NO_2 is a radical species; that is it features a single, unpaired electron conceived to be on the nitrogen centre. NO_2 necessarily has 17 valence electrons to distribute around the 3 atoms (2 xx 6 + 5). The nitrogen is formally cationic and bears a single electron, whereas one of the oxygen atoms bears a negative charge. Of course, we draw another resonance structure such that the negative charge is borne by the other oxygen:
O=dot(N)^(+)-O^(-). From left to right, each atom has 6, 4, and 7 valence electrons, and formally has a charge of 0, 1^+, and 1^- respectively. Two dotNO_2 molecules can dimerize when the single electrons on the nitrogen centres couple to form a nitrogen-nitrogen bond, i.e. O^(-)(O=)N^(+)-NO_2. Of course, there is a still a formal positive charge on each nitrogen centre. Also, of course, the nitrogen-nitrogen bond of the dimer can homolyze to give 2 equiv NO_2, which is the answer to the question I presume you have asked.
Apologies if I am barking up the wrong tree. Woof!
