Question: 77: Decrease in size from left to right in actinoid series is greater and gradual than that in lanthanoid series due to :
(1) 4 f orbitals are penultimate
(2) 4 f orbitals have greater shielding effect
(3) 5 f orbitals have poor shielding effect
(4) 5 f orbitals have greater shielding effect
Answer: Option (3)
Explanation:
In both lanthanoid and actinoid series, the atomic size decreases from left to right due to an increase in nuclear charge while electrons are added to inner f-orbitals.
In the lanthanoid series, electrons are added to the 4f orbitals.
These 4f electrons provide some shielding effect,
which partially counterbalances the increasing nuclear charge.
In the actinoid series, electrons are added to the 5f orbitals.
The 5f orbitals have a poorer shielding effect compared to 4f orbitals.
Due to the poor shielding by 5f electrons, the effective nuclear charge experienced by the outer electrons increases more significantly in actinoids.
As a result, the atomic size decreases more and in a more gradual manner across the actinoid series.
Therefore, the greater and gradual decrease in size in the actinoid series is due to
the poor shielding effect of 5f orbitals.