The actinoids show a larger number of oxidation states than the lanthanoids. The main reason is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The energy difference between 5f and 6d orbitals is smaller than that between 4f and 5d orbitals
An element shows several oxidation states when electrons from more than one subshell can be lost at a small cost in energy. In the actinoids the 5f, 6d and 7s levels are close in energy, so different numbers of electrons can be used in bonding. In the lanthanoids the 4f electrons are deeply buried, and +3 is the principal oxidation state.
Why the other options are incorrect
Option (1) The 4f orbitals are more diffuse than the 5f orbitals — Incorrect. It is the other way round: the 5f orbitals are not as buried as the 4f orbitals, so 5f electrons take part in bonding far more.
Option (3) The energy difference between 5f and 6d orbitals is larger than that between 4f and 5d orbitals — Incorrect. A larger gap would make fewer oxidation states possible, not more.
Option (4) The actinoids are more reactive than the lanthanoids — Incorrect. Reactivity towards other substances does not decide how many electrons an atom can lose; the number of oxidation states depends on the energies of the orbitals that hold the outer electrons.
NCERT (2026-27 edition):
4.6.3 Oxidation States – “There is a greater range of oxidation states, which is in part attributed to the fact that the 5f, 6d and 7s levels are of comparable energies.”