Question

lanthanoids and actinoids are placed in separate rows at the bottom of the periodic table. explain the reason.

Answer 1

Lanthanoids and actinoids belong to the f-block, Their (n-1)d and (n-2) f, are are filled across the period. These shells are inner as compared to the valence d shells. If they are included in group 3 it would interrupt the transition series.

Also, they have the same group valency (3) and exhibit similar physical and chemical properties.

Hence, to maintain the symmetry of periodic table Lanthanides and actinides are accommodated in 2 separate rows.