Essentially, resonance structures are explained by Resonance Theory. Resonance theory looks to answer the question: How do we reconcile the differences between Lewis Structures of species and their observed characteristics? There are a few rules that must be followed when determining resonance of compound.
1. Resonance occurs in species for which there are two or more valid Lewis structures.
For a species, each valid Lewis structure is called a Resonance Structure or Resonance Contributor. In these cases, you are simply moving an electron or pair of electrons to another location in a molecule. An example is HCO2-.There are two possible resonance contributors in this molecule. As shown in the example, resonance structures differ only in the placement of their valence electrons, not their atoms.
1. Resonance structures are imaginary; the one, true species is represented by the resonance hybrid.
The Resonance Hyrbid is the weighted average of all resonance structures. An example of the resonance hybrid occurs in either of these molecules.
1. The resonance hybrid looks most like the lowest energy (most stable) resonance structure. 724
Essentially, the least formal charges on the molecule, or most evenly distributed charges.
A few tips I have for drawing resonance structures based on Lewis structures for different molecules include:
1. Move away from a negative charge. If presented with a negative charge on a specific atom, it means that there is an excess of electrons on that atom. Either take electrons away from the atom, or move a bond that it is involved in.
2. Move towards a positive charge. A positive charge represents a lack of electrons, this means that you should move electrons to the atom with the charge, or move a double/triple bond to allow it to become involved in another bond.
3. Move away from a radical. A radical does not represent a charge, as it is only a single electron, but you should (in most cases) move electrons away from the radical.
Also, always remember that electrons are the only things that move in resonance structures. They can also not be added or subtracted, but only moved. Make sure not to move/add/remove any atoms, and you cannot move electrons to an atom that is already saturated with bonds for example, an sp3 hybridized Carbon as it is already saturated with Hydrogens.
Best of luck! 662