The first ionization enthalpy values (in $kJ\, mol^{-1}$) of group $13$ elements are:

Element	$B$	$Al$	$Ga$	$In$	$Tl$
Ionization Enthalpy	$801$	$577$	$579$	$558$	$589$

How would you explain this deviation from the general trend?

(N/A) In general,ionization enthalpy in a group decreases with an increase in atomic number.
This trend is observed from $B$ to $Al$. However,$Ga$ has an unexpectedly higher ionization enthalpy than $Al$.
This is because,in the case of $Ga$,there are ten $d$-electrons in its inner electronic configuration.
The $d$-electrons have poor shielding ability and therefore,shield the nuclear charge less effectively than $s$ and $p$-electrons.
As a result,the outer electron is held more strongly by the nucleus,leading to a slight increase in ionization enthalpy despite the increase in atomic size from $Al$ to $Ga$.
$A$ similar increase is observed from $In$ to $Tl$,which is due to the presence of $14$ $f$-electrons in the inner electronic configuration of $Tl$,which have an even poorer shielding effect than $d$-electrons.