The first ionization energy of gallium is greater than that of aluminum and the first ionization energy of thallium is greater than that of indium. A possible explanation for this is
◦ This is the normal trend in ionization energy.
◦ Ga follows a series of transition elements and Tl follows both a series of transition elements and inner transition elements which are poor shielders of nuclear charge.
◦ Both Ga and Tl prefer to lose three electrons rather than one.
◦ Both Ga and Tl follow transition elements which are excellent shielders of nuclear charge.