Question

Lanthanoid contraction is caused due to

• A. the imperfect shielding on outer electrons by 4f electrons from the nuclear charge
• B. By appreciable shielding on outer electrons by 4f electrons from the nuclear charge
• C. the appreciable shielding on outer electrons by 5d -electrons from the nuclear charge
• D. the same effective nuclear charge from Ce to Lu

Answer 1

Answer: (A)

The elements in Lanthanide series are $La , Ce , Pr , Nd , Pm , Sm , Eu , Gd$, Tb, Dy, Ho, Er, Tm, Y b and Lu. In Lanthanides ( $Ce - Lu )$, the atomic and ionic radii decrease steadily. This steady decrease in atomic and ionic radii is known as Lanthanide Contraction. The contraction is due to the fact that $f$ - orbitals are not capable of providing effective shielding for the valence electrons from the nuclear attraction.