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valence bond theoryZoomA-Z

In addition to the MO theory, the valence bond theory is another approximation to be used in quantum chemistry. According to this approach, atoms in a molecule are held together by one or more electron pairs which are localized between two nuclei. This arrangement of atoms and bonds is called a Lewis structure. In addition to covalent Lewis structures, others, such as ionic resonance structures, have to be considered. In these structures electron pairs are localized on single atoms. The ground state of molecules with non-localized valence electrons cannot be described by a single resonance structure. Using the VB theory, the actual bonding status of the molecule is described by an overlap of such resonance structures.