<P> In chemistry, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; 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, such as its valence--whether it may bond with other elements and, if so, how readily and with how many . For a main group element, a valence electron can exist only in the outermost electron shell; in a transition metal, a valence electron can also be in an inner shell . </P> <P> An atom with a closed shell of valence electrons (corresponding to an electron configuration s p) tends to be chemically inert . Atoms with one or two more valence electrons than are needed for a "closed" shell are highly reactive due to the following reasons: 1) It requires relatively low energy (compared to the lattice enthalpy) to remove the extra valence electrons to form a positive ion . 2) Because of their tendency either to gain the missing valence electrons (thereby forming a negative ion), or to share valence electrons (thereby forming a covalent bond). </P>

Alkali metal with only 2p and 3p electrons