Aufbau principle
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The Aufbau principle (also Aufbau rule or building-up principle), is used to determine the electron configuration of an atom, molecule or ion. The principle postulates a hypothetical process in which an atom is "built up" by progressively adding electrons. As they are added, they assume their most stable conditions (electron orbitals) with respect to the nucleus and those electrons already there.
According to the principle, electrons fill orbitals starting at the lowest available energy states before filling higher states (e.g. 1s before 2s). The number of electrons that can occupy each orbital is limited by the Pauli exclusion principle. If multiple orbitals of the same energy are available, Hund's rule says that unoccupied orbitals will be filled before occupied orbitals are reused (by electrons having different spins).
A version of the Aufbau principle can also be used to predict the configuration of protons and neutrons in an atomic nucleus.
[edit] History
The principle takes its name from the German Aufbauprinzip, "building-up principle", rather than being named for a scientist. In fact, it was formulated by the Danish physicist Niels Bohr circa 1920.
It was an early application of quantum mechanics to the properties of electrons, and explained chemical properties in physical terms. Each added electron is subject to the electric field created by the positive charge of atomic nucleus and the negative charge of other electrons that are bound to the nucleus.
