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Cambridge International AS Level Chemistry

Ionisation Energy

Ionisation energy measures the energy required to remove an electron from a gaseous atom or ion. These pages build from the definition of first ionisation energy through to successive ionisation energy patterns and the main periodic trends across a period and down a group.

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
1.1.7 – 1.4.8
9 points covered
Sub-topics
4 lessons
Revision notes available
Exam Board
Cambridge
CIE 9701 (2025-2027)

Revision Notes

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

An electron pulled clear of an atom
1

Introduction to Ionisation Energy

Revise the definition of first ionisation energy, state symbols and the factors that affect electron removal.

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Bars rising steadily then jumping, showing successive ionisation energies
2

Successive Ionisation Energies

Use successive ionisation energy data to identify electron shells, group number and large jumps between energy levels.

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Atoms shrinking across a period
3

Trends in a Period

Explain why first ionisation energy generally increases across a period, including the small drops at key points.

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Atoms growing larger down a group
4

Trends in a Group

Explain why first ionisation energy decreases down a group as shielding and atomic radius increase.

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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

Ionisation Energies Map

Cambridge International AS & A Level Chemistry · Topic 1 Ionisation energy and 9.1 Period 3

The whole section on one A4 page: ionisation energies and their four factors, successive IEs, trends, and Period 3 melting points and conductivity.

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

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Revision Game

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Study Cards

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Specification

Every specification point in this topic, in the exam board's own words. Rate how confident you are with each one.

Official Cambridge documentsSyllabus (opens the Cambridge website in a new tab)

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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

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

    Revise

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

    Revise
  4. d

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

    Revise

1.4.7

  1. 1.4.7

    deduce the electronic configurations of elements using successive ionisation energy data

    Revise

1.4.8

  1. 1.4.8

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

    Revise