Question

the first ionization energy of mg is higher than aluminium. why

Answer 1

Heya!!
First if all what is ionisation enthalpy??
Ionisation enthalpy is the energy required to remove one electron from the outermost shell of an isolated gaseous atom.
Now, E.C. of Mg is 1s2 2s2 2p6 3s2
and, E.C. of Al is 1s2 2s2 2p6 3s2 3p1
As it is seen from the above that Mg contains a fulfilled s orbital and Al contains a partially filled p orbital. The stability of a fulfilled orbital is much more than a partially filled orbital. So, in case of Mg, to extract the outermost electron, more energy is to be supplied. This corresponds to the cause that first I.E. of Mg is higher than Al

Hope it helped.