Resonance
Resonance is another kind of bond that exists between atoms. In resonance the bond that holds the compound together is not between two atoms, it is shared between more than two atoms. Looking at an example is the easiest way to understand resonace. If we try to draw sulfur dioxide (SO2), there can be two plausible formulas written. These are:
One might conclude that the molecule would be one or the other but this hypothosis does not fit the experimental data. During experimentation the bonds were determined to be the same between sulfur and both of the oxygen atoms. There were two S-O bonds that were just alike. So neither of the above formulas are correct. So the proper way to write SO2 is as follows:
The above notation symbolizes how the two bonds are the same not how the molecule can be one or the other. The actual structure is a composite of these two.