Editing Electron configuration (section)

== Energy of ground state and excited states ==
The energy associated to an electron is that of its orbital. The energy of a configuration is often approximated as the sum of the energy of each electron, neglecting the electron-electron interactions. The configuration that corresponds to the lowest electronic energy is called the [[stationary state|ground state]]. Any other configuration is an [[excited state]].

As an example, the ground state configuration of the [[sodium]] atom is 1s<sup>2</sup>&nbsp;2s<sup>2</sup>&nbsp;2p<sup>6</sup>&nbsp;3s<sup>1</sup>, as deduced from the Aufbau principle (see below). The first excited state is obtained by promoting a 3s electron to the 3p subshell, to obtain the
1s<sup>2</sup>&nbsp;2s<sup>2</sup>&nbsp;2p<sup>6</sup>&nbsp;3p<sup>1</sup> configuration, abbreviated as the 3p level. Atoms can move from one configuration to another by absorbing or emitting energy. In a [[sodium-vapor lamp]] for example, sodium atoms are excited to the 3p level by an electrical discharge, and return to the ground state by emitting yellow light of wavelength 589&nbsp;nm.

Usually, the excitation of [[valence electron]]s (such as 3s for sodium) involves energies corresponding to [[photon]]s of visible or [[ultraviolet]] light. The excitation of [[core electron]]s is possible, but requires much higher energies, generally corresponding to [[X-ray]] photons. This would be the case for example to excite a 2p electron of sodium to the 3s level and form the excited 1s<sup>2</sup>&nbsp;2s<sup>2</sup>&nbsp;2p<sup>5</sup>&nbsp;3s<sup>2</sup> configuration.

The remainder of this article deals only with the ground-state configuration, often referred to as "the" configuration of an atom or molecule.