Valance Electrons
Those electrons found in orbits farthest from the nucleus of the atom. These electrons determine the way in which the atom will combine with other atoms, and thus determine its chemical properties. 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 s2p6) 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 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
A valence electron is an electron that is the most likely to be involved in a chemical reaction. They are typically the electrons with the highest value of the principle quantum number, n.
Examples: Magnesium's ground state electron configuration is 1s22s2p63s2, the valence electrons would be the 3s electrons.
Bromine's ground state electron configuration is 1s22s2p63s2p6d104s2p5, the valence electrons would be the 4s and 4p electrons.