The ionisation enthalpies of the early actinoids are lower than those of the early lanthanoids. Which of the following explains this?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The 5f electrons are more effectively shielded from the nuclear charge than the 4f electrons.
Ionisation enthalpy depends on how firmly the nucleus holds the outer electrons.
When the 5f orbitals begin to fill, they penetrate less into the inner core than the 4f orbitals of the lanthanoids. The 5f electrons are therefore better shielded from the nuclear charge, are held less firmly and are removed more easily. This is also why they are available for bonding.
Why the other options are incorrect
Option (1) The 5f electrons are less effectively shielded from the nuclear charge than the 4f electrons. — Incorrect. This is the reverse. Less shielding would hold the electrons more firmly and raise the ionisation enthalpy.
Option (2) The contraction in size from element to element is greater in the actinoid series. — Incorrect. The statement is true, but it describes the change in size along the series; a greater contraction does not make the early actinoids easier to ionise.
Option (3) The 5f, 6d and 7s levels of the actinoids are of comparable energies. — Incorrect. This true fact explains the greater range of oxidation states of the actinoids, not their lower ionisation enthalpies.
NCERT (2026-27 edition):
4.6.4 General Characteristics and Comparison with Lanthanoids – “It is evident from the behaviour of the actinoids that the ionisation enthalpies of the early actinoids, though not accurately known, but are lower than for the early lanthanoids. This is quite reasonable since it is to be expected that when 5f orbitals are beginning 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 the 4f electrons of the corresponding lanthanoids.”