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a chemical covalent bond formed by a pair of electrons that spin in opposite directions in the region between two atomic nuclei. The molecules H2, Cl2, and HCl, for example, each contain one covalent bond that is formed by the interaction of unpaired electrons located on the 1s–1s, 3p-3p, and 1s-3p atomic orbitals. Since these molecules are diatomic, their chemical bonds have two centers. The following electronic and structural formulas may be written for the above molecules: H: H and H—H, Cl: Cl and Cl—Cl, H: Cl and H—Cl (the two dots and the valence line, or dash, denote the electron pair common to both combined atoms).
A similar description is valid for the molecules of all saturated compounds, in which the number of valence electron pairs is equal to the number of possible two-center interactions. All the chemical bonds in these compounds are simple bonds with a multiplicity equal to one.
A pair of electrons that form a single bond can also have a donor-acceptor origin. Therefore, the isoelectronic molecule CH4 and the ions BH4- and NH4+, for example, can be expressed by the same type of structural formula:
where×= B-, C, and N+, respectively.
In chemical compounds where the average number of electrons bonding each pair of atomic nuclei is not equal to two, the chemical bonds that form can exhibit varying multiplicity of less than one in compounds with an electron deficiency and greater than one in compounds with multiple bonds. In these cases a description of the electronic molecular structure usually calls for the application of the most common method of quantum chemistry—the molecular orbital method.
E. M. SHUSTOROVICH