Gallium has much higher atomic number than aluminium yet its atomic radius is lower than that of aluminium. Justify.

Explanation

Due to poor shielding of the valence electrons of Ga by the inner 3d-electrons , the effective nuclear charge of Ga is greater in magnitude than that of Al. As a result , the electron in gallium experience greater force of attraction by the nucleus than in Al and hence atomic size of Ga (135pm) is slightly less much higher than that of Al (143pm).