Atomic Structure: Fundamental Particles, Isotopes, TOF-MS and Electron Configuration

This guide covers 3.1.1, Atomic structure, the first section of Physical chemistry in OxfordAQA International AS and A-level Chemistry 9620, assessed as International AS content. It precedes Chemical equilibria, Le Chatelier’s principle and Kc elsewhere in Physical chemistry, and establishes the atomic-level model the whole course builds on.

Fundamental particles

You need to be able to describe an atom as a nucleus containing protons and neutrons surrounded by electrons, and know the relative charge and relative mass of each of the three fundamental particles. The specification also expects an appreciation that knowledge and understanding of atomic structure has evolved over time – this is not just a fact to state, but a framing the course wants you to carry into how scientific models are built and revised as new evidence emerges.

Mass number, isotopes, and the time-of-flight mass spectrometer

You need to work with mass number (A) and atomic/proton number (Z) to determine the number of protons, neutrons and electrons in atoms and ions, and to explain the existence of isotopes – atoms of the same element with different numbers of neutrons.

The specification requires understanding the principles of a simple time-of-flight (TOF) mass spectrometer, limited to four stages: ionisation, acceleration (giving all ions constant kinetic energy), ion drift, and ion detection, followed by data analysis. Unlike the OCR specification’s equivalent subtopic, which explicitly excludes knowledge of how a mass spectrometer works, OxfordAQA 9620 requires you to understand this instrument’s operating principle directly.

Two closely related applications follow from this: mass spectrometry gives accurate information about relative isotopic mass and about relative abundance, and can be used both to identify elements and to determine relative molecular mass. You need to be able to interpret simple mass spectra of elements, and calculate relative atomic mass from isotopic abundance data (limited to mononuclear ions – that is, single-atom ions rather than molecular or polyatomic ones, so mass-to-charge ratio equals mass directly).

Electron configuration and ionisation energies

You need to know electron configurations of atoms and ions up to Z = 36, expressed in terms of shells and sub-shells (orbitals s, p and d). Alongside this sits a specific, evidence-based skill: you need to be able to define first ionisation energy, write equations for first and successive ionisation energies, and explain how the pattern of first and successive ionisation energies across Period 3 (Na–Ar) and down Group 2 (Be–Ba) gives evidence for electron configuration in sub-shells and in shells.

This last point is a favourite exam theme: a graph of successive ionisation energies for a given element shows large jumps between shells (because removing an electron from a closer, lower shell requires much more energy) and smaller, more gradual increases within a shell as sub-shells fill. Being able to read a specific ionisation energy dataset and infer an element’s electron configuration and periodic-table group from the pattern of jumps is a distinct, testable skill – not simply reciting configurations from memory.

A worked example

Magnesium’s successive ionisation energies (in kJ mol⁻¹, first through twelfth) show a small but steady rise for the first two removals (from the 3s sub-shell), then a very large jump to the third ionisation energy (removing an electron from the full 2p sub-shell, much closer to the nucleus), a further large jump around the ninth removal (into the 2s sub-shell), and an enormous final jump for the last two removals (the 1s shell, closest to the nucleus). Three big jumps like this – after 2 electrons, then again a little later, then again near the end – are exactly the pattern expected for an element with electron configuration 1s² 2s² 2p⁶ 3s², and confirm magnesium sits in Group 2 with two outer-shell electrons available before the first large jump.

How TOF mass spectrometry and electron configuration connect

These two strands of 3.1.1, while assessed together as one section, serve different purposes in the wider course. TOF mass spectrometry is primarily an experimental technique for determining mass and composition data, feeding directly into 3.1.2 Amount of substance, where relative atomic mass underpins every mole calculation. Electron configuration and ionisation energy, by contrast, are conceptual tools that explain periodic trends and later underpin 3.1.3 Bonding (ionic and covalent bonding both depend on electron arrangement) and further periodicity content later in the course.

How to approach it

Learn the four-stage TOF mass spectrometer sequence (ionisation, acceleration, ion drift, ion detection) as an ordered process you can describe in words, since exam questions often ask you to explain what happens at a named stage rather than just list the stages. For ionisation energy questions, practise reading successive-ionisation- energy graphs specifically for the size and position of the biggest jumps, since that is what identifies a shell boundary – a common exam task is inferring an element’s group from a given ionisation energy dataset alone. Keep the “mononuclear ions” restriction in mind when interpreting a mass spectrum: relative atomic mass calculations from abundance data in this specification are limited to single-atom ions and do not require you to account for molecular fragmentation patterns beyond that scope. Finally, treat electron configuration up to Z = 36 as a skill to derive systematically from the periodic table’s block structure (s, p, d blocks) rather than a list to memorise element-by-element – this scales far better once you reach transition-metal configurations later in the course.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 4.3, for International AS and A-level exams May/June 2020 onwards: official specification PDF, section 3.1.1 “Atomic structure”. Verified 2026-09-02.