Question

Why does resonance occur?

Answer 1

Resonance is a molecule's way of spreading out its electron density, and that helps to minimize its ground-state energy.

We as chemists draw so-called resonance structures to depict each possible snapshot of the molecule that contribute to the overall observed resonance hybrid structure.

Consider acetate anion and the allyl carbocation:

The acetate anion has two resonance structures of the same energy;

• one in which the `pi` bond is on one oxygen,
• the other where it's on the other oxygen.

These snapshots "overlap" to give the resonance hybrid, with half-`pi`-bonds across the `"C"-"O"` internuclear distance.

The allyl carbocation is stabilized in a similar manner with two resonance structures of the same energy. The hybrid structure spreads out the negative charge to stabilize the positive charge.

Or, consider the following resonance structures of urea:

• `"I"` is the most commonly observed form, and is the lowest-energy resonance structure. It contributes the most to the observed structure.
• `"II"` is unstable because carbon only has six valence electrons.
• `"III"` is unstable because oxygen is more electronegative than nitrogen, and can withstand more of the negative charge in a double bond than nitrogen can.
• `"IV"` is the same energy as `"III"` by symmetry.