<P> In the molecule H, the hydrogen atoms share the two electrons via covalent bonding . Covalency is greatest between atoms of similar electronegativities . Thus, covalent bonding does not necessarily require that the two atoms be of the same elements, only that they be of comparable electronegativity . Covalent bonding that entails sharing of electrons over more than two atoms is said to be delocalized . </P> <P> The term covalence in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of the American Chemical Society article entitled "The Arrangement of Electrons in Atoms and Molecules". Langmuir wrote that "we shall denote by the term covalence the number of pairs of electrons that a given atom shares with its neighbors ." </P> <P> The idea of covalent bonding can be traced several years before 1919 to Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms . He introduced the Lewis notation or electron dot notation or Lewis dot structure, in which valence electrons (those in the outer shell) are represented as dots around the atomic symbols . Pairs of electrons located between atoms represent covalent bonds . Multiple pairs represent multiple bonds, such as double bonds and triple bonds . An alternative form of representation, not shown here, has bond - forming electron pairs represented as solid lines . </P> <P> Lewis proposed that an atom forms enough covalent bonds to form a full (or closed) outer electron shell . In the diagram of methane shown here, the carbon atom has a valence of four and is, therefore, surrounded by eight electrons (the octet rule), four from the carbon itself and four from the hydrogens bonded to it . Each hydrogen has a valence of one and is surrounded by two electrons (a duet rule)--its own one electron plus one from the carbon . The numbers of electrons correspond to full shells in the quantum theory of the atom; the outer shell of a carbon atom is the n = 2 shell, which can hold eight electrons, whereas the outer (and only) shell of a hydrogen atom is the n = 1 shell, which can hold only two . </P>

Where are electrons located in a covalent bond