NO2 Dot-and-Cross: The Odd Electron
Nitrogen dioxide has an odd number of outer electrons, so one is always left unpaired. Here is how to build its dot-and-cross diagram, and why that lone electron makes NO2 such a reactive radical.
An odd number of electrons
Nitrogen dioxide has 17 outer-shell electrons (N brings 5, each O brings 6). That total is odd, so once the bonds are drawn, one electron on nitrogen is left unpaired.
So NO2 is a free radical
That single unpaired electron makes NO2 a reactive free radical. It is also why NO2 readily dimerises to N2O4: the two odd electrons pair up to form the new N–N bond.
Drawing it, step by step
Show only outer-shell electrons, with dots for one atom and crosses for the other to track where each came from. Give both oxygens a full octet first, then place the leftover electron on nitrogen, which ends up electron-deficient, the signature of an odd-electron species.
Practise it yourself
Work through 24 dot-and-cross diagrams, from dative bonds to oxo-anions, in the interactive ChemBank simulation.
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