The first ionization enthalpy values (in kJ mol - 1 ) of group - 13 elements are: B Al Ga In Tl 801 577 579 558 589 How would you explain this deviation from the general trend?

Generally, on moving down the group, ionization enthalpy generally decreases due to an increase in the atomic size and shielding. Ionization energy ∝ 1 atomic size . Ionization energy ∝ 1 shielding effect . Thus, on moving down the group, ionization enthalpy decreases from B to Al . But, Ga has higher ionization enthalpy than Al . The Al follows immediately after the s - block elements, whereas, the Ga follows after the d - block elements. The shielding provided by the d - electrons are not very effective. These electrons do not shield the valence electrons very effectively. As a result, the valence electrons of Ga experience a greater effective nuclear charge than those of Al . Further, moving from Ga to In , the ionization enthalpy decreases due to an increase in the atomic size and shielding. But, on moving from In to Tl , the ionization enthalpy again increases. In the periodic table, Tl follows after the 4 f and the 5 d electrons. The shielding provided by the electrons in both these orbitals is not very effective. Therefore, the valence electron is held quite strongly by the nucleus. Hence, the ionization energy of Tl is on the higher side.