Question

Reason of lanthanoid contraction is

• A. negligible screening effect of 'f'-orbitals
• B. increasing nuclear charge
• C. decreasing nuclear charge
• D. decreasing screening effect

Answer 1

Answer: (A)

Lanthanoid contraction is the regular decrease in atomic and ionic radii of lanthanides. This is due to the imperfect shielding [or poor screening effect] of $f$-orbitals due to their diffused shape which unable to counterbalance the effect to the increased nuclear charge. Hence, the net result is a contraction in size of lanthanoids.