Ga is below AI in the periodic table, but atomic radius of Ga is less than Al. lt is because of

The atomic radius of Go is slightly less than that of Al because in going from Al to Ga, the electrons have already occupied 3d sub shell in Ga. The screening effect of these intervening electrons being poor and has less influence to decrease the effective nuclear charge, therefore the electrons in Ga experiences more forces of attraction towards nucleus to result in lower size of Ga than Al.