A-Level Chemistry: Atomic Structure & Amounts Revision

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Where is almost all the mass of an atom located?

Atomic Structure & Amounts is a core part of A-Level Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.

Key concepts

ProtonPositive subatomic particle in the nucleus; relative mass 1.

NeutronNeutral subatomic particle in the nucleus; relative mass 1.

ElectronNegative particle in shells around the nucleus; negligible mass.

Atomic numberThe number of protons, which identifies the element.

Mass numberThe total number of protons and neutrons.

IsotopeAtoms of the same element with different numbers of neutrons.

s orbitalSpherical orbital holding up to 2 electrons.

p orbitalDumbbell-shaped orbital; 3 per subshell, 6 e- max.

d orbitalComplex shape; 5 per subshell, 10 e- max.

Ionisation energyEnergy to remove 1 mol e- from gaseous atoms.

Aufbau principleElectrons fill lowest energy orbitals first.

Hund's ruleElectrons singly occupy degenerate orbitals first.

What your exam board covers

Where Atomic Structure & Amounts sits in each exam board's specification. Tap a board to see what it lists.

AQAAmount of substanceSpec 7405

Part of: Physical chemistry

Source: AQA A-level Chemistry (7405) specification — Subject content, accessed 23 June 2026.

AQAAtomic structureSpec 7405

Part of: Physical chemistry

Source: AQA A-level Chemistry (7405) specification — Subject content, accessed 23 June 2026.

Learn Atomic Structure & Amounts step by step

Free interactive mini-lessons: short screens, worked examples and quick checks. Pick your exam board:

Other boards:CCEA Edexcel Eduqas/WJEC OCR

Common mistakes to avoid

Questions where students often pick the tempting wrong answer — make sure you know the right one:

Where is almost all the mass of an atom located?✗ Electrons contribute significantly to the mass of an atom. ✓ In the nucleus, made up of protons and neutrons.

What is different between two isotopes of the same element?✗ Isotopes have different numbers of electrons, which is why their mass differs. ✓ They have the same number of protons but different numbers of neutrons.

Why is the 4s orbital filled before the 3d orbital?✗ Because the 4s shell is closer to the nucleus than the 3d shell. ✓ Because the 4s orbital has a lower energy than 3d when both are empty, so electrons fill it first.

Practise Atomic Structure & Amounts — free games

Test yourself with these quick revision games for this topic:

▶ Two Truths, One Lie ▶ Reveal Race ▶ Connections Grid ▶ Falling Words ▶ Word Web

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Atomic Structure & Amounts

Key concepts

What your exam board covers

Common mistakes to avoid

Practise Atomic Structure & Amounts — free games

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