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A-Level Chemistry May/June 2025 Q1(b)(ii): Explain why the first ionisation energy of sulfur is less than the first ionisation ene…
A-Level Chemistry · Paper 9701/23 · May/June 2025 · Question 1(b)(ii) · [2 marks]
Explain why the first ionisation energy of sulfur is less than the first ionisation energy of phosphorus.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
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The electron configuration of phosphorus is [Ne] 3s²3p³, and the electron configuration of sulfur is [Ne] 3s²3p⁴.
In a phosphorus atom, each of the three 3p orbitals is singly occupied. In a sulfur atom, one of the 3p orbitals is occupied by a pair of electrons.
This pair of electrons in the same 3p orbital in sulfur repel each other (spin-pair repulsion). This repulsion makes it easier to remove one of these electrons.
Although sulfur has a greater nuclear charge than phosphorus (which would tend to increase the ionisation energy), the effect of the spin-pair repulsion is stronger and outweighs the effect of the increased nuclear charge. Therefore, less energy is required to remove the first electron from sulfur.
How the marks are awarded
- M1 — Correctly stating that in sulfur there is a pair of electrons in a 3p orbital which repel each other, making an electron easier to remove.
- M2 — Explaining that this repulsion effect outweighs the effect of the increased nuclear charge from phosphorus to sulfur.
Common mistakes
- Only mentioning the repulsion in sulfur (scoring M1) but failing to state that this effect outweighs the increased nuclear charge (losing M2).
- Simply stating the general trend and the exception (e.g., 'IE increases across a period but there is a dip at S') without providing an explanation based on electron structure.
- Incorrectly attributing the lower ionisation energy to increased shielding; shielding changes very little between adjacent elements in the same period.
- Vaguely mentioning 'more repulsion' in sulfur without specifying that it is the spin-pair repulsion within a single 3p orbital that is the cause.
Examiner tip: When explaining exceptions to periodic trends, always compare the specific electron configurations of the elements involved to pinpoint the exact cause.
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