# 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

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

## 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

- [first ionisation energy](https://www.gradenine.co.uk/glossary/first-ionisation-energy)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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