Question

Which of the following tripositive ion has smallest size?

• A. Ce\(^{3+}\)
• B. Nd\(^{3+}\)
• C. La\(^{3+}\)
• D. Sm\(^{3+}\)
• E. Gd\(^{3+}\)

Hint:

In lanthanide series, the ionic size decreases as the atomic number increases due to the filling of the 4f orbital.

Answer 1

Answer: (E)

For tripositive ions of lanthanides, the ionic radius decreases with increasing atomic number because the electrons are being removed from the 4f orbitals, which have poor shielding. Gd\(^{3+}\), being closer to the middle of the lanthanide series, has the smallest size. 
Thus, the correct answer is (E).