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

Explanation

The atomic radus is expected to increase down the group. The decrease in atomic radius of gallium (135pm) as compared to that of aluminium (143pm) may by attributed to the presence of ten elements of the first transition series (Z=21 to 30) which have electrons in the 3d orbtials. since d-orbitals have large size than the p-orbitals. the intervening electron do not have sufficient shielding effect to counter the increase in the nuclear charge. Therefore, the effective nuclear charge in case of Ga (Z=31) is less than the expected value. This decreases it atomic radius wich otherwise is expected to increase.