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When a molecule has resonance it means its structure is always switching or changing. It doesn't always have to be 3 shapes or bond lengths that it switches between; it can be 2 or 4 ect. And it is not that 3 different structures exist of the molecule, but rather that the molecule is constantly switching between 3 different structures and moving its varied bond lengths between different atoms.
Basically, Lewis structures are an oversimplification of what the molecule actually looks like. There can be multiple ways to draw the same structure (doesn't just have to be three ways, but can be 2+) because the molecule is constantly switching which bonds are double and single. This also explains why the actual molecule's bond lengths are the average of all of the resonance structures.
A molecule that has resonance has multiple lewis dot structures with double or triple bonds that can appear in multiple places, not necessarily that it has different lewis dot structures because chemical species that experience resonance technically have the same lewis structure, just different versions.
The Lewis structure is a simplification for the structure of the molecule but it is helpful to visualize it. The bonds can be written in as many ways as the bonds can be arranged which indicates resonance. The actual molecule is an average of the resonance structures.
Having resonance does not necessarily mean that there are exactly three ways to draw the Lewis structure. Rather, resonance means that the molecule being examined has more a more complex bonding structure. A Lewis structure is a simplification of what a molecule really is, as demonstrated by the difference between experimental and theoretical bond lengths discussed in lecture when going over resonance.
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