The Aufbau principle, also known as the Aufbau rule, is a fundamental principle in chemistry that governs the filling of electrons in the atomic orbitals of an atom in its ground state. It states that electrons fill subshells of the lowest available energy before filling subshells of higher energy. This principle was formulated by Niels Bohr and Wolfgang Pauli in the early 1920s and is an early application of quantum mechanics to the properties of electrons, explaining chemical properties in physical terms
. The principle can be used to understand the location of electrons in an atom and their corresponding energy levels. It provides the order in which atomic orbitals are filled, from the lowest energy level to the highest, and is essential for determining the electron configuration of an atom
