Structure and Bonding: Discovering Resonance

Draw the Lewis dot structure of the carbonate ion, CO₃^-2.  Remember to add two electrons for the charge on the ion.

Is there any reason why the double bond in the carbonate ion cannot be located at one of the other oxygens?

From a statistical point of view, the double bond has a 0.33 chance (1 out of 3) to be located at any one of the oxygens.  So you have three possible structures for the carbonate ion.  These are called resonance structures.

At any time, can we isolate and find a double bond?  Here is the structure of the carbonate ion.  Measure the carbon-to-oxygen bond lengths (right click, click on select, then mouse click action, then click on distance).  Consecutively click on the two atoms in the bond.  Distance is shown on the lower left corner of your browser screen.  For the distance in picometers (pm):  pm = distance x 100. Explain what you find.

What happened to the double bond?  Measure and record in picometers the carbon-to-oxygen bond lengths in the three molecules given below, which illustrate the three different bond orders (BO). 

CO                                    CO₂                                 CH₃OH
triple bond                        double bond                        single bond

BO = 3                            BO = 2                                 BO = 1

_________ pm                         _________ pm                      _________ pm

How are bond order and bond length or distance related?

Measure the carbon-to-oxygen bond lengths in the three structures below.  Record in the table given below.  How do the recorded bond lengths compare to the typical single and double bond length of the molecules above?

H₂CO₃                                HCO₃^-1                                       CO₃^-2

What do you conclude about the bond orders in carbonic acid?

in the bicarbonate ion?

in the carbonate ion?

Other than in carbonic acid, a carbon-to-oxygen double bond does not occur in the two ions.  Resonance structures are formed, where the double bond is spread over the two bonds in the bicarbonate ion (bond order = ~1.5) and over the three bonds in the carbonate ion (bond order = ~1.3).  The bond length measurements show this as they are intermediate between a single and double bond length.  Resonance structures are important in many molecules and polyatomic ions.

Draw the Lewis dot structure for the nitrate ion and describe what the bond lengths will be.

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