Chapter 8, Problem 140AP

Although nitrogen dioxide $\left({\text{NO}}_{\text{2}}\right)$ is a stable compound, there is a tendency for two such molecules to combine to form dinitrogen tetroxide ${\text{(N}}_{\text{2}}{\text{O}}_{\text{4}}\text{)}$. Why? Draw four resonance structures of ${\text{N}}_{\text{2}}{\text{O}}_{\text{4}}$, showing formal charges.