1. Formal Charge
When multiple resonance structures (more than one valid Lewis structure) are possible, formal charge is used to determine the most likely structure

2. Formal Charge
Gives an indication of the extent to which atoms have gained or lost electrons in the process of covalent bond formation
Structures with lowest formal charges are likely to have the lowest energy
∴ the most likely structure…
For nonmetal central atoms, Lewis structures that minimize formal charge (0) give the best prediction.

3. FC = #valence e-s - #non-bonding e-s – ½ #bonding e-s
Formal Charge
To determine Formal Charge:
FC = #valence e-s - #non-bonding e-s – ½ #bonding e-s

4. FC = #valence e-s - #non-bonding e-s – ½ #bonding e-s
example 1: CO2

5. FC = #valence e-s - #non-bonding e-s – ½ #bonding e-s
example 2: SiCl4

6. FC = #valence e-s - #non-bonding e-s – ½ #bonding e-s
example 3: CO32-