Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The atomic radius of Niobium ( Nb ) is almost the same as that of Tantalum ( Ta ). Reason (R): The poor shielding effect of 4f electrons in Tantalum leads to an increase in effective nuclear charge, which cancels out the expected increase in size down the group. In the light of the above statements

Step-by-step solution

Niobium ( Nb ) belongs to the 4d transition series, while Tantalum ( Ta ) belongs to the 5d transition series, both residing in Group 5. Normally, atomic radius increases down a group due to the addition of a new principal quantum shell. However, before the filling of 5d orbitals in Tantalum, the 4f orbitals are filled (lanthanoid contraction). The 4f electrons provide very poor shielding for the outer electrons from the increasing nuclear charge. This results in a higher effective nuclear charge that pulls the val