When two atomic 1 s orbitals combine in the formation of H 2, the result is two sigma ( σ) orbitals.Īccording to MO theory, one sigma orbital is lower in energy than either of the two isolated atomic 1 s orbitals –this lower sigma orbital is referred to as a bonding molecular orbital. Mathematical principles tell us that when orbitals combine, the number of orbitals before the combination takes place must equal the number of new orbitals that result from the combination – orbitals don’t just disappear! We saw this previously when we discussed hybrid orbitals: one s and three p orbitals make four sp 3 hybrids. A molecular orbital describes a region of space around two or more atoms inside which electrons are likely to be found. Recall that an atomic orbital (such as the 1s orbital of a hydrogen atom) describes a region of space around a single atom inside which electrons are likely to be found. In molecular orbital theory, we make a further statement: we say that the two atomic 1 s orbitals mathematically combine to form two new orbitals. When we described the hydrogen molecule using valence bond theory, we said that the two 1 s orbitals from each atom overlap, allowing the two electrons to be shared and thus forming a covalent bond. Let’s go back and consider again the simplest possible covalent bond: the one in molecular hydrogen (H 2). In order to understand these properties, we will use the ideas of MO theory. Valence bond theory does a remarkably good job at explaining the bonding geometry of many of the functional groups in organic compounds, however, it fails to adequately account for the stability contained in alternating double and single bonds. To understand the source of this stabilization we will use molecular orbital (MO) theory. When we look at carbon-carbon double bonds (C=C), we need to look and see if they are isolated or conjugated. Alternating single and double bonds create a conjugated pi bond system across multiple atoms that lowers the energy and stabilizes the molecule or ion. It is important to train our eye to recognize structural features that have stabilizing effects.
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