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The overlapping atomic orbitals can be of different types. There are two different types of overlapping orbitals: sigma and pi. Sigma Bond s occur when the orbitals of two shared electrons overlap co-axially. Pi Bond s occur when two orbitals overlap but do not do so on the axes (i.e. the side-to-side overlap of ''p''-orbitals). For example, a bond between two ''s''-orbital electrons is a sigma bond, because two spheres are always coaxial. However, the atomic orbitals for bonding may not be "pure" atomic orbitals directly from the solution of the Schrödinger Equation . Often, the bonding atomic orbitals have a character of several possible types of orbitals. The methods to get an atomic orbital with the proper character for the bonding is called Hybridisation . Valence bond theory has been extended to Molecular Orbital Theory , which explains this Hybridisation as linear combinations of the Wavefunctions associated with each atom involved. More recently, several groups have developed what is often called Modern Valence Bond theory. This replaces the overlapping atomic orbitals by overlapping valence bond orbitals that are expanded over all Basis Functions in the molecule. The resulting energies are more competitive with energies where Electron Correlation is introduced based on a Hartree-Fock reference wavefunction. |
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