The electrons in the outermost occupied electron shell, which participate in interatomic bonding.
Valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single covalent bond both atoms contribute one valence electron to form a shared pair. For main group elements, only the outermost electrons are valence electrons. In transition metals, some inner-shell electrons are also valence electrons.
Valence electrons are important in determining how the atom reacts chemically with other atoms. Atoms with a complete (closed) shell of valence electrons (corresponding to an electron configuration s2p6) tend to be chemically inert. Atoms with one or two valence electrons more than a closed shell are highly reactive because the extra electrons are easily removed to form positive ions. Atoms with one or two valence electrons fewer than a closed shell are also highly reactive because of a tendency either to gain the missing electrons and form negative ions, or to share electrons and form covalent bonds.
Valence electrons have the ability, like electrons in inner shells, to absorb or release energy in the form of photons. This gain or loss of energy can trigger an electron to move (jump) to another shell or even break free from the atom and its valence shell. When an electron absorbs energy in the form of one or more photons, then it moves to a more outer shell depending on the amount of energy gained. When an electron loses energy (photons), then it moves to a more inner shell.