Question

Which of the following statements regarding lanthanoids is correct?

• A. Basicity of lanthanoid hydroxides increases from \(La(OH)_3\) to \(Lu(OH)_3\)
• B. Lanthanoid contraction occurs because of poor shielding by \(4f\)-electrons
• C. \(Ce^{4+}\) is less stable than \(Ce^{3+}\) because of its noble gas configuration
• D. Atomic size increases regularly from La to Lu

Hint:

Lanthanoid Contraction: \[ \text{Poor }4f\text{ shielding} \rightarrow \text{Increase in }Z_{eff} \rightarrow \text{Decrease in size} \] This is one of the highest-yield NCERT facts from the \(f\)-block chapter.

Answer 1

The Correct Option is B

Concept: This question combines:
• Lanthanoid contraction
• Shielding effect
• Basicity trends
• Stability of oxidation states These are among the most frequently tested concepts from the \(f\)-block chapter.

Step 1: Understand lanthanoid contraction. Across the lanthanoid series: \[ La \rightarrow Lu \] electrons are added to the: \[ 4f \] subshell. The shielding provided by \(4f\)-electrons is poor. As a result, the effective nuclear charge increases. Consequently: \[ \boxed{\text{Atomic and ionic radii decrease gradually}} \] This phenomenon is called: \[ \boxed{\text{Lanthanoid contraction}} \]

Step 2: Analyze Option (A). Basicity actually decreases from: \[ La(OH)_3 \] to \[ Lu(OH)_3 \] because ionic size decreases. Hence Option (A) is incorrect.

Step 3: Analyze Option (B). Poor shielding by: \[ 4f \] electrons is indeed responsible for lanthanoid contraction. Hence: \[ \boxed{\text{Option (B) is correct}} \]

Step 4: Analyze Option (C). \(Ce^{4+}\) possesses: \[ [Xe] \] configuration. This noble-gas configuration makes it unusually stable. Hence Option (C) is incorrect.

Step 5: Analyze Option (D). Atomic size decreases rather than increases. Hence Option (D) is incorrect.

Step 6: Final conclusion. \[ \boxed{\text{Option (B)}} \] is the correct answer.