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Molecular Orbital Theory vs Valence Bond Theory: Conductivity

2. (a) How does MO theory differ from Valence bond theory?
(b) Explain how the above concepts of MO theory are applied to rationalize the conductivity of :
i. metals
ii. (SN)x [a polymer]
iii. semiconductors


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(a) The Molecular Orbital Theory does a good job of predicting elctronic spectra and paramagnetism, when the Valence Bond Theory does not. The MO theory does not need resonance structures to describe molecules, as well as being able to predict bond length and energy. The major draw back is that we are limited to talking about diatomic molecules (molecules that have only two atoms bonded together), or the theory gets very complex. The MO theory treats molecular bonds as a sharing of electrons between nuclei. Unlike the V-B theory, which treats the electrons as localized baloons of electron density, the MO theory says that the electrons are delocalized. That means that they are spread out over the entire molecule.
On the other hand, the valence-bond approach considers the overlap of the atomic orbitals (AO) of the participation atoms to form a chemical bond. Due to the overlapping, electrons are localized in the bond region. The overlapping AOs can be of different types, for example, a sigma bond may be formed by the overlapping the following AOs. However, the atomic orbitals for bonding may not be "pure" ...