AIPMT 2010 Physics and Chemistry Paper

In general, the atomic and ionic radii increases on moving down a group. But the elements of second series (eg, Zr,Nb, Mo etc.) have the almost same radii as the elements of third transition series (eg, Hf, Ta, W etc).
This is because of lanthanide contraction i.e, imperfect sheilding of one 4f - electron by another.