Question

Ionisation enthalpy $\left({△}_i{H}_{l}kJmo{l}^{-1}\right)$ for the elements of group $13$ follows the order.

• A. $B>Al<Ga>In<Tl$
  
• B. $B<Al>Ga<In>Tl$
  
• C. $B>Al>Ga>In>Tl$
• D. $B<Al<Ga<In<Tl$
  

Answer 1

The correct option is A $B>Al<Ga>In<Tl$


Down the group $13$, from $BtoTl$, a regular decreasing trend in the ionisation enthalpy is not observed.

The decrease from $BtoAl$ is associated with the size.

The observed discontinuity in the $I{E}_1$ values between $Al$ and $Ga$ is due to $3d^{10}$ electrons in the penultimate or $2^{nd}$ last shell in Gallium which shields/screens the nuclear charge less effectively and outermost electrons are more attracted towards nucleus in $GathanAl$.

∴$\therefore I{E}_1\left(Ga\right)>I{E}_1\left(Al\right)$ or we can say $I{E}_1\left(Ga\right)$ is slightly greater than $I{E}_1\left(Al\right)$ and does not follow the general trend

Similarly, increase in $I{E}_1$ value from In to $Tl$ is due to poor screening effect of $4f$ electrons present in pre-penultimate shell.

i.e., $4f$ electrons shields nuclear change very less and cannot compensate for the increase in nuclear charge and therefore instead of decreasing in $I{E}_1$ value, it increases $Tl$ from In.

Hence, the correct order is $B>Al<Ga>In<Tl$