Valence Electron
An electron in the outermost occupied shell of an atom, available for bonding.
Unit
Dimensionless
In depth
Valence electrons determine an element's group, valency, bonding capacity and most of its chemistry. For main-group elements their number equals the group number modulo ten; for transition metals, d-electrons in the penultimate shell also participate, which is why variable oxidation states are common there.
Examples in the real world
Sodium has one valence electron (3s¹); chlorine has seven (3s² 3p⁵); carbon has four.