 Valence electron, 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 elements 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. An atom with a closed shell of valence electrons corresponding to an electron configuration has to p6 tends to be chemically inert. Atoms with one door to more valence electrons than are needed for a closed shell are highly reactive due to the following reasons, one it requires relatively lower energy compared to the lavish enthalpy to remove the extra valence electrons to form a positive ion, two because of their tendency either to gain the missing valence electrons thereby forming a negative ion door to share valence electrons thereby forming a covalent bond. Similar to an electron in an inner shell, a valence electron has the ability to absorb or release energy in the form of a photon, an energy gain can trigger an electron to move jump to an outer shell, this is known as atomic excitation, or the electron can even break free from its associated atoms valence shell, this is ionization to form a positive giant. When an electron loses energy thereby causing the photon to be emitted then it can move to an inner shell which is not fully occupied. Valence energy levels correspond to the principal quantum numbers n equals 1, 2, 3, 4, 5, or are labeled alphabetically with letters used in the x-ray notation k, l, m. The number of valence electrons, the number of valence electrons of an element can be determined by the periodic table group vertical column in which the element is categorized. With the exception of groups 3 to 12 the transition metals units digit of the group number identifies how many valence electrons are associated with a neutral atom of an element listed under that particular column.