lonisation enthalpies of Ce, Pr and Nd are higher than Th, Pa and U. Why?

NEETprep Answer: Ce, Pr and Nd are lanthanoids and have incomplete 4f shell while Th, Pa and U are actinoids and have 5f shell incomplete.
In the beginning, when 5f-orbitals begin to be occupied, they will penetrate less into the inner core of electrons. The 5f-electrons will therefore, be more effectively shielded from the nuclear charge than4/ electrons of the corresponding lanthanoids.

Therefore, outer electrons are less firmly heid and they are available for bonding in the actinoids.