RULES FOR RESONANCE
Drawing correct resonance structures:
In drawing resonance structures for a molecule we are only allowed to move electrons. The positions of all nuclei must remain the same.
All of the resonance structures must be proper Lewis structures; for example, we should not write a structure in which carbon has five bonds.
Physical meaning of resonance structures
- A molecule with more than one resonance structure is a hybrid or average of the individual resonance structures. It does not quickly shift back and forth between them.
- A molecule which exists as a mixture of resonance hybrids is more stable (has lower energy) than any one of the individual resonance structures and is often more stable than a molecule which cannot exist as a mixture of resonance structures.
Ranking the relative importance of individual resonance structures
Equivalent resonance structures all contribute equally to the resonance hybrid. Nonequivalent resonance structures do not contribute equally; their relative stability determines how much they contribute. More stable structures contribute more; in other words, the molecule "looks more" like the more stable structure(s).
To evaluate the relative stability of resonance structures:
- Resonance structures of an individual molecule can either be equivalent or nonequivalent. In the group of structures below, A and B are equivalent but C is not equivalent to the other two structures.
First, determine if all atoms in the structure have an octet of electrons. Structures in which all atoms (except hydrogen) have a complete octet are especially stable and make larger contributions to the hybrid. In the example given above in number 5, structure C would contribute the least since the carbon doesn't have an octet. Likewise, for the two structures below the more stable one has octets on both carbon and hydrogen.
- Next, look at the number of atoms which bear a formal charge. Structures which have fewer atoms with formal charges and lower values for these formal charges contribute more. In the group of structures below, structure Z contributes the least
- If structures have the same number of atoms with formal charges, consider the electronegativity of the atom which bears the formal charges; a more electronegative atom will be happier with a negative formal charge, while a less electronegative atom will be happier with a positive formal charge. In the structures above in part b, structure X is a more important contributor than structure Y because the negative charge is on the more electronegative atom (oxygen) in X.
Dr. Peggy Kline / Physical Science Department / Santa Monica College / last updated 8/8/00