Classification of Elements Part 1

On moving left to right in a period of the periodic table, lonisation energy (I.E.) increases due to increase in effective nuclear charge i.e. Z‌eff ‌.
But due to extra stability of fully filled and half-filled electronic configuration of Mg and P required ionisation enthalpy is more from neighbouring elements.
i.e. First ionisation enthalpy order is Al<Mg<Si<S<P.