Editing Valence electron (section)

{{Short description|Electron in the outer shell of an atom's energy levels}}
[[File:Covalent.svg|thumb|180px|Four [[covalent bond]]s. Carbon has four valence electrons and here a [[Valence (chemistry)|valence]] of four. Each hydrogen atom has one valence electron and is univalent.]]

In [[chemistry]] and [[physics]], '''valence electrons''' are [[electrons]] in the outermost [[electron shell|shell]] of an [[atom]], and that can participate in the formation of a [[chemical bond]] if the outermost shell is not [[closed shell|closed]]. In a single [[covalent bond]], a shared pair forms with both atoms in the bond each contributing one valence electron.

The presence of valence electrons can determine the [[chemical element|element]]'s [[chemistry|chemical]] properties, such as its [[valence (chemistry)|valence]]—whether it may bond with other elements and, if so, how readily and with how many. In this way, a given element's [[Reactivity (chemistry)|reactivity]] is highly dependent upon its [[Electron configuration|electronic configuration]]. For a [[main-group element]], a valence electron can exist only in the outermost [[electron shell]]; for a [[transition metal]], a valence electron can also be in an inner shell.

An atom with a closed shell of valence electrons (corresponding to a [[noble gas configuration]]) tends to be [[inert gases|chemically inert]]. Atoms with one or two valence electrons more than a closed shell are highly reactive due to the relatively [[Ionization energy|low energy]] to remove the extra valence electrons to form a positive [[ion]]. An atom with one or two electrons fewer than a closed shell is reactive due to its tendency either to gain the missing valence electrons and form a negative ion, or else to share valence electrons and form a covalent bond.

Similar to a [[core electron]], a valence electron has the ability to absorb or release energy in the form of a [[photon]]. An energy gain can trigger the electron to move (jump) to an outer shell; this is known as [[Excited state#Atomic excitation|atomic excitation]]. Or the electron can even break free from its associated atom's shell; this is [[ionization]] to form a positive ion. When an electron loses energy (thereby causing a photon to be emitted), then it can move to an inner shell which is not fully occupied.