Question

The first ionisation enthalpy of Mg is higher than AI. Why?

Answer 1

becauseis outer electron is in 3p-orbital with sprightly further from the nucleus and shielded by the 3s-orbitial and is therefore required less energy to remove than the magnesium outer electron which is closer and not shielded

Answer 2

first of all
if any element have fully filled outermost shell or half filled outermost shell is stable and require more energy to loss the electrons
then....
electronic configuration of Mg
1s^2, 2s^2, 2p^6, 3s^2
its valence shell is fully filled
but electronic configuration of Al is
1s^2, 2s^2, 2p^6, 3s^2, 3p1
so it has neither fully filled outermost shell nor half filled outermost shell

so the ionization enthalpy of Mg is higher than AI.