Explain why the first ionisation energy of chlorine is greater than that of sulfur.

IB DP Chemistry Standard Level (2023 syllabus) — S1.3 Electron configurations · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

First ionisation energies (in kJ mol⁻¹): S = 1000, Cl = 1251.

Model answer (4 marks)

Chlorine has a greater nuclear charge (17 protons) than sulphur (16 protons), so the attraction between the nucleus and the outer electrons is stronger.
Both elements have the same number of inner electron shells, so the shielding is essentially the same.
Therefore the increase in nuclear charge is felt by the valence electrons, giving Cl a greater effective nuclear charge.
Consequently the outermost electron in Cl is held more tightly and more energy is required to remove it, so the first ionisation energy of Cl is higher than that of S.

Examiner tips

  • Use the term "effective nuclear charge" to show understanding of attraction vs shielding.
  • Show that both elements have the same shielding so the difference is due to nuclear charge.
  • Explain the consequence: higher energy needed to remove an electron.
  • Keep the answer concise and to the point.

Common mistakes

  • Confusing nuclear charge with atomic number only; forgetting to mention shielding.
  • Using vague phrases like "more electrons" instead of "greater effective nuclear charge".
  • Failing to state that both elements have the same number of inner shells.

Mark scheme (4 marks)

  1. Chlorine has a greater nuclear charge (more protons: 17) than sulfur (16), so the attraction between the nucleus and the outer electrons is stronger.
  2. Both elements have the same number of inner electron shells (same shielding / shielding is similar), so the increase in nuclear charge is felt by the outer electrons.
  3. The outer (valence) electrons in Cl experience a greater effective nuclear charge than those in S, so they are held more tightly.
  4. Therefore more energy is required to remove the outermost electron from Cl than from S, giving Cl a higher first ionisation energy.

Key terms in this question

first ionisation energy

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