Aufbausäännön

Aufbausäännön, or the Aufbau principle, is a rule used in atomic physics and quantum chemistry to determine the ground-state electron configuration of atoms. It states that electrons occupy the available atomic orbitals in order of increasing energy, filling lower-energy states before higher-energy ones, while obeying the Pauli exclusion principle.

The principle is closely related to the Madelung rule (n + l rule), which estimates orbital energy based on the sum of the principal quantum number n and the azimuthal quantum number l. When two orbitals have the same n + l value, the one with the lower n is filled first. In practice the typical filling sequence proceeds roughly as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on. The Pauli exclusion principle forbids two electrons in a given orbital from sharing all quantum numbers, while Hund’s rule governs the distribution of electrons among degenerate orbitals.

Despite its usefulness, the Aufbau principle is an approximation. Electron–electron interactions, spin–orbit coupling and relativistic effects can alter the exact ordering of orbital energies, especially in heavier elements. Notable deviations occur in the first-row transition metals, such as chromium and copper, where half-filled or fully filled subshells confer extra stability, leading to configurations that differ from the simplest Aufbau sequence.

The Aufbau principle remains a foundational concept for predicting electron configurations, understanding periodic trends, and guiding discussions in inorganic chemistry and spectroscopy. It provides a practical framework for describing how atoms build up their electrons into shells and subshells.