Question

What is the correct order of size in each pair given below?
$~N{{b}^{3+}} \text{ and }T{{a}^{3+}}$
A.$~N{{b}^{3+}}=T{{a}^{3+}}$
B.$~N{{b}^{3+}}<\text{ }T{{a}^{3+}}$
C.$~N{{b}^{3+}}>\text{ }T{{a}^{3+}}$
D.None of these

Answer 1

Niobium (Nb) and Tantalum (Ta) have atomic numbers 41 and 73 respectively. Both the elements are placed in the 5th (VB) group of the periodic table. Nb is placed in the 5th period and Ta in the 6th period. The order of size of an element is the ionic radius of the ion which increases as we move down a period.

Complete answer:
Niobium and Tantalum both lose 3 electrons to gain +3 charge. As the atom becomes a cation there is more attraction between the nucleus and the outer electrons. This results in shrinking of the atomic size and subsequently the ionic radius decreases.
Ta is placed right below Nb in the periodic table. This means the atomic radii of Tantalum should be more than Niobium. But due to lanthanoid contraction their atomic radii are the same
The steady decrease in the order size of an atom with an increase in the atomic number is termed as Lanthanoid contraction. The effect of lanthanide contraction can be seen in the D-block of the periodic table.
Both Niobium and Tantalum have the atomic radii equal to 143 pm.
Thus, when they lose electrons to become $Nb^{+3}$ and $Ta^{+3}$ their order size will be the same.
Final answer: Correct Option- A) $~N{{b}^{3+}}=T{{a}^{3+}}$

Note:
The Lanthanoid contraction occurs between the elements from Lanthanum (57) to Lutetium (71). But it is responsible for the decrease in size of the ionic radii for the elements after Hafnium (72). That is why the ionic radii of Tantalum are the same as Niobium.