Electronic Configuration of Lanthanoids

Building on the general Aufbau-based approach already used for the d-block (section 4.2), the electron-filling order for the lanthanoid series is worked through element by element, starting from the Aufbau principle's basic rule that electrons are added to successive orbitals in order of increasing energy. Following the filling of the 5s, 5p and 6s orbitals in earlier periods, the Aufbau principle predicts that the 4f level should begin filling from lanthanum itself onward -- so the NAIVELY expected electronic configuration of lanthanum (La) would be [Xe]4f¹5d⁰6s². However, the ACTUAL, experimentally established electronic configuration of lanthanum is instead [Xe]4f⁰5d¹6s² -- lanthanum genuinely has zero 4f electrons and one 5d electron, not the reverse, which is precisely why lanthanum itself is conventionally classified as belonging to the d-block (alongside actinium in the analogous position for the second inner-transition series) rather than being counted as the first true lanthanoid.

Genuine filling of the 4f orbital only actually begins from cerium (Ce), the element immediately following lanthanum, whose configuration is [Xe]4f¹5d¹6s² -- notably, cerium retains one 5d electron alongside its first 4f electron, another small departure from the simplest possible filling pattern. Moving from cerium onward through the rest of the series, additional electrons are then progressively, and for the most part smoothly, added into the 4f orbitals, following the general pattern tabulated element by element (Table: electronic configuration of lanthanum and lanthanoids). …