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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. The mass spectrometry pages are 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
Sub-topics
6 lessons
1.1, 1.2, 1.3, 1.4
Exam Board
Cambridge
CIE 9701 (2025-2027)

Revision Notes

6 revision notes, in the order the specification teaches them.

An atom with its nucleus and electron shells
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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Beams of particles bending between charged plates
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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An atom beside its smaller cation and larger anion
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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A mass spectrometer with a stream of ions curving to a detector
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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An s orbital beside three p orbitals
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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An electron pulled clear of an atom
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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Summary Map

Download, print and revise the key facts of this topic at a glance.

Summary map

Download, print and revise the key facts of this topic at a glance.

Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Atomic Structure Map

Cambridge International AS & A Level Chemistry · Topic 1 Atomic structure

The whole section on one A4 page: the particles, beams in an electric field, isotopes, relative masses, mass spectra and atomic and ionic radii.

Download the map6 key ideas + a data table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Electronic Configuration Map

Cambridge International AS & A Level Chemistry · Topic 1 Electronic configuration

The whole section on one A4 page: shells, sub-shells and orbitals, the filling order and ground state, boxes and repulsion, ions and free radicals.

Download the map6 key ideas + a data table · A4 portrait · print ready

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Specification

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1.1.1

  1. a

    understand that atoms are mostly empty space surrounding a very small, dense nucleus that contains protons and neutrons

    Revise
  2. b

    understand that electrons are found in shells in the empty space around the nucleus

    Revise

1.1.2

  1. a

    identify and describe protons in terms of their relative charge and relative mass

    Revise
  2. b

    identify and describe neutrons in terms of their relative charge and relative mass

    Revise
  3. c

    identify and describe electrons in terms of their relative charge and relative mass

    Revise

1.1.3

  1. a

    understand the terms atomic and proton number

    Revise
  2. b

    understand the terms mass and nucleon number

    Revise

1.1.4

  1. a

    describe the distribution of mass within an atom

    Revise
  2. b

    describe the distribution of charge within an atom

    Revise

1.1.5

  1. a

    describe the behaviour of beams of protons moving at the same velocity in an electric field

    Revise
  2. b

    describe the behaviour of beams of neutrons moving at the same velocity in an electric field

    Revise
  3. c

    describe the behaviour of beams of electrons moving at the same velocity in an electric field

    Revise

1.1.6

  1. a

    determine the numbers of protons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge

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  2. b

    determine the numbers of neutrons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge

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  3. c

    determine the numbers of electrons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge

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1.1.7

  1. a

    state and explain qualitatively the variations in atomic radius across a period

    Revise
  2. b

    state and explain qualitatively the variations in atomic radius down a group

    Revise
  3. c

    state and explain qualitatively the variations in ionic radius across a period

    Revise
  4. d

    state and explain qualitatively the variations in ionic radius down a group

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1.2.1

  1. 1.2.1

    define the term isotope in terms of numbers of protons and neutrons

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1.2.2

  1. 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

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1.2.3

  1. 1.2.3

    state that and explain why isotopes of the same element have the same chemical properties

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1.2.4

  1. 1.2.4

    state that and explain why isotopes of the same element have different physical properties, limited to mass and density

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1.3.1

  1. a

    understand the terms shells, sub-shells and orbitals

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  2. b

    understand the term principal quantum number (n)

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  3. c

    understand the term ground state, limited to electronic configuration

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1.3.2

  1. a

    describe the number of orbitals making up s, p and d sub-shells

    Revise
  2. b

    describe the number of electrons that can fill s, p and d sub-shells

    Revise

1.3.3

  1. 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

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1.3.4

  1. 1.3.4

    describe the electronic configurations to include the number of electrons in each shell, sub-shell and orbital

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1.3.5

  1. a

    explain the electronic configurations in terms of energy of the electrons

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  2. b

    explain the electronic configurations in terms of inter-electron repulsion

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1.3.6

  1. a

    determine the electronic configuration of atoms given the atomic or proton number, using either the full electronic configuration (e.g. for Fe: 1s2 2s2 2p6 3s2 3p6 3d6 4s2) or the shorthand electronic configuration (e.g. for Fe: [Ar] 3d6 4s2)

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  2. b

    determine the electronic configuration of ions given the atomic or proton number and charge, using either the full electronic configuration or the shorthand electronic configuration

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1.3.7

  1. 1.3.7

    understand and use the electrons in boxes notation, e.g. for Fe: [Ar] with 3d and 4s boxes

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1.3.8

  1. a

    describe and sketch the shape of s orbitals

    Revise
  2. b

    describe and sketch the shape of p orbitals

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1.3.9

  1. 1.3.9

    describe a free radical as a species with one or more unpaired electrons

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1.4.1

  1. 1.4.1

    define and use the term first ionisation energy, IE

    Revise

1.4.2

  1. a

    construct equations to represent first ionisation energies

    Revise
  2. b

    construct equations to represent second and subsequent ionisation energies

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1.4.3

  1. a

    identify and explain the trends in ionisation energies across a period of the Periodic Table

    Revise
  2. b

    identify and explain the trends in ionisation energies down a group of the Periodic Table

    Revise

1.4.4

  1. 1.4.4

    identify and explain the variation in successive ionisation energies of an element

    Revise

1.4.5

  1. 1.4.5

    understand that ionisation energies are due to the attraction between the nucleus and the outer electron

    Revise

1.4.6

  1. a

    explain the factors influencing the ionisation energies of elements in terms of nuclear charge

    Revise
  2. b

    explain the factors influencing the ionisation energies of elements in terms of atomic/ionic radius

    Revise
  3. c

    explain the factors influencing the ionisation energies of elements in terms of shielding by inner shells and sub-shells

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  4. d

    explain the factors influencing the ionisation energies of elements in terms of spin-pair repulsion

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1.4.7

  1. 1.4.7

    deduce the electronic configurations of elements using successive ionisation energy data

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1.4.8

  1. 1.4.8

    deduce the position of an element in the Periodic Table using successive ionisation energy data

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