What is lanthanide contraction?

Lanthanide contraction refers to the gradual decrease in atomic and ionic radii from La to Lu across the lanthanide series.

This happens because the added electrons enter the $4f$-orbitals, which shield the nuclear charge very poorly. As a result, the effective nuclear charge increases and the size decreases.

Its important consequence

Due to lanthanide contraction, elements coming after the lanthanides become smaller than expected. Hence, elements of the second transition series and third transition series often have very similar sizes.

Check the options

A: Zr and Y have about the same radius

• $\mathrm{Y}$ and $\mathrm{Zr}$ are adjacent elements in the same period.
• This is not a consequence of lanthanide contraction.
• Incorrect.

B: Zr and Nb have similar oxidation state

• Similar oxidation states arise from similar valence electronic configurations in transition metals, not from lanthanide contraction.
• Incorrect.

C: Zr and Hf have about the same radius

• $\mathrm{Zr}$ is a $4d$ element and $\mathrm{Hf}$ is the corresponding $5d$ element.
• Normally, $5d$ elements should be larger, but due to lanthanide contraction, $\mathrm{Hf}$ becomes nearly the same size as $\mathrm{Zr}$.
• This is a classic consequence of lanthanide contraction.
• Correct.

D: Zr and Zn have the same oxidation

• This statement is unrelated to lanthanide contraction and is also chemically incorrect in general.
• Incorrect.

Final answer

The lanthanide contraction is responsible for: $\boxed{\text{Zr and Hf have about the same radius}}$

Comparison with stored correct answer

Stored correct answer = C

Derived answer = C

So, the derived answer agrees with the stored correct answer.