JEE Class main Answered

On moving down the group, ionization enthalpy decreases due to an increase in the atomic size and shielding effect.

So on moving down the group ionization enthalpy decreases from B to Al.

But Ga has higher ionization enthalpy than Al. Asin Ga there are 10 d electrons in its inner electronic configuration.

Since the d electrons shield the nuclear charge less effectively than s and p electrons.

As a result the ionization enthalpy slightly increases in spite of an increase in atomic size.

In case of Tl and In the ionization enthalpy again increases.

Tl follows the 4f and 5d electrons and shielding provided by these electrons is not very effective.

So ionization enthalpy of Tl is higher than In.