Question

What is the correct order of covalent size of the following? Nb ______ Ta
A.$Nb = Ta$
B.$Nb < Ta$
C.$Nb > Ta$
D.None of these

Answer 1

Covalent radius is one half of the distance between the nuclei of two atoms covalently bonded in a diatomic molecule which is called covalent radius atom. In other words, covalent radius may be defined as one-half of the distance between the centres of the nuclei of two atoms bonded by a covalent bond.

Complete answer:
The covalent radius is defined as the measure of the size of an atom that forms part of a covalent bond. Niobium represented as Nb having atomic number 41 and tantalum represented by Ta which has atomic number 73 belongs to the same group but both have almost equal covalent size because of lanthanide contraction.
Lanthanoid contraction which is also known as lanthanide contraction in chemistry is the decrease in the size of the atoms and ions of the lanthanides with increasing atomic number from lanthanum to lutetium. For each consecutive atom in the lanthanide series, the nuclear charge is more positive by one unit, accompanied by an increase in the number of electrons present in the 4f orbitals surrounding the nucleus. The 4f electrons will imperfectly shield each other from the increased positive charge of the nucleus, because of this the effective nuclear charge attracting each electron steadily increases through the lanthanide elements, resulting in reductions of the atomic and ionic radii from left to right.

Therefore we can say that option (A) is the right answer.

Note:
In case of actinides, the atomic size or ionic radii of actinide ions decrease steadily from Thorium to Lawrencium due to increasing nuclear charge and electrons entering inner (n-2) f orbital. This orbital has very poor shielding ability. This gradual decrease in the size of actinides with an increasing atomic number is known as actinide contraction