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

Explanation

Ce, Pr and Nd are the first three members of the 4f - series (lanthanoids), while Th, Pa and U are the first three members of the 5f - series (actinoids). When filling of 4f and 5f - orbitals starts, electrons in 5f - orbitals penetrate less into the inner core of electrons. The 5f - electrons will therefore be more effectively shielded from the nuclear charge than 4f - electrons of the corresponding lanthanoids. So outer electrons of the atoms of 5f - series are less firmly held by the nucleus and hence, have lower ionisation enthalpies.