Question

why is the first ionization enthalpy of Mg greater than Al

Answer 1

 Its outer electron is in the 3p-orbital which is slightly further from the nucleus and is shielded by the 3s-orbital and is therefore requires less energy to remove than the magnesium's outer electron which is closer and not as shielded.

Answer 2

the first ionisation enthalpy of Mg is greater than. Al because the electronic configuration of Mg is 1s2 2s2 2p6 3s2 while that of Al is. 1s2 2s2 2p6 3s2 3p1 thus due to smaller size of the s orbital and nearer to nucleus experiences much more force than the
1 electron in p orbital thus it requires more energy to remove electron from s orbital