Meaning of lanthanoid contraction

As we move across the lanthanoid series from La to Lu, electrons are added to the $4f$-subshell.

Cause of contraction

The $4f$ electrons shield the nuclear charge very poorly. Therefore, the effective nuclear charge on the outer electrons increases steadily.

Effect on size

Because of the increasing effective nuclear charge, the electrons are pulled closer to the nucleus.

Hence, across the lanthanoid series:

• atomic radii decrease
• ionic radii also decrease

Evaluation of options

• A: increase in atomic radii and decrease in ionic radii $\rightarrow$ incorrect
• B: increase in both atomic and ionic radii $\rightarrow$ incorrect
• C: decrease in both atomic and ionic radii $\rightarrow$ correct
• D: decrease in atomic radii and increase in ionic radii $\rightarrow$ incorrect

Final answer

The lanthanoid contraction means a gradual decrease in both atomic and ionic radii across the series.