The correct order of first ionisation enthalpy of group-13 elements is
The correct order of first ionisation enthalpy of group-13 elements is
$\mathrm{B}>\mathrm{Ga}>\mathrm{T} l$
$\mathrm{~B}>\mathrm{T} l>\mathrm{Ga}$
$\mathrm{Ga}>\mathrm{B}>\mathrm{T} l$
$\mathrm{~T} l>\mathrm{Ga}>\mathrm{B}$
Solution
On moving down the group from B to Tl, a regular decreasing trend in the I.E. is not observed.
\(\begin{array}{|c|c|c|c|c|c|}
\hline \text { Element } & \text { B } & \text { Al } & \text { Ga } & \text { In } & \text { Tl } \\
\hline \text { I.E. }(\mathrm{kJ} / \mathrm{mol}) & 801 & 577 & 579 & 558 & 589 \\
\hline
\end{array}\)
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 ionisation energy of both Al and Ga is nearly the same. The increase in ionisation energy from In to T1 is due to poor screening effect of 14 f electrons present in the inner shell. Thus, the correct order is, \(\mathrm{B}>\mathrm{NI}>\mathrm{Ga}\).
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