Formal Charge Example Problem

Resonance structures are all the possible Lewis structures for a molecule. Formal charge is a technique to identify which resonance structure is the more correct structure. The most correct Lewis structure will be the structure where the formal charges are evenly distributed throughout the molecule. The sum of all the formal charges should equal the total charge of the molecule.
Formal charge is the difference between the number of valence electrons of each atom and the number of electrons the atom is associated with. The equation takes the form:

• FC = e_V - e_N - e_B/2

where

• e_V = number of valence electrons of the atom as if it were isolated from the molecule
• e_N = number of unbound valence electrons on the atom in the molecule
• e_B = number of electrons shared by the bonds to other atoms in the molecule

The two resonance structures in the above picture are for carbon dioxide, CO₂. To determine which diagram is the correct one, the formal charges for each atom must be calculated.

For Structure A:

• e_V for oxygen = 6
• e_V for carbon = 4

To find e_N, count the number of electron dots around the atom.

• e_N for O₁ = 4
• e_N for C = 0
• e_N for O₂ = 4

To find e_B, count the bonds to the atom. Each bond is formed by two electrons, one donated from each atom involved in the bond. Multiply each bond by two to get the total number of electrons.

• e_B for O₁ = 2 bonds = 4 electrons
• e_B for C = 4 bonds = 8 electrons
• e_B for O₂ = 2 bonds = 4 electrons

Use these three values to calculate the formal charge on each atom.

• Formal charge of O₁ = e_V - e_N - e_B/2
  
• Formal charge of O₁ = 6 - 4 - 4/2
  
• Formal charge of O₁ = 6 - 4 - 2
  
• Formal charge of O₁ = 0
  
• Formal charge of C = e_V - e_N - e_B/2
  
• Formal charge of C₁ = 4 - 0 - 4/2
  
• Formal charge of O₁ = 4 - 0 - 2
  
• Formal charge of O₁ = 0
  
• Formal charge of O₂ = e_V - e_N - e_B/2
  
• Formal charge of O₂ = 6 - 4 - 4/2
  
• Formal charge of O₂ = 6 - 4 - 2
• Formal charge of O₂ = 0

For Structure B:

• e_N for O₁ = 2
  
• e_N for C = 0
  
• e_N for O₂ = 6
  
• Formal charge of O₁ = e_V - e_N - e_B/2
  
• Formal charge of O₁ = 6 - 2 - 6/2
  
• Formal charge of O₁ = 6 - 2 - 3
  
• Formal charge of O₁ = +1
  
• Formal charge of C = e_V - e_N - e_B/2
  
• Formal charge of C₁ = 4 - 0 - 4/2
  
• Formal charge of O₁ = 4 - 0 - 2
  
• Formal charge of O₁ = 0
  
• Formal charge of O₂ = e_V - e_N - e_B/2
  
• Formal charge of O₂ = 6 - 6 - 2/2
  
• Formal charge of O₂ = 6 - 6 - 1
  
• Formal charge of O₂ = -1

All the formal charges on Structure A equal zero, where the formal charges on Structure B show one end is positively charged and the other is negatively charged. Since the overall distribution of Structure A is zero, Structure A is the most correct Lewis structure for CO₂.