The first ionization energies of phosphorus and of sulfur are 1012 and 1000 kJ/mol.Why is IE₁ for S lower than that of P when the usual trend is for IE to increase across a row?

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Quizplus · Accepted Answer

Removing an electron from sulfur, which has a paired electron in one of its p orbitals, reduces electron-electron repulsions. This makes it slightly easier to remove an electron from sulfur compared to phosphorus, where all p electrons are unpaired and thus experience less repulsion.

The First Ionization Energies of Phosphorus and of Sulfur Are