Question

Lanthanoids are difficult to purify due to their

• A. large size
• B. similar chemical and physical properties
• C. paramagnetic nature
• D. radioactivity

Hint:

Lanthanoids are difficult to separate because they have similar ionic radii and commonly show \(+3\) oxidation state.

Answer 1

The Correct Option is B

Concept:
Lanthanoids are elements from cerium to lutetium. They mostly show the \(+3\) oxidation state. Due to lanthanoid contraction, their ionic sizes decrease gradually but remain very close to one another. Because of this, their chemical properties are very similar.

Step 1: Understand lanthanoid contraction.
Across the lanthanoid series, electrons are added to the \(4f\)-subshell. The \(4f\)-electrons shield nuclear charge poorly. So effective nuclear charge increases. As a result, ionic radius gradually decreases. This is called: \[ \text{Lanthanoid contraction} \]

Step 2: Effect on purification.
Since lanthanoids have very similar ionic radii and mostly same oxidation state \(+3\), they behave very similarly in chemical reactions. Therefore, separating one lanthanoid from another becomes difficult.

Step 3: Check the options.
Option (A) large size is not the main reason.
Option (B) similar chemical and physical properties is correct.
Option (C) paramagnetic nature is not the main reason for purification difficulty.
Option (D) radioactivity is incorrect because lanthanoids are not generally radioactive like actinoids. Hence, the correct answer is: \[ \boxed{(B)\ \text{similar chemical and physical properties}} \]