Question

Which actinoid from following has smallest ionic size in \(+3\) state?

• A. Pa
• B. Cm
• C. Fm
• D. Lr

Hint:

Actinoid contraction: ionic size falls steadily across the series.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Across the actinoid series, electrons enter the \(5f\) subshell, which shields the nuclear charge poorly. The effective nuclear charge grows, so the size of the \(M^{3+}\) ion decreases steadily. This is called the actinoid contraction.

Step 2: Key Formula or Approach:
Order the elements by atomic number: Pa \((91)\), Cm \((96)\), Fm \((100)\), Lr \((103)\).

Step 3: Detailed Explanation:
The element with the highest atomic number lies furthest along the series and has undergone the most contraction.
Lr (\(Z = 103\)) is the last member of the actinoids, so \(\text{Lr}^{3+}\) is the smallest.
Size order: \(\text{Pa}^{3+} > \text{Cm}^{3+} > \text{Fm}^{3+} > \text{Lr}^{3+}\).

Final Answer:
Lr has the smallest \(+3\) ion, option (D). \[ \boxed{\text{Lr}} \]