Is nitrogen dioxide considered as a radical or not?

Question

Nitrogen dioxide has a molecular structure •ONO in which one of the two oxygen atoms possesses an unpaired electron. So by definition, is it a radical or not? If yes, why is nitrogen dioxide much more stable than other radicals despite the dimerisation at low temperatures?

Answer 1

Yes and NO. Singly yes. But 2NO2 <--> N2O4

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Answer 2

By definition,  it is a radical.

It is stable because there is no compound into which it can react in a thermodynamically favored way.   In other words,  it's not terribly stable.  Rather  everything else that it might dimerize or polymerize into is less stable.  

At lower temperatures,  it dimerizes because there is even less thermal energy present and the lower enthalpy is the driving force.   As the temperature rises and there is more thermal energy present,  entropy becomes more of a driving force and the dimers depolymerize to a significant extent to generate the free radical form of NO2.

The bottom line is that the bond in N2O4 is weak and forms quite reversibly and with a little thermal energy will come apart.

There are other stable free radicals.   See the reference below.   These materials will remain in a free radical form rather than dimerize.   Some will remain as a radical only in the absence of oxygen.