RULES FOR RESONANCE

Drawing correct resonance structures:

  1. In drawing resonance structures for a molecule we are only allowed to move electrons. The positions of all nuclei must remain the same.
  2. 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

  1. 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.
  2. 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

  1. 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.

  2. 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).
  3. To evaluate the relative stability of resonance 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.

    1. 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

    2. 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