Flexi Says: Within a group, the ionization energy decreases as the size of the atom gets larger. The ionization energy increases as we go up the group to smaller atoms. In case of larger atoms in a group, the first electron removed is farther from the nucleus as the atomic number (number of protons) increases. Being farther away from the positive attraction makes it easier for that electron to be pulled off. As more electrons are added around a nucleus, the outer electrons become shielded from the nucleus by the inner shell electrons. This is called electron shielding.