What is Formal Charge?

An Atom's Formal Charge

• Sometimes, compounds can have multiple structures. 
• Generally, the structure that minimizes formal charge is the best structure to represent the actual molecule because it costs the lowest energy. 
• That being said, you don't have to worry about formal charge on the exam unless a question specifically asks you about you. 

How is it calculated?

• Formal charge is equal to (valence electrons) - (lone pair electrons + bonds)

Example 1: CO₂

• Formal charge for O1: (6 valence electrons) - (4 lone pair electrons + 2 bonds) = 0
• Formal charge for C: (4 valence electrons) - (0 lone pair electrons + 4 bonds) = 0
• Formal charge for O2: (6 valence electrons) - (4 lone pair electrons + 2 bonds) = 0

• Formal charge for O1: (6 valence electrons) - (2 lone pair electrons + 3 bonds) = +1
• Formal charge for C: (4 valence electrons) - (0 lone pair electrons + 4 bonds) = 0
• Formal charge for O2: (6 valence electrons) - (6 lone pair electrons + 1 bonds) = -1

Example 2: Fulminic Acid

• From question 2 on the 2017 free response:

• And here are the scoring guidelines:

• You'll notice that both structures have an atom with a formal charge of -1. 
• When this happens, the structure with the charge on the more electronegative atom is the better representation.