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Learn9701Paper 1 · Multiple ChoiceAtomic structureIonisation energies
Atomic structure · Subtopic 2 of 2

Ionisation Energies

The first ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms: X(g)→X+(g)+e−\mathrm{X(g)\rightarrow X^+(g)+e^-}. It depends on nuclear charge, distance of the outer electron from the nucleus and shielding. Across a period it generally increases (more protons, similar shielding); down a group it decreases (further away, more shielding). Dips occur at Group 13 (new p subshell, higher energy) and Group 16 (spin-pair repulsion). Successive ionisation energies show big jumps when an inner shell is broken into.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 1≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define and write equations for first and successive ionisation energies.
  • Explain trends across periods and down groups.
  • Explain the dips at groups 13 and 16.
  • Use successive ionisation energies to find an element's group.
Pause and think

Why is it harder to remove an electron from fluorine than from lithium, even though both are in period 2?

Section 2 of 14

Core idea

Start hereFingerprints of shells

Plot the energy needed to strip electrons off an atom one by one and you'll see sudden cliffs. Each cliff is a new, inner shell.

The big idea

Ionisation energy depends on nuclear charge, distance and shielding. These explain trends across periods, down groups, and the jumps in successive ionisation energies.

1. Explaining the trend across period 3

Factors affecting ionisation energy

Weigh up the three factors.

Worked exampleSuccessive ionisation energies

IEs (kJ/mol): 1st 1000, 2nd 2250, 3rd 3360, 4th 4560, 5th 7010, 6th 8500, 7th 27100.

Goal
Group and possible element (period 3).
Strategy
Find the biggest jump.
  1. jump between 6th and 7th\text{jump between 6th and 7th}
    7th electron removed from an inner shell.
  2. 6 outer electrons6\ \text{outer electrons}
    Group 16.
AnswerGroup 16: sulfur.
✓ CheckFirst IE of S is about 1000 kJ/mol ✓.
Key takeawayTwo dips in period 3 Mg → Al: 3p is higher in energy and shielded by 3s. P → S: the first paired 3p electron is repelled by its partner.
Pause and think

Why does the second IE of sodium jump so much higher than its first?

Summary

  • Charge, distance, shielding.
  • Dips at groups 13 and 16.
  • Jumps reveal shells.
Key words
First ionisation energy
Energy to remove one mole of electrons from one mole of gaseous atoms.
Shielding
Repulsion of outer electrons by inner electrons.
Spin-pair repulsion
Repulsion between two electrons in the same orbital.
HistoryEvidence for shells. Successive ionisation energy data, measured in the early 1900s, were among the first experimental evidence that electrons are arranged in shells.
Fun factHelium holds tight. Helium has the highest first ionisation energy of any element, 2372 kJ/mol, which is why it's so unreactive.
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