Aufbau principle
Sign in to saveAlso known as building-up principle, Aufbau rule
principle of atomic physics
Described at
5.15: Aufbau Principle - Chemistry LibreTexts
This page explains the Aufbau principle, which dictates that electrons fill atomic orbitals starting from the lowest energy level to the highest. It details the organization and filling of atomic …
chem.libretexts.org →Figure 5.15.1 (Public Domain; Gary Minnaert via Commons Wikimedia, File:LACMA BCAM02.jpg [commons.wikimedia.org]) In order to create ground state electron configurations for any element, it is necessary to know the way in which the atomic sublevels are organized in order of increasing energy. Figure 5.15.2 shows the order of increasing energy of the sublevels. Figure 5.15.2: Electrons are added to atomic orbitals in order from low energy (bottom of the graph) to high (top of the graph), according to the Aufbau principle. Principle energy levels are color coded, while sublevels are grouped together, and each circle represents an orbital capable of holding two electrons. (CC BY-NC 3.0 ; Christopher Auyeung via CK-12 Foundation)
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Encyclopedic overview
In atomic physics and quantum chemistry, the Aufbau principle (pronounced /ˈaʊfbaʊ/, from German: Aufbauprinzip, lit. 'building-up principle'), also called the Aufbau rule, states that in the ground state of an atom or ion, electrons first fill subshells of the lowest available energy, then fill subshells of higher energy. For example, the 1s subshell is filled before the 2s subshell is occupied. In this way, the electrons of an atom or ion form the most stable electron configuration possible. An example is the configuration 1s 2s 2p 3s 3p 4s 3d for the zinc atom, meaning that the 1s subshell has 2 electrons, the 2s subshell has 2 electrons, the 2p subshell has 6 electrons, and so on.
The configuration is often abbreviated by writing only the valence electrons explicitly, while the core electrons are replaced by the symbol for the last previous noble gas in the periodic table, placed in square brackets. For zinc, the last previous noble gas is argon, so the configuration is abbreviated to [Ar] 4s 3d, where [Ar] signifies the core electrons whose configuration in zinc is identical to that of argon.
Excerpted from Wikipedia’s “Aufbau principle” article, available under the CC BY-SA 4.0 licence.