What is "valence electron"

n. (context chemistry English) any of the electrons in the outermost shell of an atom; capable of forming bonds with other atoms

n. an electron in the outer shell of an atom which can combine with other atoms to form molecules

In chemistry, a valence electron is an electron that is associated with an atom, and that can participate in the formation of a chemical bond; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair. The presence of valence electrons can determine the element's chemical properties and whether it may bond with other elements: For a main group element, a valence electron can only be in the outermost electron shell. In a transition metal, a valence electron can also be in an inner shell.

An atom with a closed shell of valence electrons (corresponding to an electron configuration sp) tends to be chemically inert. Atoms with one or two more valence electrons than are needed for a "closed" shell are highly reactive because it requires relatively low energy (compared to the lattice enthalpy) to remove the extra valence electrons to form a positive ion. Atoms with one or two valence electrons fewer than are needed to form a closed shell are also highly reactive because of a tendency either to gain the missing valence electrons (thereby forming a negative ion), or to share valence electrons (thereby forming a covalent bond).

Like an electron in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon. An energy gain can trigger an electron to move (jump) to an outer shell; this is known as atomic excitation. Or the electron can even break free from its associated atom's valence 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.

Valence energy levels correspond to the principal quantum numbers (n = 1, 2, 3, 4 ...) or are labeled alphabetically with letters used in the X-ray notation (K, L, M, …).