What does Module 2 of OCR Chemistry A cover?

Module 2 (Foundations in chemistry) is the toolkit for the whole H432 course. It covers 2.1 Atoms and reactions (atomic structure and isotopes, compounds, formulae and equations, amount of substance and the mole, acids, and redox) and 2.2 Electrons, bonding and structure (electron configuration, ionic, covalent, dative and metallic bonding, electronegativity, molecular shapes, and crystal structures).

Which part of Module 2 is most important to master first?

Amount of substance and the mole (2.1.3) is the most reused in the whole qualification. Moles calculations underpin almost every quantitative question in all three papers, so master the mole, molar mass, the ideal gas equation, empirical formulae, titrations, percentage yield and atom economy before anything else.

How is Module 2 examined in OCR Chemistry A?

Module 2 content appears in Papers 1 and 2 and the synoptic Paper 3, because it underpins everything. Expect multiple choice and short calculation questions on the mole, oxidation numbers and shapes, structured questions on titrations and bonding, and definitions of relative masses and acid behaviour. It is assessed alongside later modules throughout the course.

What is the most common mistake in Module 2?

Getting the 4s and 3d order wrong on ionisation (4s fills first but is removed first), and mixing up units in the ideal gas equation (volume must be in cubic metres and pressure in pascals). Other frequent errors are saying a polar bond means a polar molecule, confusing strong with concentrated acids, and naming the oxidising agent as the species that is oxidised.

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OCR A-Level Chemistry A: Module 2 (Foundations in chemistry) overview

A deep-dive overview of Module 2 of OCR A-Level Chemistry A (H432): atomic structure and isotopes, compounds and equations, the mole toolkit, acids, redox, and electrons, bonding and structure.

Generated by Claude Opus 4.816 min readH432/2Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this module demands
Atoms and reactions
Acids and redox
Electrons, bonding and structure
How this module is examined
Check your knowledge

What this module demands

Module 2 of OCR A-Level Chemistry A builds the foundation for the entire qualification. It moves from the structure of the atom, through the quantitative mole toolkit and the chemistry of acids and redox, into electron configuration, bonding and molecular shape. The examiners test precise recall of definitions alongside confident calculation.

This guide walks through Module 2 in specification order and sets out the exam patterns OCR repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Atoms and reactions

Atomic structure and isotopes (2.1.1) covers protons, neutrons and electrons with their relative masses and charges, atomic and mass number, isotopes, and calculating relative atomic mass from mass spectra as a weighted mean. The recurring exam skill is the abundance calculation.

Compounds, formulae and equations (2.1.2) builds formulae from ionic charges and oxidation numbers, and constructs balanced full and ionic equations with state symbols. Spectator ions are cancelled in ionic equations.

Amount of substance and the mole (2.1.3) is the calculation engine: $n = \frac{m}{M}$ , $n = c \times V$ , the ideal gas equation $pV = nRT$ , empirical and molecular formulae, titrations, percentage yield and atom economy. Unit conversions are where marks are won or lost.

Acids and redox

Acids (2.1.4) treats acids as proton donors, distinguishes strong (full dissociation) from weak (partial dissociation) acids, and covers the reactions with metals, carbonates and bases, plus the practical skills of making a standard solution and titrating. Redox (2.1.5) assigns oxidation numbers, defines oxidation and reduction by electron transfer (OIL RIG), identifies oxidising and reducing agents, and combines half-equations.

Electrons, bonding and structure

Electron structure (2.2.1) fills shells, sub-shells and orbitals in energy order, with the $4\text{s}$ before $3\text{d}$ rule and the chromium and copper exceptions. Bonding and structure (2.2.2) classifies ionic, covalent, dative and metallic bonding, uses electronegativity to judge polarity, predicts shapes from electron-pair repulsion, and links the four crystal structures and the intermolecular forces to physical properties.

How this module is examined

A typical OCR profile for Module 2:

Multiple choice and short answer. Assigning oxidation numbers, identifying shapes and bond angles, recalling definitions of relative masses, and selecting bonding types.
Calculations. Relative atomic mass from mass spectra, moles and titrations, empirical formulae, percentage yield and atom economy, and the ideal gas equation.
Applied and explanation questions. Explaining hydrogen bonding and boiling-point trends, polarity, and acid behaviour.
Practical technique. Making a standard solution and the titration method, tested heavily in Paper 3.

Check your knowledge

A mix of recall and calculation questions across Module 2. Attempt them under timed conditions, then check against the solutions.

Define the term relative atomic mass. (2 marks)
State and explain the shape and bond angle of a water molecule. (3 marks)
Determine the oxidation number of chlorine in the chlorate(V) ion, $\text{ClO}_3^-$ . (2 marks)
Calculate the number of moles in $4.0\ \text{g}$ of sodium hydroxide ( $M_r = 40.0$ ). (1 mark)
Explain the difference between a strong acid and a weak acid. (2 marks)
State why magnesium has a higher melting point than sodium. (2 marks)

Solutions

Q1: The weighted mean mass of an atom of an element compared with one twelfth the mass of an atom of carbon-12.
Q2: Bent (V-shaped), $104.5^{\circ}$ . There are two bonding pairs and two lone pairs; lone pairs repel more than bonding pairs, reducing the angle from the tetrahedral $109.5^{\circ}$ .
Q3: Oxygen is $-2$ , so $x + 3(-2) = -1$ , giving $x = +5$ .
Q4: $n = \dfrac{4.0}{40.0} = 0.10\ \text{mol}$ .
Q5: A strong acid fully dissociates into ions in solution; a weak acid only partially dissociates, setting up an equilibrium.
Q6: Magnesium forms $2+$ ions and releases two delocalised electrons per atom, so the metallic bonding (attraction between cations and the electron sea) is stronger than in sodium with $1+$ ions.

Sources & how we know this

OCR A-Level Chemistry A (H432) specification — OCR (2015)