The valence bond (VB) model applied to F2, O2 and N2

Consider the formation of F₂. The ground state electronic configuration of F is [He]2s² 2p⁵, and the presence of the single unpaired electron indicates the formation of an F^_ F single bond. We can write down resonance structures 1.12 to describe the bonding in F₂, with the expectation that the covalent contribution will predominate.

The formation of O₂ involves the combination of two O atoms with ground state electronic configurations of 1s² 2s² 2p⁴. Each O atom has two unpaired electrons and so VB theory predicts the formation of an O=O double bond.

Since VB theory works on the premise that electrons are paired wherever possible, the model predicts that O₂ is diamagnetic. One of the notable failures of VB theory is its inability to predict the observed paramagnetism of O₂. As we shall see, molecular orbital theory is fully consistent with O₂ being a diradical. When two N atoms ([He]2s² 2p³) combine to give N₂, an N≡N triple bond results. Of the possible resonance structures, the predominant form is covalent and this gives a satisfactory picture of the bonding in N₂. In a diamagnetic species, all electrons are spin-paired; a diamagnetic substance is repelled by a magnetic field. A paramagnetic species contains one or more unpaired electrons; a paramagnetic substance is attracted by a magnetic field.