In transition elements two effects are operating, viz nuclear charge effect and screening effect which oppose each other. Due to an increase in nuclear charge from member to member in a transition series the atomic radii tend to decrease. At the same time, the addition of extra electrons in (n – 1 )d orbital provides the screening effect. As the number of ‘d’ electrons increases, the screening effect increases and this tends to increase the size. Due to these opposing tendencies, there is a small change in the atomic radii in a transition series.
In ‘s’ and ‘p’ block elements, the extra electron is added to the same ‘s’ or ‘p’ subshell which does not exert a screening effect and hence, the atomic radii considerably in a period due to an increase in nuclear charge.
