Question

Following the transition elements, (IE)1 drops abruptly in Ga, In and Tl. This is due to :-
1. Decrease in effective nuclear charge.
2. Increases in atomic radius.
3. Removal of an electron from the singly occupied np orbitals of higher energy than the ns-orbitals of Zn, Cd and Hg.
4. None is correct.
​​​​With explanation.

Answer 1

1. Decrease in effective nuclear charge.

2. Increases in atomic radius.

Explanation :

On moving down a group, ionization enthalpy generally decreases due to an increase in the atomic size and shielding. Thus, on moving down group 13, ionization enthalpy decreases from B to Al. But, Ga has higher ionization enthalpy than Al. Al follows immediately after s–block elements, whereas Ga follows after d–block elements. The shielding provided by d-electrons is 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 4f and 5d 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.