Resonance Structures
Sometimes a single Lewis structure is not enough to describe the electronic structure of a compound or an ion.
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How would you explain what a rhinoceros looks like to someone who has never seen one? You would use similes. For example, you could explain that a real rhinoceros is something, say, intermediate between a unicorn and a dragon. The explanation may be more or less apt, but if the person you are talking to knows what unicorns and dragons are, they will get a fairly reasonable idea of what a rhinoceros really is.
The reality of the nitromethane ("rhino") molecule is that the two N-O bonds are completely equivalent in all their properties: strength, N-O distance, etc.
However, the Lewis formula suggests that one bond is N-O and the other N=O. We need to explain reality with suitable "unicorns" and "dragons".
Real nitromethane molecule, with its two equivalent N-O bonds
This Lewis structure alone cannot explain the reality of nitromethane
In this case the "unicorn" and the "dragon" are almost identical. Nitromethane must be described with two very similar Lewis formulas that differ only in which of the two oxygens bears the double bond to the nitrogen. An "intermediate" formula can often be drawn to describe the "resonance" of nitromethane between the two Lewis formulas.
Something similar happens in the acetate ion. The two C-O bonds are actually equivalent, but in the Lewis formulas one is C-O and the other C=O. The acetate ion "resonates" between the two Lewis formulas. Here too, an intermediate formula can be drawn.
In many cases, the Lewis formulas between which a real structure "resonates" are not equivalent. Their contribution to the "resonance hybrid" is different depending on the following rules:
1. Resonance structures only involve the movement of electrons
(NEVER of atoms) between adjacent positions.
2. Resonance structures in which all 2nd-period atoms have complete octets are more important (they contribute more to the resonance hybrid) than structures with incomplete octets.
3. The most important structures are those with the least charge separation.
4. When a Lewis structure with complete octets cannot be drawn without charge separation, the most important structure is the one that places the negative charge on the most electronegative atom and the positive charge on the most electropositive one.
In reality, the aromatic ring of phenol has a slight excess of charge at the "ortho" and "para" positions relative to the OH group. Four Lewis formulas are needed to explain it. The predominant form is the first one, which has no charge separation. The others, although they place the positive charge on the most electronegative atom, are the only ones that explain why the ring has a slight excess of electron density at the "ortho" and "para" positions.
In nitrobenzene the reality is the opposite: the aromatic ring has a slight charge deficit at the "ortho" and "para" positions relative to the nitro group. Five Lewis formulas are needed to explain it. The predominant forms are the first two, which have the "normal" charge separation due to the structure of the nitro group. The others, although they increase the charge separation, are the only ones that explain why the ring has a slight deficit of electron density at the "ortho" and "para" positions.
