On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase.

However, a deviation can be seen. Atomic radius of $Ga$ is less than that of Al. This can be understood from the variation in the inner core of the electronic configuration.

The presence of additional $10 d$-electrons offer only poor screening effect for the outer

electrons from the increased nuclear charge in gallium. Consequently, the atomic radius of gallium ($135 pm$) is less than that of aluminium ($143 pm$).

However, regular periodicity is observed in case of ionic radius.