Ionisation energy
Cambridge A-Level Chemistry (9701) · Unit 1: Atomic structure · 9 flashcards
Ionisation energy is topic 1.4 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 1 — Atomic structure , alongside Particles in the atom and atomic radius, Isotopes and Electrons, energy levels and atomic orbitals. In one line: First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly positive gaseous ions.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 9 flashcards — 2 definitions and 7 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'first ionisation energy'
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly positive gaseous ions.
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define and use the term first ionisation energy, IE
- construct equations to represent first, second and subsequent ionisation energies
- identify and explain the trends in ionisation energies across a period and down a group of the Periodic Table
- identify and explain the variation in successive ionisation energies of an element
- understand that ionisation energies are due to the attraction between the nucleus and the outer electron
- 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
- deduce the electronic configurations of elements using successive ionisation energy data
- deduce the position of an element in the Periodic Table using successive ionisation energy data
- www.cambridgeinternational.org/alevel
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Ionisation energy
- › Always define relative atomic mass as the weighted average mass of atoms of an element compared to 1/12th the mass of a carbon-12 atom.
- › Specify the exact atomic orbitals involved in hybridisation, such as the sp hybridised orbitals of nitrogen in the N2 molecule.
- › Look for the largest 'jump' in energy; a jump after four electrons indicates the fifth is coming from a closer, full inner shell.
- › Use the syllabus definition: nucleon number is the total number of protons and neutrons in the nucleus of an atom.
- › State that shielding is 'similar' across a period because electrons are added to the same main energy level, while nuclear charge increases.
Define the term 'first ionisation energy'.
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly positive gaseous ions.
Write the equation representing the second ionisation energy of Magnesium.
The second ionisation energy equation for Magnesium is: Mg⁺(g) → Mg²⁺(g) + e⁻. This represents removing an electron from a Mg⁺ ion in the gaseous phase.
Explain the general trend in first ionisation energy across a period (e.g., Period 3).
Across a period, first ionisation energy generally increases due to increasing nuclear charge with similar shielding, leading to a stronger attraction between the nucleus and outer electrons. The atomic radius also decreases, further increasing the attraction.
Explain the general trend in first ionisation energy down a group (e.g., Group 1).
Down a group, first ionisation energy generally decreases due to increasing atomic radius and increased shielding by inner shells. This results in a weaker attraction between the nucleus and outer electrons.
Explain why successive ionisation energies for an element always increase.
Successive ionisation energies increase because each electron is being removed from an increasingly positive ion. This requires more energy to overcome the greater attraction between the nucleus and the remaining electrons.
List the four main factors influencing ionisation energy.
The main factors are: (1) Nuclear charge (2) Atomic/ionic radius (3) Shielding by inner shells/sub-shells (4) Spin-pair repulsion. These factors affect the strength of attraction between the nucleus and the valence electrons.
How can successive ionisation energy data be used to deduce the electronic configuration of an element?
Large jumps in successive ionisation energies indicate the removal of an electron from a new electron shell, closer to the nucleus. By counting the number of electrons removed before each large jump, the number of electrons in each shell can be determined, revealing the electronic configuration.
Explain how spin-pair repulsion affects ionisation energy.
Spin-pair repulsion occurs when two electrons occupy the same orbital. This repulsion makes it slightly easier to remove one of these electrons, resulting in a lower ionisation energy than expected compared to an electron in a singly occupied orbital.
Explain why the first ionisation energy of Aluminium is lower than that of Magnesium.
Aluminium's outer electron is in a 3p orbital, which is higher in energy and therefore further from the nucleus compared to Magnesium's 3s electrons. The 3p electron is also slightly more shielded. This makes the removal of the 3p electron easier, resulting in a lower first ionisation energy for Aluminium.
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Ionisation energy sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Ionisation energy deck
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