Q.What are inner transition elements?

Step 1. Define the term: inner transition elements are elements in which the differentiating (last-added) electron enters an INNER f sub-shell -- two principal quantum numbers below the atom's outermost occupied shell -- rather than an outer, valence-level orbital.

Step 2. There are exactly two such series, each of fourteen elements: the lanthanoids, running from cerium (Ce, Z=58) to lutetium (Lu, Z=71), following lanthanum, characterised by preferential filling of the 4f sub-shell; and the actinoids, running from thorium (Th, Z=90) to lawrencium (Lr, Z=103), following actinium, characterised by preferential filling of the 5f sub-shell.

Step 3. Because inserting all twenty-eight of these elements inline within the main table body would make it impractically wide, both series are conventionally displayed as two detached rows beneath the main periodic table.

Define 'inner transition' by contrasting f-sub-shell (inner) filling against d-sub-shell (outer/penultimate) filling, then name the two constituent 14-element series.

• Confusing 'inner transition elements' with 'transition elements' generally, rather than specifically identifying the f-block lanthanoid/actinoid series.