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

Explanation

The electronic configuration of Al and Ga si as follows-
`""_13Al: 1s^2 2s^2 2p^6 3s^2 3p^1`:
`""_31Ga : 1 s^2 2s^2 2p^6 3s^2 3p^6 3 d^10 4 s^2 4p^1`
The effective nuclear charge of Ga is greater than that of Al. This is due to the poor shielding of the d-electrons of Ga. Thus, the electrons in Ga experiences more attraction towards the nucleus than that experienced by the electrons of Al. Consequently, the atomic radius of Ga is lower compared to Al.