valence electron

electron in the outermost shell of an atom that participates in chemical bonding

In chemistry, valence electrons are the electrons contained in the outermost, or valence, electron shell of an atom. Valence electrons are important in determining how an element reacts chemically with other elements: The fewer valence electrons an atom holds, the less stable it becomes and the more likely it is to react.
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Noun
1. an electron in the outer shell of an atom which can combine with other atoms to form molecules
(hypernym) electron, negatron

Electrons that can be actively involved in chemical change; usually electrons in the shell with the highest value of n. For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Germanium (Ge) has the ground state electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p2; the 4s and 4p electrons are the valence electrons.