#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!
