f-block Elements -- Inner Transition Elements

The second half of this unit turns from the d-block elements studied so far to the f-block, or inner transition, elements -- so called because the differentiating electron that distinguishes each successive element within these series is added not to an outer valence shell, but to an INNER f sub-shell, two principal quantum numbers below the atom's outermost occupied shell. Within the inner transition elements, there are exactly two distinct series of fourteen elements each: the lanthanoids (previously, and still very commonly, called the lanthanides), following lanthanum and characterised by preferential filling of the 4f sub-shell; and the actinoids (previously called the actinides), following actinium and characterised by preferential filling of the 5f sub-shell.

The lanthanoid series consists of exactly fourteen elements, running from cerium (atomic number 58, symbol Ce) through to lutetium (atomic number 71, symbol Lu), immediately following lanthanum (atomic number 57) in the periodic table. The actinoid series likewise consists of fourteen elements, running from thorium (atomic number 90, symbol Th) through to lawrencium (atomic number 103, symbol Lr), immediately following actinium (atomic number 89). Because inserting all twenty-eight of these elements inline, directly within the main body of the periodic table, would make the table impractically wide for everyday printed use, both series are conventionally pulled out and displayed as two separate detached rows beneath the main table body -- exactly as shown in Figure 4.10 -- with the remainde …