Question

Which trend is correct regarding ionic radius in the 4f-series (lanthanides)?

• A. Radius increases from La\(^{3+}\) to Lu\(^{3+}\)
• B. Radius remains same
• C. Radius decreases across the series
• D. No regular trend observed

Hint:

Remember "lanthanide contraction"—as you move from La\(^{3+}\) to Lu\(^{3+}\), the size of ions decreases due to ineffective shielding by 4f electrons.

Answer 1

The Correct Option is C

Across the lanthanide series, as we move from La\(^{3+}\) to Lu\(^{3+}\), electrons are added to the 4f orbitals. These electrons do not shield each other effectively due to the poor shielding effect of f-orbitals. As a result, the effective nuclear charge increases across the series, pulling the electrons closer to the nucleus. This leads to a gradual decrease in the ionic radii—a phenomenon known as lanthanide contraction.