h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How is Valence Bond theory different from Molecular Orbital theory?

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MO and VB theory are two different approaches to describing the same thing. The major difference is that in VB theory we take N atomic orbitals and we produce N/2 valence bond wavefunctions from them and also involves concepts of promotion and hybridisation to overcome various problems. In MO theory we take N atomic orbital and produce N molecular orbital so for two atoms each with one atomic orbital we end up with two wavefunctions rather than one. The geometries of molecules ie bond angles etc are then determined by maximising the overlap integrals and doesn't require 'fudge factors' like promotion and hybridisation and is a much closer approximation to reality. For example water VB theory essentially predicts that we have two types of valence eletron, two pairs of bonding and two nonbonding lone pairs of electron. The bonding ones should all be at the same energy and the lonepairs shouild be at the same energy giving us two levels. In MO theory, we predict that each of the four pairs of electrons are at different energies and indeed this is observed by a technique called photoelectron spectroscopy that is ables to see where the molecular orbital lie in energy relative to one another (see http://www.ch.ic.ac.uk/vchemlib/course/mo_theory/h2oed.gif from one of the websites I referenced)