Al and Ga have nearly the same covalent radii, because of

Al and Ga have nearly the same covalent radii,Reason: Al  [Ne] 3s2 3p1                Ga  [Ar] 3d10 4s2 4p1From Al to Ga atomic radii should increase,but atomic size slightly decreases or almost all same .Because the poor shielding effect by the completely filled inner shell 3d10  - electronic configuration in  Ga the valency shell electrons are strongly attracted by the nucleus.Hence the size of Ga is slightly less than that of Al or almost all same as that of Al.