There are some molecules that have pi electrons that can move around from one atom to another. For example, the following molecules (A and B) are both different forms of acetate:
Structures A and B are called resonance structures (or resonance contributors). In reality, acetate actually exists somewhere in-between A and B, with the – charge being shared equally by the two oxygens.
When drawing different resonance structures, remember:
1. Only electrons move. Specifically, only pi electrons, lone-pair electrons, or negative charges can move. Do NOT move atoms.
2. You CAN move electrons toward or into an atom that does NOT have a full octet, such as carbocations.
3. If an atom already HAS a full octet, then you can move electrons into it ONLY IF you push electrons out the opposite side (electrons in, electrons out).
4. Do not move or break sigma bonds, only pi bonds.
Determining Greatest Resonance Contributor
1. The most stable resonance structure will have a full octet on every atom.
2. The most stable resonance structure will have the smallest possible number of charges.
3. The most stable resonance structure will have negative charges on the most electronegative atoms and positive charges on the least electronegative atoms.