How would you explain the lower atomic radii of `Ga` as compared to `Al`?

Explanation

The lower atomic radius of `Ga` as compared to `Al` can be explained on the basis of their electronic configuration.Aluminium has noble gas core beneath the valence shell electrons, whereas in gallium, fully filled lesser shielding `3d` electrons are present in-between the valence electrons and noble gas core. As a result, the force of attraction experienced by the valence shell electrons increases and hence the atomic radius at `Ga` decreases.