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 ElectronsLet’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 312. 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. StepByStep MethodNow 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 CCarbon (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 4NE equals 0BE equals 8We now substitute the values, and we have:formal charge = 4 – 0 – 8/2 = 0The formal charge of carbon is 0. Step 2: Calculate the Formal Charge of Oxygen on the LeftOxygen (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 6NE equals 4BE equals 4We now substitute the values, and we have:formal charge = 6 – 4 – 4/2 = 0The formal charge of oxygen (left) is 0. Step 3: Calculate the Formal Charge of Oxygen on the RightOxygen (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 6NE equals 4BE equals 4We now substitute the values, and we have:6 – 4 – 4/2 = 0The formal charge of oxygen (on the right) equals 0. Step 4: Check the Overall ChargeNow, we add all of the formal charges of the atoms in the carbon dioxide molecule. The fact that carbon dioxide does not have a superscript that pertains to charge means that the overall charge should be equal to zero. Let’s double check. All the formal charges are zero. If we add them all, the sum will be equal to zero. We know that carbon dioxide is a neutral molecule, which means its overall charge is zero. We have just confirmed this by calculating all of the formal charges together and getting a sum which is equal to zero. The values agree, which means we did it correctly. ExampleConsider another example. Let us look at the negatively charged nitrate ion. The negative sign on the upper right side of the Lewis structure means that its overall charge is 1. Let’s find the formal charges of the nitrogen and the three oxygens. We’ll label each oxygen with a number in red. Oxygen 1: Oxygen (O) is in group 16, so it has 6 valence electrons (VE). There are 2 lines attached to it, so it has 4 bonding electrons (BE), and there are 4 dots around it, so there are 4 nonbonding electrons (NE). Oxygen 2: Oxygen (O) is in group 16, so it has 6 valence electrons (VE). There is 1 line attached to it, so it has 2 bonding electrons (BE), and there are 6 dots around it, so there are 6 nonbonding electrons (NE). Oxygen 3: Oxygen (O) is in group 16, so it has 6 valence electrons (VE). There is 1 line attached to it, so it has 2 bonding electrons (BE), and there are 6 dots around it, so there are 6 nonbonding electrons (NE). Overall Charge: Now, we add all of the formal charges:0 + (+1) + (1) + (1) = 1The overall charge is equal to 1. This agrees with the correct overall charge of nitrate ion. This gives us a good hint if our calculations are correct. Lesson SummaryLet’s review. The formal charge calculates the individual charge of an atom in a molecule. This indicates whether or not the atom has a positive, negative, or neutral charge. The formal charge is determined by the valence electrons (VE), nonbonding electrons (NE), and bonding electrons (BE) in the following formula: By calculating the formal charge of each atom in a molecule, we can determine the overall charge of the molecule. Key TermsFormal charge – the charge of an atom in a moleculeLewis Structure – a drawing of a molecule that shows all the bonding and nonbonding electrons Bonding electrons – electrons that are shared between atoms in a moleculeNonbonding electrons – electrons that are not shared between atoms in a moleculeValence electrons – the electrons on the outermost shell of the atom Learning OutcomesIn this lesson, you learned how to:
