1. Define the formal charge of an atom in a molecule.
2. Determine the formal charges of atoms in a Lewis structure.

Formal Charges on Lewis Structures. The concept of formal charge provides a rational basis for choosing the more plausible Lewis structure from among several possibilities. The formal charge is the charge that an atom seems to have in a Lewis structure. When determining the formal charge all nonbonding electrons count as belonging entirely to the atom in which they are found. All bonding electrons are divided equally between the bonded atoms. Thus the formal charge of an atom in a Lewis structure is the number of valence electrons in an isolated atom minus the number of electrons assigned to that atom in a molecule. The formula for the formal charge of an atom is:

It will be good to keep in mind that the formal charge is really more a property of a structural formula than that of the species the formula represents. Formal charges do not indicate actual charges on atoms in the real molecule.

Two possible Lewis structures for BF₃ are

and

The formal charges in (1) are:

The boron atom:	formal charge = 3 – 0 – 1/2 (6) = 0
The fluorine atom:	formal charge = 7 – 6 – 1/2 (2) = 0

The formal charges in (2) are:

The boron atom:	formal charge = 3 – 0 – 1/2 (8) = –1
The fluorine atom (double bonded):	formal charge = 7 – 4 – 1/2 (4) = +1
The other fluorine atoms:	formal charge = 7 – 6 – 1/2 (2) = 0

The rule that is used to establish the more plausible structure is: A Lewis structure in which there are no formal charges is preferred over one where formal charges are present. Therefore structure (1) is preferred over structure (2).

When formal charges are assigned in a Lewis structure, the sum of the formal charges must be zero in a neutral molecule. For a polyatomic ion the formal charges must add up to the charge of the ion. For the chlorite ion (ClO₂^-), for instance:

the sum of the formal charges is –1:

The example brings out another feature of the formal charge concept. The most plausible Lewis structures will be those with negative formal charges on the more electronegative atoms. Since oxygen is more electronegative than chlorine, the formal negative charges on O atoms in this structure are more reasonable than in some other structure that would place a positive charge on the O atoms, and a negative charge on the Cl atoms.

Assign formal charges to the atoms in the following Lewis structures:

1. 

   The formal charge of C is -2 -1 0 +1 +2
   The formal charge of O is -2 -1 0 +1 +2

          Correct!

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2. 

   The formal charge of S is -2 -1 0 +1 +2
   The formal charge of is -2 -1 0 +1 +2
   The formal charge of is -2 -1 0 +1 +2

          Correct!

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1. The carbon atom: formal charge = 4 – 2 – 1/2 (6) = –1
   The oxygen atom: formal charge = 6 – 2 – 1/2 (6) = +1
   
   
2. The sulfur atom: formal charge = 6 – 2 – 1/2 (6) = +1
   The oxygen atom on the left: formal charge = 6 – 4 – 1/2 (4) = 0
   The oxygen atom on the right: formal charge = 6 – 6 – 1/2 (2) = –1

Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.

1. Assign formal charges to the atoms in the following Lewis structures:
   1. 
   2. 
2. Assign formal charges to the atoms in the following possible Lewis structures.
   1. 
   2.