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In chemistry, resonance is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula.

Resonance Structures

Resonance structures are all the valid Lewis structures of a particular species that have the same connectivity for the atoms in the molecule. However, the arrangement of the electrons differ. Resonance structures are indicated by drawing a double headed arrow between the two (or more) structures. The actual structure is an average of all of the resonance structures of the molecule. When drawing resonance structures for a given species, it is important to note the overall charge. All the resonance structures of a molecule must have the same overall charge.

Make sure Carbon follows the octet rule.

Choosing the Best Resonance Structure

The best resonance structure is more stable and has minimum formal charge. If there is a negative formal charge, the structure with the negative charge on the more electronegative atom will be the best structure because electronegative atoms can better accommodate the negative charge. If there is a positive formal charge, the structure having the positive charge on the least electronegative atom is best.

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