CBSE Class 12-science Answered

There is an increase in radii from the first (3d) to the second (4d) series of the elements but the radii of the third (5d) series are virtually the same as those of the corresponding members of the second series. This phenomenon is called Lanthanoid contraction. The possible reason for that is the intervention of the 4f orbitals which must be filled before the 5d series of elements begins. The imperfect shielding of one 4f electron by another result in the decrease in the size of the entire 4f ⁿ orbitals as the nuclear charge increases along the series.