The first ionisation energy of sulphur is lower than that of phosphorus mainly because ____.
Correct answer C. sulphur has one pair of electrons in a 3p orbital, and their mutual repulsion makes one easier to remove
Explanation
Phosphorus has three unpaired 3p electrons (3p³). In sulphur (3p⁴) two electrons share one 3p orbital, and spin-pair repulsion raises their energy, so removing one needs less energy.
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