Nitric oxide is a free radical as, it has an unpaired electron, which is sometimes denoted by a dot in its chemical formula (∙N=O or ∙NO). A radical is an atom, molecule, or ion that has an unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerise. ‌‌
∙N=O
Nitric‌oxide
Whereas CO2,CN−and NO2−have all paired electrons.