In the f-block elements, which of the following subshells is progressively filled? (n = number of the outermost shell)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (n − 2)f
In the f-block elements the 4f and 5f orbitals are progressively filled. Compare each with the outermost shell n:
Lanthanoids (period 6): outermost shell n = 6; 4f = (6 − 2)f.
Actinoids (period 7): outermost shell n = 7; 5f = (7 − 2)f.
In both series the filling f subshell lies two shells inside the outermost shell: (n − 2)f.
Why the other options are incorrect
Option (1) (n − 1)d — Incorrect. The d orbitals of the penultimate shell are filled in the d-block (transition) elements.
Option (3) (n − 1)f — Incorrect. For n = 6 this would be 5f; the lanthanoids fill 4f, one shell further in.
Option (4) np — Incorrect. The outermost p subshell is filled in the p-block elements.
NCERT (2026-27 edition):
Unit 4: The d- and f-Block Elements – “The f-block consists of elements in which 4 f and 5 f orbitals are progressively filled. They are placed in a separate panel at the bottom of the periodic table.”