How to Draw Resonance Structures, Rules, Examples, Problems
There are rules to follow drawing resonance structures step by step. For molecules and ions, we can draw several resonance structures and their stability is different from one structure to another structure and you should have the ability to identify stability of each structure.
Rules to draw resonance structures
- Locations of nuclei of atoms should not be changed with structures. Structures are changed only with their electrons
configuration.
- Number of total electron pairs should be same in every structure. Total electron pairs can be simplified as bonds and lone pairs.
Otherwise we can say, number of electron pairs = σ bonds + π bonds + lone pairs. If unpaired electrons exist, they also should be same number in every molecules. (example NO2). - Energy of different structures should be same or close.
- Higher electronegative atoms have more chance to hold negative charges to obtain more stability to the molecule or ion.
As an example, when sulfur and oxygen is in the molecule, oxygen has the higher ability to hold the negative charge
(S2O32-).
Guidelines to follow
To draw all resonance structures, take the lewis structure we drawn by using VESPR rule.
In resonance structures, it does not require to show transformation of electrons by arrows. But, to identify each resonance structures, it is good to show arrows. In following examples, arrows are used to show electrons transformation.
Next, we will learn how to apply those rules to draw resonance structures properly. Lets draw resonance structures of nitrate ion.
Resonance structures of nitrite ion (NO3-)
Nitrogen atom has the greatest possibility to be the middle atom than oxygen atom according to theory of an atom which has a greater valence should be the middle atom. Therefore, three oxygen atoms are located around the nitrogen atom. When we draw the structure of the nitrite ion by following "VSEPR rule". Then it is as the figure 1.a .
Keep octal of oxygen and nitrogen atoms
Remember that, octal of oxygen and nitrogen atoms cannot be exceeded because both oxygen and nitrogen atoms are located in the second period and have only s and p orbitals.
In the nitrite ion, there is a -1 charge. According to the rule number 4, an oxygen atoms should keep negative charges because electronegativity of oxygen is higher than nitrogen. In NO3-, there are two -1 charges on two oxygen atoms and +1 charge on nitrogen atom.
Now we have one structure. Now we try to draw more structures by changing the bonds and lone pairs keeping locations of atoms without changing (rule number 1).
Bond to lone pair, lone pair to bond
A lone pair is converted to a bond. With that, a bond should be converted to a lone pair by keeping the octal of nitrogen and oxygen atoms. So it follows rule number 2 which says number of total electron pairs should be same in every structure.
With conversion of a bond to a lone pair and lone pair to a bond, double bond becomes a single bond and single bond become double bond respectively. Study the resonance structures of NO2- ion (Figure 1.b)
Three resonance structures
Three resonance structures can be drawn. In all resonance structures (common for all drawn resonance structures), two oxygen atoms have -1 charge and nitrogen atom has a +1 charge.
Stability of NO3- resonance structures
You see, all drawn three resonance structures are similar because,
- There are one double bond between nitrogen and oxygen atom and two single bonds nitrogen and oxygen atoms.
- Always there are -1 charges on two oxygen atoms and +1 charge on nitrogen atom.
Therefore all three resonance structures have equal stability. In next examples, you may see, different isomers' stabilities for some other molecules are not same as NO3- .
Sulfur dioxide (SO2) molecule resonance structures
 molecule resonance structures.jpg)
We can draw three resonance structures for SO2 molecule. You can see first two structures have charges on atoms. But in third structure, there are no charges on atoms. So we can understand, structure three is more stable than other two structure.
Nitrogen dioxide (NO2) molecule
 resonance structures.jpg)
Nitrogen dioxide also show two resonance structures. But, their stability is same.
How do you apply the rule, negatively charges should be on most electronegative atoms in this two resonance structures?
You see in both resonance structures, we have marked the negative charge on oxygen atom and positive charge on nitrogen atom.
Resonance structures nitrous oxide (N2O) molecule
 resonance structures.jpg)
We can draw three resonance structures for N2O.
Nitrite ion (NO2-)
Nitrite ion is a -1 charge. So that negative charge should be kept on oxygen atom. Again we see, in most stable structures negative charges should be put on oxygen atoms.
 resonance structures.jpg)
