Ques. 1AIPMT 2004
Lanthanoids are
- 14 elements in the sixth period (atomic no. 90 to 103) that are filling 4f sublevel
- 14 elements in the seventh period (atomic number = 90 to 103) that are filling 5f sublevel
- 14 elements in the sixth period (atomic number = 58 to 71) that are filling the 4f sublevel
- 14 elements in the seventh period (atomic number = 50 to 71) that are filling 4f sublevel.
Answer
(3) 14 elements in the sixth period (atomic number = 58 to 71) that are filling the 4f sublevel
Solution
As per NCERT, the lanthanoids are the 14 elements following lanthanum, from cerium (Z = 58) to lutetium (Z = 71).
They lie in the sixth period, and the differentiating electron enters the 4f subshell.
Elements with Z = 90 to 103 (thorium to lawrencium) are the actinoids of the seventh period, filling the 5f subshell.
Answer: (3)