n electron revolving around the nucleus of an atom has potential energy, boller centrifugal energy, magnetic energy and rotational energy. These energies determine the total energy or energy level of an electron, measured as electron volt.
An electron volt can be defined as the amount of energy required to move an electron through a potential of one volt i.e., the amount of energy gained or lost when an electron falls through or moves against the potential difference of one volt.
1 eV = 1.6 x 10-19 Joules
Each shell has an energy level. An electron in the outermost orbit is loosely bound to the nucleus and has higher energy. Similarly, the electron revolving in the inner shell very close to the nucleus is very tightly bound and has only a small amount of energy. Thus, it cannot be easily isolated from an individual atom.
Hence, the valence electrons in the outermost shell can be easily extracted from the atom. These isolated electrons involve in chemical reactions and bond the atoms together.