Question

Lanthanoid contraction is caused due to :

• A. the same effective nuclear charge from $Ce$ 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 1

The correct option is B the imperfect shielding on outer electrons by 4f electrons from the nuclear charge

Lanthanoid contraction is caused due to the imperfect shielding on outer electrons by 4f electrons from the nuclear charge. With an increase in the atomic number, the positive charge on nucleus increases by 1 unit and one more electron enters in same 4f subshell.

There is imperfect shielding of one electron by another electron in same 4f subshell. Due to this, the effective nuclear charge increases. The valence shell is pulled slightly towards the nucleus.