Q41Short answer
Ionisation enthalpies of Ce, Pr and Nd are higher than Th, Pa and U. Why?
Show answer
Hint : It is because 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 than 4f electrons of the corresponding lanthanoids. Therefore outer electrons are less firmly held and they are available for bonding in the actinoids.