Explain why the first ionisation energy of sulphur is lower than that of phosphorus.

The general trend is that the first ionisation energies increases across the period from Na to Ar. But the first ionisation energy of sulphur is lower than phosphorus because the electronic configuration is less stable. Like when you get on the bus, you would sit on whatever seat you can where you don’t have to sit next to someone until all of the free two-seats are used up. The same is with electrons - they fit into the orbitals one at a time until the only orbitals available are ones with electrons already in them. There is extra repulsion between electrons when two fit into the same orbital. The electronic configuration of phosphorus (3p3) means the electrons are in a 'maximum unpairing' configuration, which is very stable. The extra electron of sulphur (3p4) is added to an orbital which already contains an electron and the repulsion between these two electrons makes it easier to remove this electron and therefore the first ionisation energy is lower.