The overall decrease in atomic and ionic radii from lanthanum tolutetium is a unique feature in the chemistry of the lanthanoids. It has farreaching consequences in the chemistry of the third transition series ofthe elements.
This contraction is attributed to the imperfect shielding of one electron byanother in the same sub-shell. However, the shielding of one 4f electron byanother is less than one d electron by another with the increase in nuclearcharge along the series. There is fairly regular decrease in the sizes withincreasing atomic number.
The cumulative effect of the contraction of the lanthanoid series, knownas lanthanoid contraction, causes the radii of the members of the thirdtransition series to be very similar to those of the corresponding membersof the second series. The almost identical radii of Zr (160 pm) and Hf(159 pm), a consequence of the lanthanoid contraction, account fortheir occurrence together in nature and for the difficulty faced in theirseparation.