# Calculating Formal Charge: Definition & Formula

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• 0:00 What is Formal Charge?
• 2:55 Step-By-Step Method
• 6:12 Example
• 8:39 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

It is important to know what the valence electron contribution is from each atom in a molecule. In order to determine this, we calculate the formal charge of the atoms. In this lesson, we will discuss how to determine formal charge.

## What Is Formal Charge?

Whenever we look at our accounts, it is good to know if we are way over budget (negative), just right (neutral), or if we can afford to spend more (positive). It's the same way in the subatomic level when we look at electrons shared between atoms in a molecule. An atom can have the following charges: positive, negative, or neutral, depending on the electron distribution. Just like we can calculate if our account is positive, negative, or balanced, there is a way to calculate an atom's charge in a molecule.

We can determine what the electron distribution in a molecule is by figuring out the formal charge. The formal charge is the charge of an atom in a molecule. By adding all of the formal charges of all of the atoms in the molecule, you can determine if the overall charge of the molecule is positive, negative, or neutral. The way to determine the formal charge is by using the following equation:

#### Lewis Structure

To visualize what the formula says, we'll first look at the Lewis Structure, which is a drawing of the molecule that shows all the bonding and nonbonding electrons. Let us take a look at the example of the carbon dioxide Lewis Structure shown here.

The Lewis structure of carbon dioxide shows that when we say bonding electrons, we need to count the lines on the structure. One line corresponds to two electrons. The nonbonding electrons, on the other hand, are the unshared electrons. These are shown as dots. One dot is equal to one nonbonding electron. The valence electrons are the electrons on the outermost shell of the atom.

#### Valence Electrons

Let's recall a simple way to count valence electrons based on their group number in the periodic table. There is a pattern for counting valence electrons based on the group number an atom has. For example, sodium (Na) is in group 1, therefore, it has 1 valence electron. Oxygen (O) is in group 16, therefore it has 6 valence electrons. Bromine (Br) is in group 17, so it has 7 valence electrons. An exception that we need to take note of is for Helium (He). It is in group 18; however, it has only 2 valence electrons because its maximum number of electrons is 2. All the other noble gases in group 18, such as Neon (Ne), Argon (Ar), and the rest, have 8 valence electrons. We left out groups 3-12. That's because no pattern exists for the valence electrons for these elements, and there is a different and more complex way of counting their valence electrons.

## Step-By-Step Method

Now that we know the formula for determining the formal charge, let's put this into practice and go over how to determine the formal charge for each atom in a molecule step by step. The formula for the formal charge is:

Let us start with something simple, like carbon dioxide. Carbon dioxide has one carbon atom and two oxygen atoms. Its Lewis structure looks like this:

Step 1: Calculate the Formal Charge of C

Carbon (C) is in group 14, so that means it has 4 valence electrons. There are no dots around carbon, so that means it has no nonbonding electrons. There are 4 lines around carbon, and one line is equal to two bonding electrons. Carbon, therefore, has 8 bonding electrons.

VE equals 4
NE equals 0
BE equals 8

We now substitute the values, and we have:

formal charge = 4 - 0 - 8/2 = 0

The formal charge of carbon is 0.

Step 2: Calculate the Formal Charge of Oxygen on the Left

Oxygen (O) is in group 16, so that means it has 6 valence electrons. There are 4 dots around oxygen, so that means it has 4 nonbonding electrons. There are 2 lines attached to oxygen, and one line is equal to two bonding electrons. Oxygen, therefore, has 4 bonding electrons.

VE equals 6
NE equals 4
BE equals 4

We now substitute the values, and we have:

formal charge = 6 - 4 - 4/2 = 0

The formal charge of oxygen (left) is 0.

Step 3: Calculate the Formal Charge of Oxygen on the Right

Oxygen (O) is in group 16, so that means it has 6 valence electrons. There are 4 dots around oxygen, so that means it has 4 nonbonding electrons. There are 2 lines attached to oxygen, and one line is equal to two bonding electrons. Oxygen, therefore, has 4 bonding electrons.

VE equals 6
NE equals 4
BE equals 4

We now substitute the values, and we have:

6 - 4 - 4/2 = 0

The formal charge of oxygen (on the right) equals 0.

Step 4: Check the Overall Charge

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