Question

Zr (Z = 40) and Hf (Z = 72) have similar atomic and ionic radii because of:

• A. Diagonal relationship
• B. Lanthanoid contraction
• C. Having similar chemical properties
• D. Belonging to same group
• E.

Hint:

Lanthanoid contraction is why Zr and Hf are often found together in nature and are extremely difficult to separate; their identical sizes lead to almost identical chemical behaviors.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
Atomic radius usually increases as we move down a group in the periodic table due to the addition of new electron shells. However, between the second (\(4d\)) and third (\(5d\)) transition series, this trend is interrupted by a phenomenon known as Lanthanoid contraction.

Step 2: Key Formula or Approach
Lanthanoid contraction occurs due to the poor shielding effect of \(4f\) electrons. As we move from Lanthanum (\(Z=57\)) to Lutetium (\(Z=71\)), the \(4f\) shell is filled. These electrons do not shield the nucleus effectively, causing the outer electrons to be pulled closer.

Step 3: Detailed Explanation
1. Zirconium (Zr) is in the \(4d\) series, and Hafnium (Hf) is in the \(5d\) series.
2. Before Hf, the 14 lanthanoid elements are filled, where electrons enter the \(4f\) subshell.
3. The poor shielding of the \(4f\) electrons causes an increase in the effective nuclear charge acting on the outer electrons of Hf.
4. This results in a contraction of the atomic size of Hf, making it nearly identical to the size of Zr (Zr \(\approx\) 160 pm, Hf \(\approx\) 159 pm).

Step 4: Final Answer
The similarity in radii is due to Lanthanoid contraction.