The correct order of ionic radius of lanthanides is:  

CONCEPT:

Lanthanide Contraction and Ionic Radius

• The lanthanides are elements with atomic numbers 57 (La) to 71 (Lu), where 4f orbitals are progressively filled.
• Lanthanide contraction refers to the gradual decrease in ionic radii of the Ln³⁺ ions with increasing atomic number.
• This occurs because:
  • The poor shielding effect of 4f electrons increases the effective nuclear charge.
  • This increased nuclear attraction pulls the electrons closer, reducing the ionic size.
• Thus, as we move from left to right in the lanthanide series, the ionic radius decreases.

EXPLANATION:

Nd³⁺ > Gd³⁺ > Ho³⁺ > Tm³⁺

• Nd³⁺ (atomic number 60) has the largest radius among the given ions.
• Gd³⁺ (atomic number 64) is smaller than Nd³⁺.
• Ho³⁺ (atomic number 67) is smaller than Gd³⁺.
• Tm³⁺ (atomic number 69) is even smaller.
• Hence, the correct order based on decreasing ionic radius is:

Therefore, the correct answer is Option 3: Nd³⁺ > Gd³⁺ > Ho³⁺ > Tm³⁺