## Comparison between Main and Inner transition elements

**Main transition elements** are the elements of the d-block, occupying three horizontal periods (fourth, fifth, and sixth). Their energy levels are completely filled with electrons except the outer two energy levels. The d-sublevels of these elements are successively filled. Examples include Iron, Copper, and Manganese.

**Inner transition elements** are the elements of the f-block, occupying two horizontal series (lanthanides and actinides). Their energy levels are completely filled with electrons except the outer three energy levels. The f-sublevels of these elements are successively filled. Examples include Uranium and Cerium.