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?

Answer:

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.