The shape of orbital influences the ionization potential. As s-electrons remain closer to the nucleus than p-,d-, and f-electrons of the same valence shell, the ionization energy decreases in the order given below:
s > p > d > f
For instance, the first ionization energy of aluminum is lower than that of magnesium.
The electronic configuration of magnesium is [Ne]3s2 and that of aluminum is [Ne] 3s2 3p1. Thus, one has to remove 3p-electron in case of aluminum and 3s electron in the case of magnesium. But it is easier to remove the p electron than the s-electron. Thus, the first ionization energy of aluminum is lower than that of magnesium.