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.
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