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
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define first ionisation energy (IE) and use the term.
- Construct equations showing the first, the second and further ionisation energies.
- Identify and explain how ionisation energies change going down a group and along a period of the Periodic Table.
- Identify how the successive ionisation energies of one element vary, and explain the pattern.
- Understand that ionisation energy arises from the nucleus attracting the outer electron.
- Explain which factors affect the ionisation energies of elements: nuclear charge, spin-pair repulsion, atomic or ionic radius, and shielding from inner shells and inner sub-shells.
- Use data on successive ionisation energies to deduce an element's electronic configuration.
- Use data on successive ionisation energies to deduce where an element sits in the Periodic Table.
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
- › Look for the largest 'jump' in energy; a jump after four electrons indicates the fifth is coming from a closer, full inner shell.
- › State that shielding is 'similar' across a period because electrons are added to the same main energy level, while nuclear charge increases.
- › Always include gaseous state symbols (g) when writing equations for ionisation energy or electron affinity.
- › Explain the two dips in first ionisation energy across Period 3: from Mg to Al the electron is removed from a 3p orbital, higher in energy and shielded by the 3s electrons; from P to S it is removed from a doubly occupied 3p orbital, where paired electrons repel.
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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