Aufbau Principle

The Aufbau principle describes the rule by which electrons are understood to fill the available energy orbitals within an atom, stating that electrons occupy the lowest-energy orbitals available before filling any higher-energy orbitals, building up an atom's full electron configuration one electron at a time in a predictable, well-defined order. The principle takes its name from the German word Aufbau, meaning building up or construction, and it developed during the 1920s alongside the broader emergence of quantum mechanical models of the atom, particularly through the work of physicist Niels Bohr and others extending early quantum theory to explain the structure of the periodic table. Combined with the Pauli exclusion principle, which limits how many electrons a single orbital can hold, and Hund's rule, which governs how electrons distribute themselves among orbitals of equal energy, the Aufbau principle allows the predicted electron configuration of any given atom to be built up systematically. The principle remains a standard, foundational tool taught in introductory chemistry for predicting the electron configuration of atoms and thereby explaining much of the periodic table's own recurring structure and the chemical behavior of different elements, even though real atoms are known to show a modest number of specific exceptions to its simplest form.