Azide is the anion with the formula N₃^-. It is the conjugate base of hydrazoic acid (HN₃). N₃^- is a linear anion that is isoelectronic with CO₂ and N₂O. Per valence bond theory, azide can be described by several resonance structures, an important one being N^-=N⁺=N^-. Azide is also a functional group in organic chemistry, RN₃. The dominant application of azides is as a propellant in air bags.