According to strict structural definitions, the inner transition block involves the progressive fill — d and f Block Elements Chemistry Question

💡 Solution & Explanation

The Lanthanoid series consists of elements where the $4f$ subshell is progressively filled. Because an $f$ -subshell has exactly $7$ orbitals, and each orbital holds $2$ electrons (Pauli Exclusion Principle), the maximum capacity is $14$ electrons. Thus, there are exactly $14$ elements in the series (from Cerium, $Z=58$, to Lutetium, $Z=71$).