Which of the following represents actinoid series?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Th to Lr
The f-block elements are placed in two rows at the bottom of the periodic table. In them the electrons enter an inner f subshell:
Lanthanoids: the 14 elements that follow lanthanum, from cerium (Ce, Z = 58) to lutetium (Lu, Z = 71); the 4f orbitals are filled.
Actinoids: the 14 elements that follow actinium, from thorium (Th, Z = 90) to lawrencium (Lr, Z = 103); the 5f orbitals are filled.
Lanthanum and actinium are usually studied along with these series because their properties are very similar, but the series themselves begin with the next element.
NCERT (2026-27 edition):
The Inner Transition Elements (f-Block)
