Chemistry - Mole Concept Question with Solution | TestHub

In general, the atomic and ionic radii increases on moving down a group. But the elements of second transition series (eg, $\mathrm{Zr},\mathrm{Nb}$, 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 ie, imperfect shielding of one $4\mathrm{f}$-electron by another.