Atomic structure
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
The topic on one screen
- Protons and neutrons in the nucleus, electrons in shells. Mass number = protons + neutrons; isotopes differ only in neutron number.
- Relative atomic mass = the mean mass of an atom divided by 1/12 the mass of a carbon-12 atom.
- TOF mass spectrometer: ionise → accelerate → ion drift (separate by m/z) → detect. Ions must be charged to be accelerated and to make a current.
- At the detector, each positive ion gains an electron; the current produced is proportional to that ion's abundance.
- Ar from a spectrum: (sum of isotope mass x abundance) / (total abundance).
- Ionisation energy rises across a period (more nuclear charge, similar shielding) and falls down a group (outer electron further out and more shielded).
Where students actually lose marks
In the Ar definition, 'mean mass' and comparison to 1/12 the mass of a carbon-12 atom were crucially missed. Give the full definition, not just 'average mass of the atoms'.
June 2024 Paper 1 examiner report (Q01.2)
In mass spectra, students knew a current is produced but not why: the ion gains an electron at the detector, and the current is proportional to abundance. State both.
June 2023 Paper 1 examiner report (Q09.3)
In the relative-mass calculation, common errors were giving the mass of one atom rather than one mole, and not giving the mass number as an integer.
June 2024 Paper 1 examiner report (Q01.4)
Try it — exam-style
A sample of boron contains 19.9% boron-10 and 80.1% boron-11. Calculate the relative atomic mass of boron to 1 decimal place.
Define relative atomic mass.
In a TOF mass spectrometer, explain how the ions produce a signal at the detector and what the size of the signal tells you.
Explain why the first ionisation energy of magnesium is higher than that of sodium.
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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