Chemistry - PERIODIC TABLE Question with Solution | TestHub

The lanthanide contraction causes elements in the same group but different periods to have similar radii. However, the effect of lanthanide contraction takes place after La, so $Y<La$ is correct.

A. Nb and Ta are in the same group. Due to lanthanide contraction, Ta's radius is very similar to Nb, but slightly larger. So, $Nb<Ta$.

B. N³⁻ and Na⁺ are isoelectronic. N³⁻ has fewer protons (7) than Na⁺ (11), so N³⁻ is larger than Na⁺. Thus, $N^{3-}>N{a}^{+}$.

C. Mo and W are in the same group. Due to lanthanide contraction, W's radius is very similar to Mo, but slightly larger. So, $Mo<W$.