Explain the atomic radius of elements of the boron family.

(N/A) On moving down the group, for each successive member, one extra shell of electrons is added, and therefore, the atomic radius is expected to increase.
However, a deviation is observed. The atomic radius of $Ga$ $(135 \text{ pm})$ is less than that of $Al$ $(143 \text{ pm})$.
This can be explained by the variation in the inner core of the electronic configuration. The presence of additional $10$ $d$-electrons offers only poor screening (shielding) effect for the outer electrons from the increased nuclear charge in gallium.
Consequently, the effective nuclear charge increases, causing the atomic radius of gallium to be smaller than that of aluminium. However, regular periodicity is observed in the case of ionic radius.