$2{\mathrm{NO}}_2\rightleftharpoons {\mathrm{N}}_2{\mathrm{O}}_4$ The dimerization of ${\mathrm{NO}}_2$ is accompanied with:

The nitrogen atom in the ${\mathrm{NO}}_2$ molecule possesses one unpaired electron. In order for this unpaired electron to combine with another to produce the ${\mathrm{N}}_2{\mathrm{O}}_4$ molecule, the molecule will often dimerize. Because ${\mathrm{NO}}_2$ has an unusual number of electrons, it is paramagnetic. It's colored because it has an unusual electron in it. In contrast, ${\mathrm{N}}_2{\mathrm{O}}_4$ is dimer is colorless and less magnetic because it lacks an unpaired electron.