Position of Lanthanoids in the Periodic Table

Although the true, formal position of the lanthanoid elements within the periodic table, based strictly on electron-filling order, is group number 3 and period number 6 (directly below lanthanum's own group-3 position and immediately following it), the chapter explains why these fourteen elements are, in standard practice, instead grouped together and displayed separately at the bottom of the table rather than being shown inline within the main period-6 row.

The justification rests on three connected points. First, in the sixth period, immediately after lanthanum, the next electrons added are preferentially filled not into an outer valence orbital but into the INNER 4f sub-shell -- so these fourteen elements following lanthanum share the distinguishing feature that their differentiating electron is buried in an inner shell rather than contributing directly to the atom's outermost, chemically dominant valence shell. Second, and following directly from this shared inner-shell-filling pattern, all fourteen of these elements display closely similar chemical properties to one another -- far more similar, element to element, than is typical moving across an ordinary period. Third, essentially all fourteen lanthanoids share the same common oxidation state, +3 (developed fully in section 4.5.3) -- a further mark of the close family resemblance that a normal, group-by-group periodic arrangement is designed to capture but that a purely electron-filling-order placement would obscure. …