Question
Lanthanoid contraction is caused due to
A.
the same effective nuclear charge from $$Ce\,{\text{to}}\,Lu$$
B.
the imperfect shielding on outer electrons by $$4f$$ electrons from the nuclear charge
C.
the appreciable shielding on outer electrons by $$4f$$ electrons from the nuclear charge
D.
the appreciable shielding on outer electrons by $$5d$$ electrons from the nuclear charge
Answer :
the imperfect shielding on outer electrons by $$4f$$ electrons from the nuclear charge
Solution :
The configuration of Lanthanides show that the additional electron enters the $$4f$$ subshell. The shielding of one $$4f$$ electron by another is very little or imperfect. The imperfect shielding of $$f$$ electrons is due to the shape of $$f$$ orbitals which is very much diffused. Thus as the atomic number increases, the nuclear charge increases by unity at each step. While no comparable increase in the mutual shielding effect of $$4f$$ occurs. This causes a contraction in the size of the $$4f$$ subshell. as a result atomic and ionic radii decreases gradually
from $$La\,to\,Lu.$$