The electronic configurations of the actinoids cannot be assigned with certainty because of
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → small energy difference between 5f and 6d levels
The actinoids have the outer configuration 7s2 with a variable number of electrons in the 5f and 6d subshells. Their 5f, 6d and 7s levels have comparable energies (the same fact that gives the actinoids their wide range of oxidation states).
With so small a gap between 5f and 6d, it is hard to fix which of the two subshells an electron enters, so the configurations of the actinoids are only believed, not assigned with certainty.
Why the other options are incorrect
Option (1) overlapping of inner orbitals — Incorrect. The doubt is about how electrons are shared between the 5f and 6d subshells, which is decided by their energies, not by any overlap of inner orbitals.
Option (2) free movement of electrons over all the orbitals — Incorrect. Electrons in an atom occupy definite orbitals; they do not move freely over all orbitals. The uncertainty is limited to the 5f/6d choice.
NCERT (2026-27 edition):
4.6.1 Electronic Configurations – “All the actinoids are believed to have the electronic configuration of 7s2 and variable occupancy of the 5f and 6d subshells.”
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.”