Electron Configuration of Multielectron Atoms

The Pauli exclusion principle, Hund’s rule of maximum multiplicity, and the aufbau principle can be extended to envisage the electron configuration of any element.

Consider writing the electron configuration for sodium. The core electron distribution in sodium is exactly that of the preceding element, neon. The single valence electron occupies the 3s orbital.

Neon belongs to the eighteenth column of the periodic table — the noble gases. The electron configurations of these elements facilitate the condensed depiction of the electron configuration for other elements. For any element, the core electron configuration is the same as that of the noble gas that precedes it in the periodic table.

The electron configuration of sodium, for example, can be written as neon core, 3s¹.

The core electron configuration of potassium is 1s² 2s² 2p⁶ 3s² 3p⁶, leaving one valence electron. Now, does the nineteenth electron enter the 3d subshell? 

Recall that the 4s subshell has substantial penetrating ability, which often leads to it having a lower energy than the 3d subshell does. The aufbau principle, therefore, would hold that the 4s subshell fills prior to the 3d subshell. The core of the preceding