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Atomic Structure revision — GCSE & A-Level Chemistry

Atomic structure underpins the rest of Chemistry. You need to know the relative mass and charge of protons, neutrons and electrons, how to deduce electron configuration, and how isotopes give rise to a relative atomic mass.

At A-Level the topic expands into mass spectrometry, time-of-flight calculations, successive ionisation energies and sub-shell notation (1s, 2s, 2p…). Strong understanding here sets up bonding, periodicity and equilibrium later in the course.

At GCSE

At GCSE you state the relative mass and charge of protons, neutrons and electrons, deduce electron configuration in shells (e.g. Cl = 2,8,7), and explain isotopes and relative atomic mass from isotopic abundance.

At A-Level

At A-Level you write sub-shell electron configurations (1s, 2s, 2p…), interpret time-of-flight mass spectra (calculations on flight time and m/z), and explain trends in successive ionisation energies as evidence for shells and sub-shells.

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Atomic structure — key points:
- Atom: tiny dense nucleus of protons (+1) and neutrons (0), with electrons (-1) orbiting in shells/sub-shells.
- Atomic number (Z) = number of protons = number of electrons in a neutral atom.
- Mass number (A) = protons + neutrons.
- Isotopes have the same Z but different A; they have identical chemical properties (same electrons) but slightly different physical properties.
- Relative atomic mass (Ar) = weighted mean mass of an atom of an element / (1/12 mass of a 12C atom).
- Electron configuration (GCSE): 2,8,8… (e.g. Cl = 2,8,7). (A-Level): 1s², 2s², 2p⁶, 3s², 3p⁵ for chlorine.
- A-Level: time-of-flight mass spectrometry — ionise, accelerate, drift through field-free region, detect. Time taken depends on m/z.
- First ionisation energy = energy to remove one mole of electrons from one mole of gaseous atoms to form one mole of 1+ ions. Trends: decreases down a group (more shielding, greater distance), generally increases across a period (greater nuclear charge).

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Example flashcards

Q: Define relative atomic mass.
A: The weighted mean mass of an atom of an element compared with 1/12 the mass of a carbon-12 atom.
Q: Write the electron configuration of sulfur (Z = 16).
A: 1s² 2s² 2p⁶ 3s² 3p⁴.
Q: Why do isotopes have identical chemical properties?
A: They have the same number and arrangement of electrons; chemical reactions involve electrons only.
Q: Why does first ionisation energy decrease down Group 1?
A: Outer electron is in a higher shell, further from the nucleus and more shielded, so less strongly attracted.

Quick summary

Atomic Structure is a high-yield Chemistry topic for GCSE and A-Level students (AQA, Edexcel, OCR). At A-Level you write sub-shell electron configurations (1s, 2s, 2p…), interpret time-of-flight mass spectra (calculations on flight time and m/z), and explain trends in successive ionisation energies as evidence for shells and sub-shells. Examiners reward precise definitions and applied explanations — focus on the core ideas and the small set of terms that come up every series.

Key terms

Proton
Neutron
Electron
Isotope
Atomic number
Mass number
Relative atomic mass
Ionisation energy
Mass spectrometer

Atomic Structure FAQs

Why is the mass of an electron usually ignored?+

An electron is about 1/1836 the mass of a proton, so its contribution to the overall atomic mass is negligible.

How does a mass spectrometer work in outline?+

The sample is vaporised and ionised (usually to 1+), accelerated by an electric field, drifts through a field-free region, and is detected. Lighter ions arrive sooner; the time of flight gives m/z.

What causes the small drop in ionisation energy between Group 2 and Group 13?+

The outer electron in Group 13 is in a p sub-shell, which is slightly higher in energy than the s sub-shell, so it is removed more easily.

What is the difference between an atom and an ion?+

An atom has equal protons and electrons (no overall charge). An ion has lost or gained electrons and so has a positive or negative charge.

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