Editing Energy level (section)

{{Short description|Different states of quantum systems}}
{{Use American English|date=January 2019}}[[Image:Energy levels.svg|thumb|right|Energy levels for an [[electron]] in an [[atom]]: ground state and [[excited state]]s. After absorbing [[energy]], an electron may "jump" from the ground state to a higher energy excited state.]] 
{{Quantum mechanics|cTopic=Fundamental concepts}}
A [[quantum mechanics|quantum mechanical]] system or [[particle]] that is [[bound state|bound]]—that is, confined spatially—can only take on certain discrete values of energy, called '''energy levels'''. This contrasts with [[classical mechanics|classical]] particles, which can have any amount of energy. The term is commonly used for the energy levels of the electrons in atoms, [[ion]]s, or [[molecule]]s, which are bound by the electric field of the [[atomic nucleus|nucleus]], but can also refer to energy levels of nuclei or [[molecular vibration|vibrational]] or [[rotational energy]] levels in molecules. The energy spectrum of a system with such discrete energy levels is said to be [[Quantization (physics)|quantized]].

In [[chemistry]] and [[atomic physics]], an electron shell, or principal energy level, may be thought of as the [[orbit]] of one or more [[electrons]] around an [[atom]]'s [[atomic nucleus|nucleus]]. The closest shell to the nucleus is called the "1 shell" (also called "K shell"), followed by the "2 shell" (or "L shell"), then the "3 shell" (or "M shell"), and so on further and further from the nucleus. The shells correspond with the [[principal quantum number]]s ({{math|''n''}} = 1, 2, 3, 4,&nbsp;...) or are labeled alphabetically with letters used in the [[X-ray notation]] (K, L, M, N,&nbsp;...).

Each shell can contain only a fixed number of electrons: The first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on. The general formula is that the ''n''th shell can in principle hold up to 2''n''<sup>2</sup> electrons.<ref name="madsci">[http://www.madsci.org/posts/archives/1999-03/921736624.Ch.r.html Re: Why do electron shells have set limits ?] madsci.org, 17 March 1999, Dan Berger, Faculty Chemistry/Science, Bluffton College</ref> Since electrons are [[electrostatic field|electrically attracted]] to the nucleus, an atom's electrons will generally occupy outer shells only if the more inner shells have already been completely filled by other electrons. However, this is not a strict requirement: atoms may have two or even three incomplete outer shells. (See [[Madelung rule]] for more details.) For an explanation of why electrons exist in these shells see [[electron configuration]].<ref name="corrosionsource.com">[http://www.corrosionsource.com/handbook/periodic/e_subshells.htm Electron Subshells]. Corrosion Source. Retrieved on 1 December 2011.</ref>

If the [[potential energy]] is set to zero at [[infinity|infinite]] distance from the atomic nucleus or molecule, the usual convention, then [[bound states|bound electron states]] have negative potential energy.

If an atom, ion, or molecule is at the lowest possible energy level, it and its electrons are said to be in the ''[[ground state]]''. If it is at a higher energy level, it is said to be ''[[excited state|excited]]'', or any electrons that have higher energy than the ground state are ''excited''. An energy level is regarded as [[degenerate energy level|degenerate]] if there is more than one measurable quantum mechanical [[quantum state|state]] associated with it.