An exception to this trend occurs in Group 13, and gallium is smaller than aluminum. Justify this exception to the expected periodic trend by invoking the electron configurations of each element.

Reduction in effective nuclear charge due to the filling of 3d sub-shell

Explanation:

Although, according to the trend in the periodic table, the size of shell go on increasing but an anomaly in Group 13 s observed in case of Aluminium and Gallium. Gallium should have a bigger shell configuration according to the trend but it is small as compared due to the first filling of 3d sub-shell by electron as we can see from their electronic configurations as:

Aluminium = [Ne] 3s² 3p¹

Gallium = [Ar] 3d10 4s2 4p1

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