Explain why the first ionisation energy of sulfur is lower than that of phosphorus, despite sulfur having a greater nuclear charge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Both elements are in period 3, so they have the same number of shells and similar shielding.
Phosphorus has a half‑filled 3p sub‑level (3p³), giving a particularly stable arrangement.
Sulphur has a 3p⁴ configuration, where one 3p orbital contains a pair of electrons.
The electron–electron repulsion between the paired electrons in sulphur’s 3p orbital makes one electron easier to remove, so the first ionisation energy of sulphur is lower than that of phosphorus.
Phosphorus has a half‑filled 3p sub‑level (3p³), giving a particularly stable arrangement.
Sulphur has a 3p⁴ configuration, where one 3p orbital contains a pair of electrons.
The electron–electron repulsion between the paired electrons in sulphur’s 3p orbital makes one electron easier to remove, so the first ionisation energy of sulphur is lower than that of phosphorus.
Examiner tips
- State that both are in period 3 and have similar shielding. Mention phosphorus’s half‑filled 3p³ stability. Explain sulphur’s 3p⁴ pairing and the resulting repulsion. Link the repulsion to the lower ionisation energy.
Common mistakes
- Confusing nuclear charge with effective nuclear charge. Forgetting that both elements have the same number of shells. Not mentioning the half‑filled 3p stability of phosphorus.
Mark scheme (4 marks)
- Both sulfur and phosphorus are in the same period, so they have the same number of electron shells / similar shielding.
- Phosphorus has a half-filled 3p sub-level (3p³), which is a particularly stable arrangement.
- Sulfur has a 3p⁴ configuration in which one 3p orbital contains two paired electrons.
- Inter-electronic / electron–electron repulsion between the paired electrons in sulfur's 3p orbital makes one electron easier to remove, lowering the first ionisation energy relative to phosphorus.
Key terms in this question
first ionisation energy · nuclear charge
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
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- Decode the mark scheme abbreviations →
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