Resonance structures
Resonance structures
It is often
observed that a single Lewis structure is inadequate for the representation of
a molecule in conformity with its experimentally determined parameters. For example,
the ozone, O3 molecule can be equally represented by the structures I and II
shown below:
In both
structures we have a O-O single bond and a O=O double bond. The normal O-O and
O=O bond lengths are 148 pm and 121 pm respectively. Experimentally determined
oxygen-oxygen bond lengths in the O3 molecules are same (128 pm). Thus the
oxygen-oxygen bonds in the O3 molecule are intermediate between a double and a
single bond. Obviously, this cannot be represented by either of the two Lewis
structures shown above.
The concept
of resonance was introduced to deal with the type of difficulty experienced in
the depiction of accurate structures of molecules like O3. According to the concept
of resonance, whenever a single Lewis structure cannot describe a molecule
accurately, a number of structures with similar energy, positions of nuclei,
bonding and non-bonding pairs of electrons are taken as the canonical
structures of the hybrid which describes the molecule accurately. Thus for O3,
the two structures shown above constitute the canonical structures or resonance
structures and their hybrid i.e., the III structure represents the structure of O3 more accurately. This is also called resonance hybrid. Resonance is
represented by a double headed arrow.
Some of the
other examples of resonance structures are provided by the carbonate ion and
the carbon dioxide molecule.
In general, it may be stated that
- Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single canonical structure; and,
- Resonance averages the bond characteristics as a whole.
Thus the
energy of the O3 resonance hybrid is lower than either of the two canonical
forms I and II.
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