Why is there only one double bond in ozone and not 2?

Answer

Hi Jan,

If you draw a Lewis structure of ozone, it will indeed take one double bond to give all oxygen atoms an octet configuration. Simultaneously, it is necessary to place a negative charge at the other end (one electron more than usual) and give the central oxygen atom a positive charge (one electron less than usual).

Lewis structures, however, are only approximations of reality: they are a 2D representation of a molecule that is three-dimensional. Moreover, Lewis structures are not very good at representing delocalized electrons: in reality the electrons are spread over the three oxygen atoms, with a slightly higher electron density at the ends. (The molecular orbital theory is more suitable for representing electron density in molecules.)

You can partially overcome the limitation of the classical Lewis structures by drawing two Lewis structures as shown in the figure below. We then speak of resonance canons. The “real” structure of ozone is then somewhere between these two extremes.