Question:
Ionisation enthalpies of Ce, Pr and Nd are higher than Th, Pa and U. Why?
Solution:
Th, Pa and U, 5f electrons start filling and they have penetration lower than 4f electrons for Ce, Pr and Nd. Removing 4f electrons will be difficult, so ionization enthalpy for Th, Pa and U are lower than Ce, Pr and Nd.