Question

Lanthanoid contraction is due to:

• A. Negligible screening effect of $4f$ orbitals.
• B. Negligible screening effect of $5f$ orbital.
• C. Decreasing nuclear charge.
• D. Increasing screening effect.

Answer 1

Answer: (A)

Lanthanide contraction :
There is a decrement in the size of the atoms and ions of the $f$-block elements with increasing atomic number from $La$ having atomic number $57$ through $Lu$ having atomic number $71.$ The radius is almost same as that of the previous period in the periodic table. Weak screening effect by the $4f$ - electrons cause Lanthanide contraction. The effective nuclear charge on the valence electron increases and this decreases the size of the atoms.