Question

Why lanthanoids and actinoids placed separately at the bottom of the periodic table?

Answer 1

Lanthanoids and actinoids belong to f block. Their (n-1)d and (n-2)f are filled across the period These shells are inner as compared to valence shell . If they are included in group 3 it would interrupt the transition series hence to maintain the symmetry of periodic table Lanthanoids and actinoids are separately placed in periodic table