Fig. 7
A Lewis structure-like presentation of \({\text{NO}_{2} + \text{NO} \longrightarrow \text{N}_{2}\text{O}_{3}}\) highlights that multigraphs with atom-type dependent degrees are not sufficient to represent all molecules of interest. To represent NO2, both an unpaired electron (shown as a semi-edge ending in a small black ball), an N atom with vertex degree \(4<{{\,\mathrm{val}}}({N})=5\) and an oxygen atom with vertex degree \(7>{{\,\mathrm{val}}}({O})=6\) are required. Similarly, NO is a neutral stable radical, with an unpaired electron at N. The product N2O3 has no unpaired electrons, but exhibits an O and an N atom with a deviant vertex degree and thus a net charge. Differences between nominal valency and actualy vertex degree are indicated by the charge symbols \(\oplus\) and \(\ominus\). In general, the net charge at a vertex v is given by \({{\,\mathrm{val}}}(\alpha (v))-\deg (v)\)