NEET 2021 Chemistry Question 85 Solution

Zirconium ($Zr$, atomic number 40) is a member of the second ($4d$) transition series in Group 4. Hafnium ($Hf$, atomic number 72) is located directly beneath $Zr$ in the third ($5d$) transition series. In moving from $Zr$ to $Hf$, 32 protons are added to the nucleus, including the 14 elements of the lanthanoid series ($_{58}\text{Ce}$ to $_{71}\text{Lu}$).

In the lanthanoids, electrons enter the inner $(n-2)f = 4f$ orbitals. Because $f$-orbitals have highly diffuse angular shapes, they exert exceptionally poor screening (shielding) of the nuclear charge on the outer $5d$ and $6s$ electrons. Consequently, as the nuclear charge increases by 14 units, the effective nuclear charge ($Z_{\text{eff}}$) experienced by outer electrons increases steadily, pulling the electron clouds inwards.

This contraction in size (lanthanoid contraction) almost exactly cancels the expected expansion due to the addition of an extra quantum shell ($n=5 \to n=6$): - Atomic radii: $Zr = 160\text{ pm}, \quad Hf = 159\text{ pm}$ - Ionic radii ($M^{4+}$): $Zr^{4+} = 79\text{ pm}, \quad Hf^{4+} = 78\text{ pm}$ Hence, $Zr$ and $Hf$ possess almost identical physical and chemical properties and occur together in nature.