The main reason for larger number of oxidation states exhibited by the actinoids than the corresponding…
The main reason for larger number of oxidation states exhibited by the actinoids than the corresponding lanthanoids is:
more energy difference between $5 f$ and $6 d$ orbitals than between $4 f$ and $5 \mathrm{~d}$ orbitals
lesser energy difference between $5 \mathrm{f}$ and $6 \mathrm{~d}$ orbitals than between $4 f$ and $5 \mathrm{~d}$ orbitals
larger atomic size of actinoids than the lanthanoids
greater reactive nature of the actinoids than the lanthanoids
Solution
In actinides, both $4 f$ and $5 f$ orbitals are present. These orbitals poorly shields the nuclear charge due to which the energy difference between the $5 f$ and the $6 d$ orbitals will be very low due to which the actinides will show a large number of oxidation states.
Caution
The elements in which the extra electron enters 5 - orbitals of $(n-2)^{\text {th }}$ shell (5f-block elements, actinides) shows the maximum oxidation state i.e. +7.