An approximate scheme for constructing an atomic electron configuration by filling lower-energy atomic orbitals before higher-energy ones, subject to the Pauli exclusion principle. Orbital ordering can change on ionization and has exceptions in interacting chemical atoms. Selecting the free-ion ground term within a specified open-shell configuration additionally requires Hund's rules or a more detailed interaction calculation.
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The Aufbau principle is a fundamental concept in quantum chemistry and atomic physics that describes the process by which electrons populate atomic orbitals in a particular order. According to this principle, electrons fill atomic orbitals starting from the lowest energy level and move to higher energy levels only after the lower ones are filled. The general guideline for this filling order is summarized by the "n + l" rule, where "n" is the principal quantum number and "l" is the azimuthal quantum number.
Boring rule that says that less energetic atomic orbitals are filled first.
Much more interesting is actually determining that order, which the Madelung energy ordering rule is a reasonable approximation to.