Understanding the Valence Bond Theory of Atomic Structure

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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 and a dienophile in the Diels-Alder reaction.  Substituent effect on the electronic level will also contribute to whether the bonding will be easier or not..  This is a remarkable effect of atoms and groups of atoms that can affect the rate of chemical reactions by changing the energy levels of the HOMO and LUMO orbitals by raising and lowering their energetic level.  This will depend on the type of substituent whether it is electron rich or electron poor. 

The valence bond theory is a currently successful theory in predicting the manner of bonding between atoms and between molecules.  Molecules that do not have degenerate levels of the HOMO and the LUMO have chemistries that are governed by the single HOMO and the single LUMO. 

The energy gap between the HOMO and the LUMO of the same compound is important, for example, in solid state chemistry.  In the solid state atoms and molecules have instead of single HOMO and single LUMO orbitals they have bands that are called the valence band which depicts the highest occupied electronic level.  In addition the atom also has a conduction band that reperesent the lowest unoccupied molecular orbitals in terms of bands in the solid state. 

The gap between these two bands will determine if the material will be a conductor or an insulator.  A large energetic gap between them will cause the material to an insulator while a small gap will cause the material to be either a conductor or a semi-conductor.  This will depend on the extent to which the valence band is filled with electrons.  A completely filled valence band with low energy gap between the valence band and the conductance band will make this compound a good semi-conductor.  Partially filled valence band with low energy gap between the valence band and the conductance band will make this material a normal conductor.  A semiconductor material will show elevated conductance with the elevation of the temperature.  A normal conductor will show initially high conductance with elevated temperature but with the continuous elevation of the temperature the conductance will decrease because of the increased collisions between electrons in the valence band.

Valence shell of atoms usually has electrons in several orbitals of the same principal quantum number n.  In a molecule like methane or CH4 in which all the bonds between the carbon atom and the four hydrogen atoms are identical a concept of hybridization of atomic orbitals must be used in order to rationalize the existence of the same bonding between the carbon atom and the four hydrogen atoms.

Carbon atom has a valence electronic shell that has 2s orbital that is occupied with two electrons and three p orbitals that are partially filled with electrons.  There are two electrons in the p orbitals of the carbon aom.  A mixing process of the energetically not equivalent valence orbitals on carbon to generate four equivalent orbitals that are designated sp3 orbitals lead to a successful explanation of the bonding in CH4 in which there are four identical bonds between the carbon and the four hydrogen atoms. 

This concept of orbitals hybridization on a central atom to form various numbers of hybridized orbitals in order to account for bonding in molecules such as PCl5 is a satisfactory explanation to the formation of the identical bonds around the central atom.  This method works quiet well also for transition metals bonding with various ligands.       

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