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.
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