The correct order of atomic radii in group 13 elements is

The atomic radii as well as ionic radii increases on moving down the group 13 elements because of the successive addition of one extra shell of electrons. However, there is an anomaly in case of atomic radius . Atomic radius Ga is lesser as compared to that of Al. Gallium (Ga) with electronic configuration, [Ar]₁₈ 3d¹⁰  4s² 4p¹ has extra d-electrons which do not screen the nucleus effectively. Consequently, electrons of Ga are more attracted by nucleus. Thus, the increasing order of atomic radii of the group 13 elements is B(85pm) < Ga (135pm) < Al (143pm) < In (167pm) < Tl (170pm).