Explain why the ionisation energy of a group III atom is lower than that of group II.
Mg has an electronic configuration of 1s22s22s63s2, Al has an electronic configuration of 1s22s22s63s23p1.
Shielding in Al is higher than that of Mg since the 3s orbital (a spherical orbital) in Mg is full.
Effective nuclear charge on the outermost electrons in Al decreases. (less tightly held by the nucleus)
The Ionisation Energy decreases for Al.