The Aufbau Principleâ€Definition, Rules and Exceptions

The Aufbau principle, simply put, means electrons are added to orbitals as protons are added to an atom. The term comes from the German word aufbau, which means built up or construction. Lower electron orbitals fill before higher orbitals do, building up the electron shell. The end result is that the atom, ion, or molecule forms the most stable electron configuration. The Aufbau principle outlines the rules used to determine how electrons organize into shells and subshells around the atomic nucleus. Electrons go into the subshell having the lowest possible energy.An orbital can hold at most 2 electrons obeying the Pauli exclusion principle.Electrons obey Hunds rule, which states that electrons spread out before they pair up if there are two or more energetically equivalent orbitals (e.g., p, d). Aufbau Principle Exceptions Like most rules, there are exceptions.  Half-filled and completely filled d and f subshells add stability to atoms, so the d and f block elements dont always follow the principle. For example, the predicted Aufbau configuration for Cr is 4s23d4, but the observed configuration is actually 4s13d5. This actually reduces electron-electron repulsion in the atom, since each electron has its own seat in the subshell. Aufbau Rule Definition A related term is the Aufbau Rule, which states that the filling of different electron subshells is by order of increasing energy following the (n 1) rule. The nuclear shell model is a similar model that predicts the configuration of protons and neutrons in an atomic nucleus.