Question

Identify the correct decreasing order of ionic radii of lanthanoids.

• A. Sm>Gd>Ce>Pm
• B. Gd>Pm>Ce>Sm
• C. Ce>Pm>Sm>Gd
• D. Pm>Sm>Ce>Gd

Hint:

Lanthanoid contraction causes a regular decrease in ionic radii across the lanthanoid series.

Answer 1

The Correct Option is C

Step 1: Understand lanthanoid contraction.
In lanthanoids, ionic radii decrease gradually from left to right due to poor shielding effect of \(4f\)-electrons, a phenomenon known as lanthanoid contraction.
Step 2: Apply the concept to given elements.
Cerium (Ce) appears earlier in the series, followed by promethium (Pm), samarium (Sm), and gadolinium (Gd). Hence, ionic radius decreases in the same order.
Step 3: Conclusion.
Therefore, the correct decreasing order of ionic radii is: \[ \text{Ce}>\text{Pm}>\text{Sm}>\text{Gd} \]