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Cambridge International AS Level Chemistry

Topic 1
Atomic Structure

Topic 1 covers the particles in the atom, isotopes, the arrangement of electrons in shells, sub-shells and orbitals, and ionisation energy as evidence for electronic structure. These pages cover all four parts, with the mass spectrometry pages included because they build on isotopes (the spectra themselves are examined in Topic 22).

Exam Papers
Papers 1 & 2
9701/1 and 9701/2 (AS Level)
Learning Outcomes
1.1.1 – 1.4.8
4 parts
Topic Parts
4 of 4 live
1.1, 1.2, 1.3, 1.4
Exam Board
Cambridge
CIE 9701 (2025-2027)

Revision Notes

Work through each part of Cambridge Topic 1 in a structured sequence.

Particles
Atomic
Structure
1

1.1 / 1.2 Atomic Structure and Isotopes

Protons, neutrons and electrons, atomic and nucleon number, particle numbers in atoms and ions, and isotopes (1.1.1 to 1.1.6, 1.2.1 to 1.2.4).

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Deflection
Charged
Particles
2

1.1 Charged Particles in Electric Fields

How beams of protons, neutrons and electrons behave in an electric field, and what the deflections show about charge and mass (1.1.5).

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Trends
Atomic and
Ionic Radii
3

1.1 Atomic and Ionic Radii

Atomic radius across a period and down a group, cations and anions compared with their atoms, and the isoelectronic ions of Period 3 (1.1.7, 9.1.1).

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Instrument
Mass
Spectrometry
4

1.2 Mass Spectrometry

How a mass spectrometer separates isotopes, relative isotopic and atomic mass, RAM from abundances and diatomic spectra (with 2.1.2; spectra examined in Topic 22).

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Orbitals
Electron
Configuration
5

1.3 Electrons in Atoms

Shells, sub-shells and orbitals, s and p orbital shapes, filling order and the configurations of atoms and ions using s p d notation and electrons in boxes (1.3.1 to 1.3.9).

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Energy
Ionisation
Energy
6

1.4 Ionisation Energy

First and successive ionisation energies, the factors that decide them, and the trends across a period and down a group as evidence for electronic structure (1.4.1 to 1.4.8).

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Specification Coverage

Topic 1 Atomic Structure – Cambridge International AS Level Chemistry

The following Cambridge learning outcomes state what candidates should be able to do for Topic 1. Wording is taken from the Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027).

1.1 Particles in the atom and atomic radius

1.1.1
understand that atoms are mostly empty space surrounding a very small, dense nucleus that contains protons and neutrons; electrons are found in shells in the empty space around the nucleus
1.1.2
identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
1.1.3
understand the terms atomic and proton number; mass and nucleon number
1.1.4
describe the distribution of mass and charge within an atom
1.1.5
describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field
1.1.6
determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge
1.1.7
state and explain qualitatively the variations in atomic radius and ionic radius across a period and down a group

1.2 Isotopes

1.2.1
define the term isotope in terms of numbers of protons and neutrons
1.2.2
understand the notation (A over Z)X for isotopes, where A is the mass or nucleon number and Z is the atomic or proton number
1.2.3
state that and explain why isotopes of the same element have the same chemical properties
1.2.4
state that and explain why isotopes of the same element have different physical properties, limited to mass and density

1.3 Electrons, energy levels and atomic orbitals

1.3.1
understand the terms: • shells, sub-shells and orbitals • principal quantum number (n) • ground state, limited to electronic configuration
1.3.2
describe the number of orbitals making up s, p and d sub-shells, and the number of electrons that can fill s, p and d sub-shells
1.3.3
describe the order of increasing energy of the sub-shells within the first three shells and the 4s and 4p sub-shells
1.3.4
describe the electronic configurations to include the number of electrons in each shell, sub-shell and orbital
1.3.5
explain the electronic configurations in terms of energy of the electrons and inter-electron repulsion
1.3.6
determine the electronic configuration of atoms and ions given the atomic or proton number and charge, using either of the following conventions: e.g. for Fe: 1s2 2s2 2p6 3s2 3p6 3d6 4s2 (full electronic configuration) or [Ar] 3d6 4s2 (shorthand electronic configuration)
1.3.7
understand and use the electrons in boxes notation, e.g. for Fe: [Ar] with 3d and 4s boxes
1.3.8
describe and sketch the shapes of s and p orbitals
1.3.9
describe a free radical as a species with one or more unpaired electrons

1.4 Ionisation energy

1.4.1
define and use the term first ionisation energy, IE
1.4.2
construct equations to represent first, second and subsequent ionisation energies
1.4.3
identify and explain the trends in ionisation energies across a period and down a group of the Periodic Table
1.4.4
identify and explain the variation in successive ionisation energies of an element
1.4.5
understand that ionisation energies are due to the attraction between the nucleus and the outer electron
1.4.6
explain the factors influencing the ionisation energies of elements in terms of nuclear charge, atomic/ionic radius, shielding by inner shells and sub-shells and spin-pair repulsion
1.4.7
deduce the electronic configurations of elements using successive ionisation energy data
1.4.8
deduce the position of an element in the Periodic Table using successive ionisation energy data