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Formal charge is the charge assigned to an atom in a molecule. 

What's the point?

  • 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

Structure A:

  • 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

Structure B:

  • 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

Structure A is a better representation of CO₂ because it minimizes formal charge.

Example 2: Fulminic Acid

  • 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. 
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