Which one of the following nitrogen oxides may dimerize?

Nitrogen dioxide (NO_2$) is an odd-electron molecule containing 17$ valence electrons.

Due to the presence of an unpaired electron on the nitrogen atom, it is highly reactive and paramagnetic.

To achieve stability by pairing the odd electron, $NO_2$ undergoes dimerization to form dinitrogen tetroxide (N_2O_4$), which is a diamagnetic and even-electron molecule.<br><br>2NO_2 \rightleftharpoons N_2O_4$

The other oxides (N_2O$,$N_2O_3$,$N_2O_5$) are even-electron molecules and do not dimerize.<br><br>Answer:$NO_2