Valence electron

To define valence electron of an atom first let us give definition of valence. Valence of an atom in a molecule is the number of bond concerned with this atom in molecule. Just for example, in methane carbon atom is bonded with 4 hydrogens. Each hydrogen is connected with carbon by single bond. Four hydrogen atom bonded with one carbon by 4 single bond. Hence, the number of bond concerned with carbon is 4. Hence, the valence of carbon atom in methane molecule is four. The valence electron is the number of electron available in outer shell of atom that can participate in the forming of bond. In an atom there may have a large number of electron surrounding the nucleus.But all of the electrons do not participate in bond formation and are not available ifor bond formation. In methane though carbon have number six electrons, only 4 electrons involved in formation of bond. These electrons available for bond forming in outer shell of atom is called valence electron. In carbon the valance electron is four. In a molecule each valence electron of an atom may not involved in bond forming. For example, the outer shell of hydrogen atom contains only one electron and the outer shell of chlorine contains 7 electrons. Hence, the valence electron of hydrogen is 1 and chlorine is 7. But in HCl molecule number one valence electron of chlorine atom is involved in bond formation. The valence electron that forms bond is called bonding electron. The rest of the electrons that do not take place in bond formation, but available for bond formation is called non bonding electron.