N2O₄ has a covalent bond. Covalent bonds are formed when two atoms share electrons in order to complete their outermost electron shell. In the case of N2O₄, each nitrogen atom shares two electrons with each of the two oxygen atoms. This creates a stable molecule in which all of the atoms have a full outermost electron shell.

In contrast, ionic bonds are formed when one atom donates electrons to another atom. This creates two oppositely charged ions which are attracted to each other. Ionic bonds are typically found in compounds between a metal and a non-metal. For example, sodium chloride (NaCl) is an ionic compound because the sodium atom donates one electron to the chlorine atom, creating sodium and chloride ions.