The metallic radii of the third (5d) series of transition metals are nearly the same as those of the corresponding members of the second series.

Question

 How would you account for the following : 

The metallic radii of the third (5d) series of transition metals are nearly the same as those of the corresponding members of the second series.

Answer 1

In moving down a group, there is increase in number of shells leading to increase in size. So, size of elements of 4d series is larger than those of 3d series. The similar atomic radii of second and third transition series is due to phenomenon called lanthanoid contraction, associated with intervention of 4f-orbitais which must be filled before the 5d series of elements begin.