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)\[B\text{ }>\text{ }Al\text{ }<\text{ }Ga\text{ }>\text{ }In\text{ }<\text{ }Tl\]

Answer:

  (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.