Why do transition elements from titanium to copper exhibit variable oxidation states?

Transition elements from titanium to copper exhibit variable oxidation states due to the similar energy levels of their 3d and 4s orbitals. This allows electrons to be removed from both orbitals, resulting in multiple stable oxidation states. For example, iron can exist in +2 and +3 oxidation states, which is a common characteristic studied in A-Level Inorganic Chemistry.

How do transition elements form colored compounds?

Transition elements form colored compounds due to d-d electron transitions. When light is absorbed, electrons in the d orbitals are excited to higher energy levels. The specific wavelengths of light absorbed correspond to the colors observed. This property is particularly emphasized in the Cambridge International A Levels Chemistry curriculum when studying the electronic structure of transition metals.

What role do transition elements play as catalysts?

Transition elements act as catalysts due to their ability to change oxidation states and form complex ions. This allows them to provide alternative reaction pathways with lower activation energies. For instance, iron is used in the Haber process for ammonia synthesis, and vanadium(V) oxide is used in the Contact process for sulfuric acid production. These applications are crucial topics in A-Level Chemistry.

How do transition elements form complex ions?

Transition elements form complex ions by coordinating with ligands through dative covalent bonds. The presence of vacant d orbitals allows these metals to accept electron pairs from ligands, forming stable complexes. This ability to form complex ions is a key aspect of transition metal chemistry covered in the Cambridge International A Levels syllabus.

Why is "Equating coordination number with the number of ligand molecules." a common mistake in General characteristic chemical properties of the first set of transition elements, titanium to copper?

Why it happens: Ignoring bidentate ligands. How to avoid it: Coordination number = number of dative BONDS; a bidentate ligand (e.g. en) makes two. Source: 9701 Examiner Reports 2022-2024

Why is "Keeping coordination number 6 when Cl⁻ replaces water." a common mistake in General characteristic chemical properties of the first set of transition elements, titanium to copper?

Why it happens: Forgetting Cl⁻ is larger. How to avoid it: Bulky Cl⁻ usually gives 4-coordinate tetrahedral complexes (e.g. [CuCl₄]²⁻).

Why is "Adding a separate indicator to a manganate(VII) titration." a common mistake in General characteristic chemical properties of the first set of transition elements, titanium to copper?

Why it happens: Treating it like an acid–base titration. How to avoid it: MnO₄⁻ is self-indicating — the end point is the first permanent pink.

Why is "Confusing the Fe²⁺ and Fe³⁺ hydroxide colours." a common mistake in General characteristic chemical properties of the first set of transition elements, titanium to copper?

Why it happens: Both are iron. How to avoid it: Fe(OH)₂ green; Fe(OH)₃ red-brown.

Chemistry of transition elementsCambridge International A Level Chemistry 9701

General characteristic chemical properties of the first set of transition elements, titanium to copper

Q: What are the general characteristic chemical properties of the first set of transition elements from titanium to copper?

The first set of transition elements, from titanium to copper, exhibit several characteristic chemical properties. These include variable oxidation states, the formation of colored compounds, the ability to form complex ions, and catalytic activity. These properties arise from the presence of partially filled d orbitals, which allow for diverse electron configurations and bonding scenarios.

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Transition Elements: Characteristic Chemistry — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Variable oxidation states, ligands and complex ions, ligand-substitution colour changes, the reactions with NaOH and NH₃, and redox reactions including the manganate(VII) titration.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

28.2 — Describe variable oxidation states and use them in redox reactions.
28.2 — Define ligand, complex ion and coordination number, and describe ligand-substitution reactions.
28.2 — Describe the reactions of aqueous transition-metal ions with NaOH and NH₃, and a redox titration.

Variable oxidation states, ligands and complexes

Close 4s/3d energies give several oxidation states; ligands dative-bond to form complex ions.

Because the 4s and 3d sub-shells are close in energy, transition metals can lose different numbers of electrons, giving variable oxidation states (e.g. Mn from +2 to +7, Fe +2/+3, Cu +1/+2).

A ligand is a species with a lone pair that forms a dative (coordinate) bond to a central metal ion, giving a complex ion. The coordination number is the number of dative bonds:

Monodentate (one bond each): H₂O, NH₃, Cl⁻, OH⁻, CN⁻.
Bidentate (two bonds each): 1,2-diaminoethane (en), ethanedioate (C₂O₄²⁻).

The oxidation state of the metal in a complex follows from the overall charge: $\text{overall charge} = (\text{oxidation state of metal}) + \sum(\text{ligand charges})$ e.g. in [Fe(CN)₆]³⁻: Fe + 6(−1) = −3, so Fe is +3.

Variable oxidation states (Mn +2 to +7).
Ligand = lone-pair donor (dative bond); complex ion forms.
Coordination number = number of dative bonds.

See the full worked example for general characteristic chemical properties of the first set of transition elements, titanium to copper →

Ligand substitution and colour changes

Swapping ligands changes ΔE and so the colour; bulky Cl⁻ also changes shape.

In ligand substitution one ligand replaces another around the metal, often with a striking colour change. For copper(II):

$[\text{Cu(H}_2\text{O)}_6]^{2+} \;(\text{pale blue}) + 4\text{NH}_3 \rightarrow [\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+} \;(\text{deep blue}) + 4\text{H}_2\text{O}$

$[\text{Cu(H}_2\text{O)}_6]^{2+} \;(\text{pale blue}) + 4\text{Cl}^- \rightarrow [\text{CuCl}_4]^{2-} \;(\text{yellow}) + 6\text{H}_2\text{O}$

With NH₃ only four water ligands are replaced (the colour deepens). With concentrated chloride, the bulky Cl⁻ ligands also change the coordination number from 6 to 4 (octahedral → tetrahedral), giving a larger colour change.

Excess NH₃: pale blue → deep blue [Cu(NH₃)₄(H₂O)₂]²⁺.
Conc Cl⁻: pale blue → yellow [CuCl₄]²⁻ (octahedral → tetrahedral).
Changing the ligand changes ΔE and the colour.

See the full worked example for general characteristic chemical properties of the first set of transition elements, titanium to copper →

Reactions with NaOH/NH₃ and redox

Hydroxide precipitates identify the ion; MnO₄⁻/Fe²⁺ is a self-indicating titration.

With aqueous NaOH (or a little NH₃), aqueous transition-metal ions give coloured hydroxide precipitates:

Cu²⁺ → blue Cu(OH)₂
Fe²⁺ → green Fe(OH)₂ (darkens in air to brown)
Fe³⁺ → red-brown Fe(OH)₃

Redox. Variable oxidation states make transition-metal ions useful oxidising/reducing agents. The classic redox titration uses acidified manganate(VII): $\text{MnO}_4^- + 8\text{H}^+ + 5\text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}$ MnO₄⁻ is self-indicating: purple MnO₄⁻ is decolourised (to almost-colourless Mn²⁺) while Fe²⁺ remains, so the end point is the first permanent pale pink.

Cu(OH)₂ blue; Fe(OH)₂ green; Fe(OH)₃ red-brown.
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O.
MnO₄⁻ is self-indicating (first permanent pink).

See the full worked example for general characteristic chemical properties of the first set of transition elements, titanium to copper →

How it’s examined

Paper 4 favours ligand-substitution equations with colours, oxidation states in complexes, hydroxide-precipitate identification, and a MnO₄⁻ titration calculation. Examiner reports flag keeping coordination number 6 for [CuCl₄]²⁻ and adding a separate indicator to a manganate(VII) titration.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 28.2; 9701 Examiner Reports 2022-2024; 9701/41 & 9701/42 May/Jun 2024 question papers and mark schemes. Last reviewed 2026-05-20.

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Step-by-step worked examples — General characteristic chemical properties of the first set of transition elements, titanium to copper

Step-by-step solutions to past-paper-style questions on general characteristic chemical properties of the first set of transition elements, titanium to copper, written exactly the way a tutor would explain them at the board.

1Ligands and coordination number (2 marks)
Getting started• complex ions
Question
State the coordination number and overall charge of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ and $[\text{CuCl}_4]^{2-}$ .
Step-by-step solution
1. Step 1
  [Cu(H₂O)₆]²⁺: 6 H₂O ligands → coordination number 6; charge from Cu²⁺ + 6 neutral H₂O = 2+.
2. Step 2
  [CuCl₄]²⁻: 4 Cl⁻ ligands → coordination number 4; Cu²⁺ + 4(−1) = 2−.
Answer
[Cu(H₂O)₆]²⁺: c.n. 6, charge 2+. [CuCl₄]²⁻: c.n. 4, charge 2−.
Examiner tip
Coordination number = number of dative bonds (ligand atoms), not the number of ligand molecules if a ligand is bidentate.
2Oxidation state in a complex (2 marks)
Getting started• oxidation state
Question
Deduce the oxidation state of the metal in (a) $[\text{Fe(CN)}_6]^{3-}$ and (b) $\text{MnO}_4^-$ .
Step-by-step solution
1. Step 1
  (a) Fe + 6(−1) = −3 → Fe = +3.
2. Step 2
  (b) Mn + 4(−2) = −1 → Mn = +7.
Answer
(a) Fe(III); (b) Mn(VII).
3Ligand substitution colour changes (4 marks)
Building confidence• ligand substitution
Question
Describe what happens when (a) excess aqueous ammonia and (b) concentrated HCl are added to $[\text{Cu(H}_2\text{O)}_6]^{2+}$ , with equations and colours.
Step-by-step solution
1. Step 1
  Start: [Cu(H₂O)₆]²⁺ is pale blue.
2. Step 2
  (a) Excess NH₃ replaces 4 water ligands → deep blue.
  $[\text{Cu(H}_2\text{O)}_6]^{2+} + 4\text{NH}_3 \rightarrow [\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+} + 4\text{H}_2\text{O}$
3. Step 3
  (b) Conc HCl replaces all 6 water with 4 Cl⁻ → yellow (tetrahedral).
  $[\text{Cu(H}_2\text{O)}_6]^{2+} + 4\text{Cl}^- \rightarrow [\text{CuCl}_4]^{2-} + 6\text{H}_2\text{O}$
Answer
NH₃: pale blue → deep blue [Cu(NH₃)₄(H₂O)₂]²⁺. Conc HCl: pale blue → yellow [CuCl₄]²⁻.
Examiner tip
Replacing water with Cl⁻ also changes the coordination number (6 → 4) and shape (octahedral → tetrahedral).
4Reactions with aqueous NaOH (3 marks)
Building confidence• precipitates
Question
State the colour of the precipitate when NaOH(aq) is added to (a) Cu²⁺(aq), (b) Fe²⁺(aq) and (c) Fe³⁺(aq).
Step-by-step solution
1. Step 1
  Cu²⁺ → Cu(OH)₂: blue precipitate.
2. Step 2
  Fe²⁺ → Fe(OH)₂: green precipitate (darkens in air).
3. Step 3
  Fe³⁺ → Fe(OH)₃: red-brown precipitate.
Answer
Cu(OH)₂ blue; Fe(OH)₂ green; Fe(OH)₃ red-brown.
Examiner tip
These hydroxide colours are common identification marks; the equation is Mⁿ⁺ + nOH⁻ → M(OH)ₙ.
5Manganate(VII) redox titration (4 marks)
Stretch• redox, titration
Question
Write the half-equations and overall equation for the reaction of acidified MnO₄⁻ with Fe²⁺, and state the colour change at the end point.
Step-by-step solution
1. Step 1
  Reduction:
  $\text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}$
2. Step 2
  Oxidation (×5):
  $5\text{Fe}^{2+} \rightarrow 5\text{Fe}^{3+} + 5e^-$
3. Step 3
  Overall:
  $\text{MnO}_4^- + 8\text{H}^+ + 5\text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}$
4. Step 4
  End point: the first permanent pale pink (purple MnO₄⁻ no longer decolourised) — MnO₄⁻ is self-indicating.
Answer
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O; end point = first permanent pale pink.
Examiner tip
MnO₄⁻ is self-indicating (purple → colourless Mn²⁺); the end point is the first lasting pink.
6Variable oxidation states (3 marks)
Stretch• oxidation states
Question
Explain why transition elements show variable oxidation states, and give the range shown by manganese.
Step-by-step solution
1. Step 1
  The 4s and 3d sub-shells are very close in energy, so successive electrons are removed with similar amounts of energy.
2. Step 2
  Different numbers of electrons can be lost, giving several stable oxidation states.
3. Step 3
  Manganese ranges from +2 to +7 (e.g. Mn²⁺ to MnO₄⁻).
Answer
4s and 3d are close in energy, so variable numbers of electrons are lost; Mn shows +2 to +7.

Key Formulae — General characteristic chemical properties of the first set of transition elements, titanium to copper

The formulae you need to memorise for general characteristic chemical properties of the first set of transition elements, titanium to copper on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

Charge of a complex ion
$\text{overall charge} = (\text{metal oxidation state}) + \sum(\text{ligand charges})$
$ligand charges$
0 for neutral ligands (H₂O, NH₃), −1 for Cl⁻/CN⁻/OH⁻
When to use
Finding the overall charge of a complex, or the metal's oxidation state within it.

Key Definitions and Keywords — General characteristic chemical properties of the first set of transition elements, titanium to copper

Definitions to memorise and the exact keywords mark schemes credit for general characteristic chemical properties of the first set of transition elements, titanium to copper answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Ligand
Examiner keyword
A species with a lone pair that forms a dative (coordinate) bond to a central metal ion.
Complex ion
Examiner keyword
A central metal ion surrounded by ligands bonded to it by dative bonds.
Coordination number
Examiner keyword
The number of dative (coordinate) bonds from ligands to the central metal ion (e.g. 6 in [Cu(H₂O)₆]²⁺).
Ligand substitution
Examiner keyword
A reaction in which one ligand replaces another around the central metal ion, often with a colour change.
Variable oxidation states
Examiner keyword
Transition metals form ions of more than one oxidation state because the 4s and 3d electrons are close in energy.

Common Mistakes and Misconceptions — General characteristic chemical properties of the first set of transition elements, titanium to copper

The traps other students keep falling into on general characteristic chemical properties of the first set of transition elements, titanium to copper questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Equating coordination number with the number of ligand molecules.
9701 Examiner Reports 2022-2024
Why it happens
Ignoring bidentate ligands.
How to avoid it
Coordination number = number of dative BONDS; a bidentate ligand (e.g. en) makes two.
✕Keeping coordination number 6 when Cl⁻ replaces water.
Why it happens
Forgetting Cl⁻ is larger.
How to avoid it
Bulky Cl⁻ usually gives 4-coordinate tetrahedral complexes (e.g. [CuCl₄]²⁻).
✕Adding a separate indicator to a manganate(VII) titration.
Why it happens
Treating it like an acid–base titration.
How to avoid it
MnO₄⁻ is self-indicating — the end point is the first permanent pink.
✕Confusing the Fe²⁺ and Fe³⁺ hydroxide colours.
Why it happens
Both are iron.
How to avoid it
Fe(OH)₂ green; Fe(OH)₃ red-brown.

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