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[[File:Electron shell 001 Hydrogen.svg|thumb|180px| Example of the [[Hydrogen atom]]: it has one electron shell and one [[electron]].]]

An '''electron shell''', or main energy level, is the part of an [[atom]] where [[Electron cloud|electrons]] are found orbiting the [[Atomic nucleus|atom's nucleus]].

In [[chemistry]] and [[atomic physics]], an electron shell may be thought of as an [[orbit]] followed by [[Electron cloud|electrons]] around an [[Atomic nucleus|atom's nucleus]]. All atoms have one or more electron shell(s), all of which have varying numbers of electrons.  

Each electron shell has one or more electron sub-shells, or sub-levels. Electron shells make up the [[electron configuration]] of an [[atom]]. The number of [[electron]]s that can be in a certain shell is equal to <math>2n^2</math>, where <math>n</math> is the number of a shell.

The name for electron shells came from the [[Bohr model]], which states that electrons [[orbit]] the [[Nucleus (physics)|nucleus]] at certain distances so that their [[orbit|orbits]] form "shells". This term was presented by [[Niels Bohr|Niels Henrik David Bohr]].

==Valence shell==
The valence shell is the outermost shell of an [[atom]] in its neutral state. The valence shell contains the [[electron]]s most likely to be involved in [[chemical reaction|reactions]]. 

In a [[noble gas]], an atom tends to have eight electrons in its outer shell (except [[helium]], which is only able to fill its shell with two electrons). Noble gases serve as the model for the [[octet rule]], which states that atoms tend to form bonds or create ions so that they can have eight electrons in their outermost shell. This is mostly applicable to the [[Group (periodic table)|main-group element]]s of the second and third [[Periodic table|periods]].

For [[Complex (chemistry)|coordination complexes]] containing [[transition metal]]s, the valence shell consists of 18 electrons. This is referred to as the ''eighteen electron rule''.

{| class="wikitable"
|+Possible number of electrons in shells 1-7
! Shell
! Electrons
|-
| 1
|  2
|-
| 2
|8
|-
| 3
|18
|-
| 4
|32
|-
| 5
|32
|-
|6
|18
|-
|7
|8
|-
|}

== Sub-shells ==

Electron sub-shells are identified by the letters s, p, d, f, g, h, i, etc., corresponding to the [[:en:Azimuthal quantum number|azimuth quantum numbers]]: 0, 1, 2, 3, 4, 5, 6, etc. Each shell is able to hold 2, 6, 10, 14, and 18 electrons respectively. 

== References ==
{{reflist}}
* Tipler, Paul & Ralph Llewellyn (2003).  ''Modern Physics'' (4th ed.).  New York: W. H. Freeman and Company.  {{ISBN|0-7167-4345-0}}

[[Category:Physics]]
[[Category:Quantum mechanics]]
[[Category:Chemical bonding]]