3.7: Molecular Orbital Theory

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VSPER and valence bond theory have their advantages. They are easily applied using atomic properties like valence electron configuration and electronegativity. They can easily and accurate predict many molecular properties such as molecular shape and bond angles, bond lengths and strengths, polarity, and chirality. Like any theory, there are disadvantages and places where VSPER and valence bond theory fail to explain experimental data. For example, diatomic oxygen (O₂) is a very simple molecule. Both the Lewis structure and valence bond theory would predict that O₂ has a double bond and each O atom has two lone pairs of electrons, and is a diamagnetic molecule (all the electrons are paired). Experiment shows that O₂ is in fact paramagnetic (has unpaired electrons). VSEPR and valence bond theory are not always useful at predicting reactivity. For example, those theories can predict that CO has a triple bond and one lone pair on both O and C. However those theories can't predict if C or the O of CO would react with a Lewis acid.

A video showing the paramagnetic behavior of liquid oxygen

Molecular orbital theory is another bonding theory that can work in places where VSEPR and valence bond fail. Similar to valence bond theory, molecular orbital theory involves the mixing of atomic orbitals, but rather than mixing multiple atomic orbitals on the central atom to form hybrid orbitals atomic orbitals from multiple atoms are mixed together to form molecular orbitals.