How would you explain the lower atomic radius of $Ga$ as compared to $Al$?

(N/A)

Element	Atomic radius $(pm)$
Aluminium $(Al)$	$143$
Gallium $(Ga)$	$135$

Although $Ga$ has one additional electron shell compared to $Al$, its atomic radius is smaller. This is due to the poor shielding effect of the $3d$-electrons. The $d$-electrons provide ineffective shielding of the nuclear charge, leading to a higher effective nuclear charge $(Z_{eff})$ experienced by the valence electrons in $Ga$ compared to $Al$, which pulls the valence shell closer to the nucleus.