Question

Why does NO2 dimerise?

Answer 1

In NO2 structure there is an unpaired electron left on Nitrogen atom, so it is reactive it dimerises to pair up its electron and gains stability as N2O2.

Answer 2

No2 contain odd number of valence electrons. It behaves as a typical odd molecule.On dimerisation,

it is converted to stable N2O4 molecule with even number of electrons.