• # question_answer 55)   Ionisation enthalpy $({{\Delta }_{i}}{{H}_{i}}kJmo{{l}^{-1}})$ for the elements of group 13 follows the order : (a) $B\text{ }>\text{ }Al\text{ }>\text{ }Ga\text{ }>\text{ }In\text{ }>\text{ }Tl$                                              (b) $B\text{ }<\text{ }Al\text{ }<\text{ }Ga\text{ }<\text{ }In\text{ }<\text{ }Tl$ (c) $B<Al>Ga<ln>Tl$                                    (d)$B\text{ }>\text{ }Al\text{ }<\text{ }Ga\text{ }>\text{ }In\text{ }<\text{ }Tl$

(d) On moving down the group from B to$Tl$, a regular decreasing trend in the ionisation energy values is not observed. $\begin{array}{*{35}{l}} B~~~~~Al~~~~Ga~~~~In~~~~~Tl \\ 801~~~577~~~579~~~558~~~589~~~~~kJ\text{ }mo{{l}^{-1}} \\ \end{array}$ 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 ionization 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 14 f-electrons present in the inner shell.