Aufbau principle

In <a href="/facts/Atomic_physics/7Jlm4I7k">atomic physics</a> and <a href="/facts/Quantum_chemistry/H9AXV0lx">quantum chemistry</a>, the Aufbau principle , also called the Aufbau rule, states that in the <a href="/facts/Ground_state/ML7cfCug">ground state</a> of an <a href="/facts/Atom/FoQ4kGN5">atom</a> or <a href="/facts/Ion/3bePpDna">ion</a>, <a href="/facts/Electron/L0KBo5cW">electrons</a> first fill <a href="/facts/Electron_shell/QxI30eh9">subshells</a> of the lowest available <a href="/facts/Energy/wd8PrQM7">energy</a>, 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 <a href="/facts/Electron_configuration/TQjRr22P">electron configuration</a> possible. An example is the configuration 1s2 2s2 2p6 3s2 3p3 for the <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a> 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 <a href="/facts/Valence_electron/mXPA15VP">valence electrons</a> explicitly, while the <a href="/facts/Core_electron/LQuGLKcG">core electrons</a> are replaced by the symbol for the last previous <a href="/facts/Noble_gas/j0yHMEAE">noble gas</a> in the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a>, placed in square brackets. For phosphorus, the last previous noble gas is neon, so the configuration is abbreviated to [Ne] 3s2 3p3, where [Ne] signifies the core electrons whose configuration in phosphorus is identical to that of neon.
Electron behavior is elaborated by other principles of <a href="/facts/Atomic_physics/7Jlm4I7k">atomic physics</a>, such as <a href="/facts/Hund%27s_rule/SqQFS8vR">Hund's rule</a> and the <a href="/facts/Pauli_exclusion_principle/lsBKGeHD">Pauli exclusion principle</a>. Hund's rule asserts that if <a href="/facts/Degenerate_orbitals/ulrnMXTR">multiple orbitals of the same energy</a> are available, electrons will occupy different <a href="/facts/Atomic_orbital/tKhgl6mL">orbitals</a> singly and with the same <a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a> before any are occupied doubly. If double occupation does occur, the Pauli exclusion principle requires that electrons that occupy the same orbital must have different <a href="/facts/Spin_(physics)/4Jvp7e8P">spins</a> (+1⁄2 and −1⁄2).
Passing from one <a href="/facts/Chemical_element/0jducgWD">element</a> to another of the next higher <a href="/facts/Atomic_number/bgAr581S">atomic number</a>, one <a href="/facts/Proton/clCjepXP">proton</a> and one electron are added each time to the neutral atom.
The maximum number of electrons in any <a href="/facts/Electron_shell/QxI30eh9">shell</a> is 2n2, where n is the <a href="/facts/Principal_quantum_number/8SzSWV6w">principal quantum number</a>.
The maximum number of electrons in a subshell is equal to 2(2l + 1), where the <a href="/facts/Azimuthal_quantum_number/vHSIzbFC">azimuthal quantum number</a> l is equal to 0, 1, 2, and 3 for s, p, d, and f subshells, so that the maximum numbers of electrons are 2, 6, 10, and 14 respectively. In the <a href="/facts/Ground_state/ML7cfCug">ground state</a>, the electronic configuration can be built up by placing electrons in the lowest available subshell until the total number of electrons added is equal to the atomic number. Thus subshells are filled in the order of increasing energy, using two general rules to help predict electronic configurations:

<ol><li>Electrons are assigned to subshells in order of increasing value of n + l.</li>
<li>For subshells with the same value of n + l, electrons are assigned first to the subshell with lower n.</li></ol>
A version of the aufbau principle known as the <a href="/facts/Nuclear_shell_model/1Qgep495">nuclear shell model</a> is used to predict the configuration of protons and <a href="/facts/Neutron/oRCM1geb">neutrons</a> in an <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nucleus</a>.

Aufbau principle open-in-new

Aufbau principle