The Lewis structure for certain molecules or ions can be drawn in more than one way. For example, for NO2- the number of valence electrons is 5 + 2 (6) + 1 = 18 e- (or 9 pairs), and we find that there are two equally valid Lewis structures that can be drawn:
Which one is correct? Well, you would expect that the doubly bonded oxygen-nitrogen distance to be slightly less than the singly bonded distance. Actually, what is found experimentally is that both N-O distances are equivalent. The true structure of the molecule is a combination of the two. Anytime you have more than one valid structure for a molecule or ion, you have what are known as resonance structures. So in this case, both resonances structures contribute equally to the final structure of the molecule. Sometimes you will have multiple resonance structures which do not contribute equally to the final structure of the molecule. In these cases it can be helpful to know which structure has the greatest contribution to the final structure. If you have many possible resonance forms, you choose the most likely resonance form by calculating the formal charge on each atom in each resonance form. In these situations it is helpful to calculate the formal charge on each atom in each possible resonance structure, and use the formal charges to determine the most representative structure.
Formal charge = Group number - number of nonbonding e-- (number of bonding e-) / 2.
In the example below, we calculate the formal charge on each atom in a Lewis structure.
What are the formal charges on each atom in NO2-?
The sum of the formal charges must equal that of the compund or ion. So, 0 + 0 - 1 = -1 as expected for NO2-.
To use the formal charge to determine most representative resonance forms we follow:
Rules for determining most representative resonance form
- Resonance forms with the least number of atoms with non-zero formal charge are preferred.
- Resonance forms with low formal charges are favored over high formal charge. (e.g., ±1 is favored over ±2).
- Resonance forms with negative formal charge or most electronegative atoms are favored.
- Resonance forms with the same sign charge on adjacent atoms are not favored.
For example, N2O has number of 2 ( 5 ) + 6 = 16 valence electrons or 8 pairs. We can draw the three valid Lewis structures below, labeled A, B, and C:
For each structure we can calculate the formal charges below on each atom:
| A | B | C | |
|---|---|---|---|
| N1 | 5 - 2 - ( 6 ) / 2 = 0 | 5 - 4 - ( 4 ) / 2 = - 1 | 5 - 6 - ( 2 ) / 2 = -2 |
| N2 | 5 - 0 - ( 8 ) / 2 = + 1 | 5 - 0 - ( 8 ) / 2 = + 1 | 5 - 0 - ( 8 ) / 2 = + 1 |
| O | 6 - 6 - ( 2 ) / 2 = - 1 | 6 - 4 - ( 4 ) / 2 = 0 | 6 - 2 - ( 6 ) / 2 = + 1 |
Examining the formal charges above we see that Formula C is less representative because it has a -2 charge, and formula B is less representative because it has a -1 charge on N and 0 charge on O. Oxygen is more electronegative and should get -1 charge, therefore Formula A is most representative.
Homework from Chemisty, The Central Science, 10th Ed.
8.45, 8.47, 8.49, 8.51, 8.53, 8.55, 8.57, 8.59, 8.61, 8.63