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.
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.
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 .
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).
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 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.
You see, all drawn three resonance structures are similar because,
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- .
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 also show two resonance structures. But, their stability is same.
You see in both resonance structures, we have marked the negative charge on oxygen atom and positive charge on nitrogen atom.
We can draw three resonance structures for N2O.
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.