A level Chemistry · Edexcel 9CH0 · Topic 1C
Ionisation energies and periodicity
Key points
- First ionisation energy: energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions, e.g. Na(g) → Na⁺(g) + e⁻.
- It depends on nuclear charge, distance of the outer electron from the nucleus and shielding by inner electrons.
- Down a group IE falls: the outer electron is further away and more shielded, despite the extra protons.
- Across a period IE generally rises: more protons, similar shielding, smaller atoms.
- Dips: Group 2 → 3 (e.g. Mg → Al) because the electron comes from a higher-energy p sub-shell; Group 5 → 6 (P → S) because of repulsion between paired electrons in a p orbital.
- Successive ionisation energies jump when an electron is removed from a shell closer to the nucleus, which reveals the group number.
- Periodicity: melting points across Period 3 rise to Si (giant covalent), then fall sharply for simple molecular P₄, S₈, Cl₂ and Ar.
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Ionisation energies measure how hard it is to remove electrons, and their patterns are the main experimental evidence for shells and sub-shells. The same ideas explain the periodic trends in radius and melting temperature.
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