Question

Which of the two have almost similar size?
(A) $_{22}Ti$ and $_{40}Zr$
(B) $_{41}Nb$ and $_{73}Ta$
(C) $_{39}Y$ and $_{57}La$
(D) $_{20}Ca$ and $_{31}Ir$

Answer 1

In general, we have the trends for the periodic table with respect to the size and other properties of the atoms. The size of element increases down the group for transition elements but for the Lanthanides, size decreases from third to fourth series due to lanthanoid contraction.

Complete step-by-step answer: Let us see the general trends of modern periodic table and the term ‘lanthanoid contraction’ in detail;
The general trend for the size of the element in the modern periodic table-
The size of the element increases as we go down the table from top to bottom (increase in number of shells) and decreases as we move along the period from left to right (increase in number of electrons in the same shell).
Lanthanoid contraction-
The steady decrease in size of some of the rare elements in their atomic and ionic forms with increasing atomic number from 57 i.e. lanthanum to 71 i.e. lutetium. This is caused due to the poor shielding effect of 4f electrons present within them drawing the 6s electrons towards the nucleus.
Thus, Nb and Ta have almost equal radius i.e. sizes.

Therefore, option (B) is correct.

Note: Do note that sometimes lanthanoid contraction is also spelled as lanthanide contraction; do not get confused as both are the same.
Actually, lanthanide contraction is the decrease in the size which is more than expected in general. So, the proper data can be described for specific sizes of those elements.