by Tarek Musslimani

Created on: October 20, 2010

The valence bond theory is a quantum mechanics theory that, like the molecular orbitals theory, tries to explain the chemical bonding between atoms.  Each of these theories have different perspectives on how the bond forms.  The valence bond theory is a theory which assumes that the most important electrons of the atom for the bonding between atoms are those electrons that reside in the outermost electronic shell of the atom.   The electrons that are located in the inner shells of the atom are supposed to inert so that they do not participate in the bonding process between the atoms.

This theory assumes that the electrons in the outermost electronic shell govern the chemical and physical properties of the atom under consideration.  It assumes also that the inner electrons are considered as neutralized charges by the positively charged protons of the nucleus.  This model can qualitatively explain the manner of bonding between atoms.  Also in molecules with the higher energy electrons can also explain if the compound will be reactive or inert.  This is done based on the energetic level of the electrons that occupy the highest molecular orbital in the compound.  This can tell also if the molecule will be a good Lewis base or not and if it will be a good Lewis acid or not based on the energy of the highest unoccupied molecular orbital.

All these characters will be determined based on the energy level of the outermost orbital of the compound that is occupied with electrons and the energy level of the unoccupied orbital that is energetically the highest.  In terms of molecular orbitals theory, it is the highest orbital that is occupied and the lowest orbital that is unoccupied that will determine the chemistry of the molecule under investigation, namely these orbitals will determine if the compound will be a good Lewis acid or not and if it will be a good Lewis base or not. 

HOMO-LUMO interaction is an acceptable theory nowadays for the explanation of whether bonding will occur between two molecules or not.  It is the HOMO of one molecule that can interact with the LUMO of the other compound.  The matching in energy and of the mutual symmetry between the two compounds will determine if a bond is likely to occur between the molecules or not.  An example of an application to this theory is the interaction between a diene

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