Resonance in   Carbon Dioxide

Fig. 1.1: Resonance structures of carbon dioxide

In the above figure we see the second and third resonance structures average out to the first, and so the average of all the resonance structures is a double bond. That is, if you look at the first, the two structures on the right add up to a total of 4, which when averaged out between the two of them, becomes 2. Thus it is common to write carbon dioxide as having two double bonds, and that resonance structure is the correct structure of carbon dioxide.