(solved)Question 3.19 of NCERT Class XI Chemistry Chapter 3

The IE order is expected to be B > Al > Ga > In > Tl

but observed to be B > Tl > Ga > Al > In

first write the electronic configuration of each

B is $\displaystyle [He]2s^22p^1$

Al is $\displaystyle [Ne]3s^23p^1$

Ga is $\displaystyle [Ar]4s^23d^{10}4p^1$

In is $\displaystyle [Kr]5s^24d^{10}5p^1$

Tl is $\displaystyle [Xe]6s^24f^{14}5d^{10}6p^1$

Shielding order is s > p > d > f

Anomaly of Gallium: The order of Ga and Al is opposite of what is expected.

Gallium[Z = 31] has a greater nuclear charge but the 3d electrons of Ga provide very poor shielding, so the 4p electron experiences a greater effective nuclear charge and is more tightly bound to the nucleus, leading to an abnormally high IE. In fact, the size of Ga atom is smaller than that of Al because of the same reasons.

anomaly of Thallium: Thallium[Z = 81] has a greater nuclear charge but the 5f and 5d electrons provide very poor shielding, so the 6p electron experiences a greater effective nuclear charge and is more tightly bound to the nucleus. This explains the anomaly of its higher IE.

The IE of thallium is so high (because of poor shielding and greater nuclear charge) that it comes next to boron.

The Position of Indium: Indium is screened by d electrons in exactly the same way as in the case of Ga. Because of that the effect of screening is neither reduced, nor increased. Therefore, Indium follows the expected trend and has its IE after Ga and Al.