Question

₁₃Al and ₃₁Ga belong to group IIIA. Their atomic radii of both are 1.24 Å. What is the reason for only a very small increase in size from Al to Ga?

Answer 1

Al: 1s² 2s² 2p⁶ 3s² 3p¹

Ga: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹

In Gallium 3d level gets filled up. During this transition there is a gradual contraction in size (10 electrons being added to the inner d level). This contraction move and offsets the expanding effect of the additional electronic shells. The result is that there is practically little difference between the atomic radii of aluminium and gallium.