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Formal Charge Example Problem

Lewis Structures and Formal Charge

By Todd Helmenstine

This is a resonance Lewis structure for carbon dioxide.

This is a resonance Lewis structure for carbon dioxide.

Todd Helmenstine

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 = eV - eN - eB/2

where
eV = number of valence electrons of the atom as if it were isolated from the molecule
eN = number of unbound valence electrons on the atom in the molecule
eB = 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:

eV for oxygen = 6
eV for carbon = 4

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

eN for O1 = 4
eN for C = 0
eN for O2 = 4

To find eB, 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.

eB for O1 = 2 bonds = 4 electrons
eB for C = 4 bonds = 8 electrons
eB for O2 = 2 bonds = 4 electrons

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

Formal charge of O1 = eV - eN - eB/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 = eV - eN - eB/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 = eV - eN - eB/2
Formal charge of O2 = 6 - 4 - 4/2
Formal charge of O2 = 6 - 4 - 2
Formal charge of O2 = 0

For Structure B:

eN for O1 = 2
eN for C = 0
eN for O2 = 6

Formal charge of O1 = eV - eN - eB/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 = eV - eN - eB/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 = eV - eN - eB/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.

More information about Lewis structures:

Lewis Structures or Electron Dot Structures
How to Draw a Lewis Structure
Exceptions to the Octet Rule
Draw a Lewis Structure of Formaldehyde - Lewis Structure Example Problem
How to Draw a Lewis Structure - Octet Exception Example Problem

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