The effect of lanthanoid contraction in the lanthanoid series of elements by and large means:

Concept: Lanthanoid Contraction

As we move across the lanthanoid series (La → Lu), the nuclear charge increases by 1 for each element while electrons are added to the inner 4f subshell.

4f electrons provide poor shielding of the nuclear charge compared to outer electrons, because:

• 4f orbitals have diffuse, complex shapes
• They do not effectively shield the outer electrons from the increasing nuclear charge

Effect: The effective nuclear charge experienced by the outer electrons increases progressively → the outer electrons are pulled closer → both atomic and ionic radii decrease across the series.

$\text{La}^{3+} (117 \text{ pm}) > \text{Ce}^{3+} > \cdots > \text{Lu}^{3+} (86 \text{ pm})$

This is called lanthanoid contraction — a steady decrease in both atomic and ionic radii from La to Lu.

Answer: Option (4) — decrease in both atomic and ionic radii