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Date: 23-8-2016
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The valence bond (VB) model of bonding in H2
Valence bond theory considers the interactions between separate atoms as they are brought together to form molecules. We begin by considering the formation of H2 from two H atoms, the nuclei of which are labelled HA and HB, and the electrons of which are 1 and 2, respectively.
When the atoms are so far apart that there is no interaction between them, electron 1 is exclusively associated with HA, while electron 2 resides with nucleus HB. Let this state be described by a wavefunction ψ1.
When the H atoms are close together, we cannot tell which electron is associated with which nucleus since, although we gave them labels, the two nuclei are actually indistinguishable, as are the two electrons. Thus, electron 2 could be with HA and electron 1 with HB. Let this be described by the wavefunction ψ2.
Equation 1.1 gives an overall description of the covalently bonded H2 molecule; ψcovalent is a linear combination of wavefunctions ψ1 and ψ2. The equation contains a normalization factor, N (see Box 1.4). In the general case where: