Question

Account for the fact that the atomic radius of two group 5 elements, niobium (period 5) and tantalum (period 6) are the same.

Answer 1

Due to Lanthanide contraction, the atomic radius of two group elements are same. As we move along the lanthanides series, the nuclear charge increases by one unit. The new electron is added to the 4f orbitals but the shape of f orbital is very diffused. So the mutual shielding effect of f electrons is very little.

It has been observed that the normal increase in size from Sc->Y->La disappears after the lanthanides. Thus the pairs of elements such as Zirconium, Hafnium Niobium, tantalum and molybdnum and Tungsten have almost same size. As a result they have same chemical properties .