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

Explanation

Ce, Pr and Nd belong to4f series ( langthanoids) wheras Th, Pa and U belong to 5f series ( actinoids). When filling of 4f and 5f starts, electrons in 5f penetrate less into the inner core. Therefore, they are more effectively shielded from the nuclear charge than 4f electrons of langthanoids. As a result, outer electrons of 5f series experience less force of attraction by the nucleus and hence have lower ionization enthalpies.