Question

Ionisation enthalpy $\left({\Delta }_i{H}_{1}kJmo{l}^{-1}\right)$ for the elements of Group $13$ follows the order:

• A. $B>Al>Ga>In>Tl$
• B. $B<Al>Ga<In>Tl$
• C. $B<Al>Ga>In>Tl$
• D. $B>Al<Ga>In<Tl$

Answer 1

Answer: (D)

$B>Al<Ga>In<Tl$

On moving down the group from $B$ to $Tl$, a regular decreasing trend in the I.E. is not observed.

Element	$B$	$Al$	$Ga$	$In$	$Tl$
I.E. (kJ/mol)	801	577	579	558	589

In Ga, there are ten d-electrons in the penultimate shell which screen the nuclear charge less effectively and thus, outer electron is held firmly. As a result, the ionisation energy of both Al and Ga is nearly the same. The increase in ionisation energy from In to Tl is due to poor screening effect of 14f electrons present in the inner shell.

Thus, the correct order is, $B>Al<Ga>In<Tl$