an electron in an outer shell of an atom that can form chemical bonds with other atoms

an electron in the outermost, or valence, shell of an atom

Electrons that can be actively involved in chemical change; usually electrons in the shell with the highest value of n ( electrons in the "outermost" shell of an atom). For example, sodium's ground state electron configuration is 1s2 2s2 2p6 3s1; the 3s electron is the only valence electron in the atom. Valence electrons determine the chemical properties of an atom and are the only electrons that participate 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.