Classification of Elements and Periodicity in Properties — Chemistry STD 11 Science — Question

3. lanthanoids.

Explanation:

The sixth period (n = 6) contains 32 elements and electrons enter 6s, 4f, 5d and 6p orbitals, in the order of filling up of the 4f orbitals begins with cerium (Z = 58) and ends at lutetium (Z = 71) to give the 4f-inner transition series which is called the lanthanoid series.