkali — an acid–base equilibrium, not a redox change: chromium stays at +6.
Zinc in acid reduces Cr³⁺ further to blue Cr²⁺.
24 · Worked example — thiosulfate titration
n(S₂O₃²⁻) = 2.24 × 10⁻³ mol
I₂ : S₂O₃²⁻ = 1 : 2 → n(I₂) = 1.12 × 10⁻³ mol
Cu²⁺ : I₂ = 2 : 1 → n(Cu²⁺) = 2.24 × 10⁻³ mol
c(Cu²⁺) = 0.0896 mol dm⁻³
Add starch only near the end point, when the solution is straw‑coloured.
Colours worth memorising
Cu²⁺(aq) pale blue · Cu(OH)₂ pale blue ppt · [Cu(NH₃)₄(H₂O)₂]²⁺ deep blue · [CuCl₄]²⁻ yellow
Fe²⁺ pale green · Fe(OH)₂ green ppt · Fe³⁺ yellow‑brown · Fe(OH)₃ red‑brown ppt
MnO₄⁻ purple · Mn²⁺ almost colourless · Cr₂O₇²⁻ orange · Cr³⁺ green
Marks lost here
— Describing a ligand exchange without naming both the colour change and any change of shape.
— Using HCl to acidify a manganate(VII) titration.
— Forgetting the 5 : 1 ratio of Fe²⁺ to MnO₄⁻.
— Treating Kstab as a rate: it measures stability, not speed.
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Transition Elements — Frequently Asked Questions
Why are transition metal complexes coloured?
The ligands split the five d orbitals into two energy levels. An electron absorbs a photon of visible light to jump from the lower to the upper set; the colour you see is the complementary colour of the light absorbed. A d⁰ or d¹⁰ ion such as Sc³⁺ or Zn²⁺ has no possible d–d transition and is therefore colourless.
Why is zinc not a transition element?
A transition element must form at least one stable ion with a partially filled d sub-shell. Zinc only forms Zn²⁺, which is 3d¹⁰ — completely full — so it shows none of the characteristic transition properties.
What is a stability constant and what does a large value mean?
Kₛₜₐᵇ is the equilibrium constant for a ligand exchange reaction. A large value means the incoming ligand forms a much more stable complex than the one being displaced, so the exchange goes essentially to completion — which is why ammonia and EDTA displace water so readily.
