Rules for drawing
reasonable resonance structures for a given molecule
- All resonance structures must be valid Lewis structures.
- In all possible resonance structures the atomic nuclei must not
change their positions.
- All atoms must not change their hybridization.
- Only electron distribution may be changed.
- All resonance structures must have the same number of unpaired electrons.
- All atoms involved in the resonance (electron sharing) and the atoms directly
bonded to them must lie in (or nearly in) the same plane.
Rules for assessing the relative importance of
resonance structures for a given molecule
- The "true" electronic structure is a superposition of all reasonable
resonance structures. The equivalent structures contribute equally and
different structures contribute according to their relative importance.
- Structures in which first row atoms (B, C, N, O, F) have filled their valence
shells (the octet rule) are generally more important than structures where the octet rule
is not satisfied. However, the non-octet structures may be valid
and contribute significantly, especially if the alternative structures
have formal charges that are inconsistent with the electronegativity differences.
- Ordinarily, the structures with more covalent bonds are more important than those
with fewer such bonds.
- Importance of the structure is decreased by an increase in charge separation.
Structures containing formal charges of more than ±2 on an atom usually contribute
very little. Structures with two like charges on adjacent atoms are especially
- Structures with negative charges assigned to a more electronegative atom are more
important than those in which the charge is on a less electronegative atom.
Similarly positive charges are best carried on atoms of low electronegativity.
- Structures requiring strongly distorted bond angles (or lengths) are not important.