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Why is the ionisation potential of sulphur lower than phosphorus?

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There are three competing factors for which the ionisation potential of sulphur lower than phosphorus: 
1. The effective nuclear charge 
2. The size of the atom and the force of attraction according to Coulomb's law 
3. A pair of electrons in an orbital 

In case of phosphorous and sulfur, phosphorous has the greater ionization energy which means that it takes more energy to remove an electron from a phosphorous atom, which means that the electrons are lower in energy, even though the atom is larger and sulfur has a greater effective nuclear charge. 

This is what we call as stable half filled configuration of phosphorus.

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