A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: Ionic Bonding and Formulas of Ionic Compounds — mark scheme explained

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

The short answer

Understanding ionic bonding is fundamental to chemistry, as it explains how atoms combine to form stable compounds through the transfer of electrons. This section delves into the nature of ionic bonding, the formation of ions, and the construction of formulas for ionic compounds.

The question

Predict the charge on a sodium ion and explain why it forms this charge.

[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 prediction questions, marks are awarded for correctly identifying the charge on an ion. For construction questions, marks are given for balancing the charges and writing the formula correctly. For explanation questions, marks are allocated for clear and concise descriptions that demonstrate a deep understanding of ionic bonding principles.

What the command words demand

Predict: Determine the charge on an ion based on its position in the Periodic Table.
Construct: Formulate the correct formula for an ionic compound given the ions involved.
Explain: Provide a detailed description of how ionic bonds form and why certain charges are typical for specific elements.
Identify: Recognize and name common polyatomic ions in chemical formulas or equations.

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 element and its group in the Periodic Table.0 marks
Sodium (Na) is in Group 1.
Determine the typical charge for elements in Group 1.1 mark
Group 1 elements lose one electron to form +1 ions.
Explain why sodium forms a +1 ion.2 marks
Sodium loses one electron to achieve a stable electron configuration, resembling the nearest noble gas (neon).

Final answer: +1

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

Another worked example

Construct the formula for aluminum oxide (Al 2 O 3 ).

4 marks

Determine the charges on the ions involved.1 mark
Aluminum (Al) forms Al 3+ and oxygen (O) forms O 2- .
Balance the charges to form a neutral compound.2 marks
To balance +3 from Al with -2 from O, we need two aluminum ions and three oxygen ions.
Write the formula with the cation first and the anion second.1 mark
The formula is Al 2 O 3 .

Final answer: Al 2 O 3

Work through every step correctly and you earn all 4 marks.

Common mistakes

Misinterpreting the charges on ions based on their group in the Periodic Table.
Why it happens: Students may confuse the typical charges of elements, especially for transition metals which can form multiple charged ions.
Fix: Always refer to the periodic table and remember the common charges for each group. For transition metals, consider the specific compound formed.
Forgetting to balance the charges when constructing formulas for ionic compounds.
Why it happens: Students may write formulas without ensuring that the total positive charge equals the total negative charge.
Fix: Always check that the charges are balanced by finding the lowest common multiple (LCM) of the charges and adjusting the number of each ion accordingly.
Incorrectly writing polyatomic ions in formulas.
Why it happens: Students may forget to use parentheses for polyatomic ions when more than one is needed in a formula.
Fix: Enclose polyatomic ions in parentheses and use subscripts to indicate the number of ions. For example, write (SO 4 ) 3 instead of SO 12 .
Confusing the order of cations and anions in formulas.
Why it happens: Students may write the anion first, which is incorrect according to standard chemical notation.
Fix: Always write the cation (positive ion) first followed by the anion (negative ion).
Failing to explain the formation of ionic bonds clearly.
Why it happens: Students may struggle to articulate how electrons are transferred and why this results in a stable compound.
Fix: Practice explaining the process step-by-step. Emphasize that metals lose electrons to form cations, non-metals gain electrons to form anions, and the resulting electrostatic attraction forms a stable ionic bond.
Incorrectly identifying common polyatomic ions.
Why it happens: Students may not recognize or remember the formulas and charges of common polyatomic ions.
Fix: Memorize the formulas and charges of common polyatomic ions such as sulfate (SO 4 2- ), hydroxide (OH - ), nitrate (NO 3 - ), carbonate (CO 3 2- ), and ammonium (NH 4 + ).

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
Predict Ion Charge	Predict the charge on a sodium ion and explain why it forms.	3
Construct Ionic Formula	Work out the ionic charges and balance them to write the correct compound formula.	4
Predict Ionic Charge	Predict the charge on a sulfur ion and explain why it forms this charge.	3
Construct Ionic Formula	Balance the charges of iron(III) and sulfate ions to write the correct formula.	5
Total across these question types		15

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