A molecule or ion exhibits resonance when two or more acceptable Lewis diagrams can be drawn to describe the bonding for the same arrangement of atoms. The true distribution of electrons is thought to be an average of all the diagrams.

Nitrite Ion

For the nitrite ion, two Lewis diagrams can be drawn that show alternate positions for one double bond and one single bond. Thus, each bond is the average of the single bond and double bond. To indicate this numerically we average 2 and 1 and say that the nitrite ion has a bond order of 1.5 for each N—O bond. The bond order obtained from the CAChe System PM3 MOPAC calculation is 1.47, which agrees closely with the prediction.

A graphic showing bond order shows the bond colored halfway by each atom color because it is no longer a single bond or a double bond.

Carbonate Ion

The carbonate ion exhibits resonance with three Lewis diagrams. Four electron pairs are distributed over three bonds, giving a predicted bond order of 4/3 or 1.33. The bond order calculated by the CAChe System is 1.27, very close to the predicted value. The double bond is distributed or delocalized approximately in thirds over the three C—O bonds.

A graphic showing bond order shows the bond colored halfway by each atom color because it is no longer a single bond or a double bond.

Bond Lengths

Resonance, or delocalization of the bonding electrons, in carbonate ion is reflected in the bond length. The C—O bond length of 1.29 Å in carbonate ion is intermediate between the C—O single bond length of 1.43Å and the C=O bond length of 1.22 Å.

A graphic showing bond order shows the bond colored halfway by each atom color because it is no longer a single bond or a double bond.