Formal Charge

Formal charge is a charge present on an individual atom(s) of a polyatomic molecule. It is a fraudulent charge that associates with only a single atom of the structure. We call it fake because the real charge of a molecule or compound distributes itself throughout the structure of the species. Formal charge exists because of deficiencies in the configuration of an atom which participates in the compound formation. Formal charge requires calculation, which we will learn hereafter. From here, we will pick up topics like formal charge formula, how to calculate formal charge, etc.  Formal charge gives us an important aspect of chemistry.

Formal Charge Formula 

Now that we know what is formal charge, we will move onto the formal charge formula. Mathematically, the formal charge formula stands as follows:

Formal Charge= Valence Electrons - 0.5Bonding Electrons - Nonbonding Electrons

We half the value of bonding electrons because the bond exists between two electrons.

Now that we know the formal charge formula, we can move onto an example and understand how to calculate formal charge of a polyatomic molecule. 

How To Calculate Formal Charge Of SO2?

Here we will understand how to calculate formal charge of SO2. For calculating formal charge of SO2, we have to unlock its Lewis structure. Therefore, the first step of calculating formal charge is drawing the Lewis structure. The Lewis structure of SO2 is as follows:

Numbers 1,2,3,4 Indicate The Index of the Oxygen Atom.

Atom

Valence electrons in the free state

No. of non-bonding electrons in Lewis structure

No. of pairs of bonding electrons in Lewis structure

Formal Charge

Sulphur (S)

6

0

12

6 – 0 – 12/2 = 0

Oxygen (O) – 1

6

4

4

6 – 4 – 4/2 = 0

Oxygen (O) – 2

6

6

2

6 – 6 – 2/2 = -1

Oxygen (O) – 3

6

4

4

6 – 4 – 4/2 = 0

Oxygen (O) - 4

6

6

2

6 – 6 – 2/2 = -1


The above example shows us how to calculate formal charge of SO2. Therefore, we need to know the Lewis structure of the molecule or ion, and then we apply the formula for calculating formal charge.

Importance Of Formal Charge 

Now that we know what is formal charge and we are familiar with the process for calculating formal charge, we will learn about its importance. 

  • The formal charge is a theoretical concept, useful when studying the molecule minutely. It does not indicate any real charge separation in the molecule. This concept and the knowledge of ‘what is formal charge' is vital.

  • Formal charge is crucial in deciding the lowest energy configuration among several possible Lewis structures for the given molecule. Therefore, calculating formal charge becomes essential.

  • Knowing the lowest energy structure is critical in pointing out the primary product of a reaction. This knowledge is also useful in describing several phenomena.

  • The structure of least energy is usually the one with minimal formal charge and most distributed real charge. 

Besides knowing what is formal charge, we now also know its significance. 

Difference Between Formal Charge And Oxidation State 

Often people confuse the concepts of formal charge and oxidation state. Although both these concepts probe into electron distribution, their perspectives are different, and therefore, the results are different too. It is essential to keep in mind the subtle difference between these concepts. In case of formal charge, we assume that electrons present in a bond equally distribute between both atoms. Thus, following this concept, and the formula which arises from it, we come to a value known as formal charge. However, for the oxidation state, we look into the differences in electronegativity of the two atoms. Thus, the atom having a greater tendency to attract electrons gets an advantage over the bond. Therefore, these concepts are fundamentally different, and one should not mix them up.

Solved Formal Charge Examples 

  1. Calculate the formal charge on the following:

  1. O atoms of O3

  2. Cl atom in HClO4- ion

  3. S atom in HSO4- ion

Ans: 

We are showing how to find formal charge of the species mentioned.

  1. Formal charge on O1: 6 – 6/2 – 2 = +1

Formal charge on O2: 6 – 4/2 – 4 = 0

Formal charge on O3: 6 – 2/2 – 6 = -1


  1. Formal charge on Cl atom of HClO4 ion: 7 – 8/2 – 0 = 3


  1. Formal charge on S atom of HSO4- ion: 6 – 8/2 – 0 = 2

Fun Facts On Formal Charge 

  • In organic chemistry, convention governs that formal charge is essential for depicting a complete and correct Lewis-Kekulé structure. However, the same does not apply to inorganic chemistry.  

  • The structure variation of a molecule having the least amount of charge is the most superior. 

  • The differences between formal charge and oxidation state led to the now widely followed and much more accurate valence bond theory of Slater and the molecular orbital theory of Mulliken.

FAQ (Frequently Asked Questions)

1. Define formal charge. Why is it important to calculate it?

Ans: A method of formal charge definition is to state that it is a theoretical charge present on individual atoms of a polyatomic molecule. The formal charge on an atom exists because of unfulfilled orbital configuration. It is theoretical and considered fake as the real, physical charge on a molecule or ion is distributed throughout its structure. However, its calculation is pretty essential as it leads us to make several predictions. The lesser the formal charge on a particular possible structure of a molecule, the more stable it is. Therefore, it is more likely that this structure will dominate a chemical reaction. In this way, we can predict the major product of a reaction. The concept of formal charges also helps us justify many phenomena.

2. How to calculate formal charges?

Ans: The formal charge calculation is a process that works on the principle that the two atoms participating in a bond equally share the electrons between them. Therefore, for each atom, we find the valence electrons present in their free state, the number of electrons that are not participating in bonding, and the number of electrons present in the bond. After that, we follow the formula: Formal Charge = Valence electrons – No. of non-bonding electrons – ½ (No. of bonding electrons). Hence, if we put the respective values of the variables we will be able to find out the formal charge.