Decrease in size from left to right in actinoid series is greater and gradual than that in lanthanoid series…
Decrease in size from left to right in actinoid series is greater and gradual than that in lanthanoid series due to:
$4 f$ orbitals are penultimate
$4 f$ orbitals have greater shielding effect
$5 f$ orbitals have poor shielding effect
$5 \mathrm{f}$ orbitals have greater shielding effect
Solution
More is the number of poor shielders, greater will be $\mathrm{Z}_{\text {eff }}$ and hence, smaller is the size. Actinoids consists of more no. of poor shielders $(3 d, 4 d, 5 d, 4 f)$. Therefore, size continuously decreases left to right.