• # question_answer 29) The first ionisation enthalpy values (in $KJ\,\,mo{{l}^{-1}}$) of group 13 elements are : $\underset{801}{\mathop{B}}\,\,\,\,\,\,\,\,\,\,\underset{577}{\mathop{Al}}\,\,\,\,\,\,\,\,\,\underset{579}{\mathop{Ga}}\,\,\,\,\,\,\,\,\,\underset{558}{\mathop{\ln }}\,\,\,\,\,\,\underset{589}{\mathop{Tl}}\,$ How will you explain this deviation from the general trend?

The general trend for normal groups is that ionisation enthalpy decreases regularly on moving down the group. However, in group 13, it is observed that ${{\Delta }_{i}}{{H}_{1}}$ of $Ga$is slightly higher than ${{\Delta }_{i}}{{H}_{1}}$ of Al and ${{\Delta }_{i}}{{H}_{1}}$ of $\text{Tl}$ is higher than those of$\text{Tl}$, $Ga$and In. These deviations are explained as follows: Al occupies its position after. s-block elements while and In occupy their positions after $d$-block elements and $Tl$ after d- and $f$-block elements. The d- and$f$-electrons do not shield the outer shell electrons effectively. Thus, outer electrons are held tightly by the nucleus and higher amount of energy is needed for the removal. Hence, ${{\Delta }_{i}}{{H}_{1}}$ of $Ga$ is higher than$\text{Al}$. On moving down in the group from $Ga$to in, the increased shielding effect outweighs the increased nuclear charge and thus, ${{\Delta }_{i}}{{H}_{1}}$ decreases. There after the effect of nuclear charge outweighs the shield effect of $5d$- and $4f$-electrons and hence${{\Delta }_{i}}{{H}_{1}}$ of $Tl$ increases.