The O – N – O bond angle in NO2 is around 134°. One way to explain this is by making use of resonance structure and hybridisation theory. Let us consider the following resonance structure of NO2:
Nitrogen atom needs three hybridised orbitals to accommodate two sigma bonds and a single electron, so it has sp2 hybridisation. Each of the three sp2 orbitals in nitrogen has one electron and the p orbital also has one electron. After two sigma bonds, only one sp2 orbital has one electron and the p orbital has one electron as well. The p orbital is involved in pi bond with one oxygen atom.
Normally, the sp2 orbitals are at 120° angle with each other. This would have been the case had there been three pair of electrons distributed uniformly. However, after bonding since only one sp2 orbital has one electron, we have two pair of electrons and a single electron. There is greater repulsion between the two pair of electrons in comparison to the pair of electrons and single electron which increases the angle from 120° to 134°.