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:
The elements Zirconium (Zr) and Hafnium (Hf) belong to Group 4 of the periodic table, located in the 4d and 5d transition series, respectively.
Normally, atomic and ionic radii increase significantly as we move down a group due to the addition of a new principal electron shell.
However, the atomic radius of Zr (160 pm) is almost identical to that of Hf (159 pm).
Step 2: Key Formula or Approach:
The approach here is to correlate the anomalous atomic size trend with the electronic configuration and the shielding effect of inner electrons.
We analyze the effect of filling the 4f subshell before the 5d subshell in the periodic table.
Step 3: Detailed Explanation:
Before the 5d transition series begins, the 4f orbitals are progressively filled with 14 electrons, forming the lanthanide series.
The shape of f-orbitals is highly diffused, which makes f-electrons very poor at shielding the outer valence electrons from the pull of the nucleus.
As the nuclear charge increases across the lanthanide series, the poor shielding by the 4f electrons causes the effective nuclear charge (\(Z_{\text{eff}}\)) to increase significantly.
This results in a steady and significant decrease in the atomic and ionic radii along the lanthanide series, a phenomenon termed "Lanthanoid contraction."
By the time we reach Hafnium (Z = 72), this contraction has exactly offset the expected increase in size due to the addition of the new 6s and 5d shells.
Consequently, Hf ends up having a size very similar to its upper group neighbor, Zr.
Step 4: Final Answer:
The similar radii are caused by the Lanthanoid contraction.