A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: Transition Metal Characteristics and Complex Formation — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 3 July 2026

The short answer

In A-Level Chemistry, understanding the characteristics of transition metals is crucial for grasping their unique properties and applications. Transition metals are elements that have an incomplete d sub-level in either their atoms or ions. This section focuses on four key characteristics: complex formation, formation of coloured ions, variable oxidation states, and catalytic activity.

The question

Identify the ligands and co-ordination number in the complex [Cu(NH 3 ) 4 ]SO 4 .

[Paraphrased for study — not reproduced from any exam paper.]

3 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For identification questions, marks are typically awarded for correctly naming ligands, central metal ions, and determining co-ordination numbers. For explanation questions, marks are given for clear and concise descriptions that demonstrate a deep understanding of the topic, including relevant principles and relationships.

What the command words demand

Identify: Recognize and name specific components, such as ligands or oxidation states, in a given complex.
Explain: Provide a detailed description of the concept, including relevant principles and relationships, such as why transition metals form coloured ions.
Determine: Find or derive a specific value or quantity based on given information, such as the co-ordination number or oxidation state.
Describe: Give a clear and concise account of a process or phenomenon, such as how transition metals act as catalysts.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.

Identify the central metal ion.0 marks
The central metal ion is Cu 2+ (copper(II)).
Identify the ligands.1 mark
The ligands are ammonia (NH 3 ) molecules.
Determine the co-ordination number.2 marks
There are four NH 3 molecules bonded to the Cu 2+ ion, so the co-ordination number is 4.

Final answer: Ligands: NH 3 , Co-ordination number: 4

Work through every step correctly and you earn all 3 marks.

Another worked example

Determine the oxidation state of iron in the complex [Fe(CN) 6 ] 4- .

3 marks

Identify the charge on the complex.1 mark
The complex has a -4 charge.
Determine the charge on each ligand.1 mark
Each cyanide (CN - ) ion has a -1 charge.
Set up an equation to solve for the oxidation state of iron (Fe).0 marks
Let x be the oxidation state of Fe. The total charge from six CN - ions is -6.x + (-6) = -4
Solve for x.1 mark
x - 6 = -4x = 2

Final answer: Oxidation state of Fe: +2

Work through every step correctly and you earn all 3 marks.

Common mistakes

Misidentifying ligands in a complex.
Why it happens: Students may confuse ligands with other ions or molecules present in the compound, leading to incorrect identification.
Fix: Carefully identify the central metal ion and the molecules or ions directly bonded to it. Ligands are those that form co-ordinate bonds with the metal.
Incorrectly determining the co-ordination number.
Why it happens: Students may count the total number of ligand molecules instead of the number of co-ordinate bonds formed with the central metal.
Fix: Count only the number of co-ordinate bonds between the central metal and the ligands to determine the co-ordination number.
Failing to correctly calculate oxidation states.
Why it happens: Students may set up incorrect equations or make arithmetic errors when solving for the oxidation state of a transition metal in a complex.
Fix: Set up an equation that accounts for the charge on the complex and the charges on the ligands. Solve the equation step-by-step to find the oxidation state of the central metal.
Not explaining why transition metals form coloured ions clearly.
Why it happens: Students may provide vague or incomplete explanations, failing to reference the energy differences between d orbitals and the absorption of light in the visible region.
Fix: Practice explaining that the presence of partially filled d orbitals in transition metals leads to the absorption of specific wavelengths of light, resulting in the characteristic colours observed.
Confusing catalytic activity with other properties of transition metals.
Why it happens: Students may mix up the concepts of catalytic activity, complex formation, and variable oxidation states, leading to incorrect answers in questions involving catalysts.
Fix: Remember that transition metals act as catalysts by providing a surface for reactants to adsorb, lowering the activation energy. Practice explaining this role clearly with examples from industry.
Incorrectly identifying the central metal ion in a complex.
Why it happens: Students may confuse the central metal ion with other ions or atoms present in the compound, leading to incorrect identification.
Fix: Carefully identify the central metal ion by looking at the formula of the complex and recognizing the transition metal element. The central metal is typically the atom or ion surrounded by ligands.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Identify Complex Ligands	State the ligands and work out the co-ordination number in a given complex ion.	3
Oxidation State Determination	Work out the oxidation state of the central metal ion in a complex.	3
Explain Colour In Complexes	Explain why a transition metal complex appears coloured using d-orbital splitting and light absorption.	4
Explain Catalysis	Explain how transition metals catalyse reactions, using a named industrial process as an example.	4
Total across these question types		14

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.