how to calculate formal charge of an atom

How to Calculate Formal Charge? Formula & Examples

How to calculate formal charge? A formal charge is a charge present on individual atoms in a polyatomic molecule. It is a fraudulent charge associated with only a single atom of the structure. The real charge of a molecule or compound is distributed throughout the structure of the species, which is why we call it fake.

Formal charge exists because of deficiencies in the configuration of an atom that participates in the compound formation. Calculation of formal charge is the subject of this lesson. From here, we will pick up topics like formal charge formula, how to calculate the formal charge, etc. Formal charge gives us an essential aspect of chemistry.

Formal Charge Formula 

Now that we know a formal charge, we will move on to the formal charge formula. Mathematically, the formal charge formula stands as follows:

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

Since the bond exists between two electrons, we half the value of bonding electrons.

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

How To Calculate Formal Charge Of SO2?

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

how to calculate the formal charge
how to calculate the formal charge

Numbers 1,2,3,4 represent the Oxygen Atom Index

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 the formal charge of SO2. The Lewis structure of a molecule or an ion must be known in order to apply the formula for calculating formal charge.

Importance Of Formal Charge 

Now that we know what the formal charge is and are familiar with calculating a 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. Understanding “what is formal charge” is crucial.
  • The formal charge is crucial in deciding the lowest energy configuration among several possible Lewis structures for the given molecule. Therefore, calculating formal charges becomes essential.
  • Knowing the lowest energy structure is critical in pointing out the immediate product of a reaction. This knowledge is also helpful 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 a formal charge is, we now also know its significance.

Difference Between Formal Charge And Oxidation State 

The concepts of formal charge and oxidation state are often confused. Despite the fact that both of these concepts investigate electron distribution, their perspectives differ, and therefore, the results are different. It is essential to keep in mind the subtle difference between these concepts. In the case of formal charge, we assume that electrons present in a bond are equally distributed between both atoms.

Thus, following this concept and the formula which arises from it, we come to a value known as a formal charge. Nevertheless, for the oxidation state, we examine the differences in electronegativity between 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 

  • Convention governs that formal charge is essential for depicting a complete and correct Lewis-Kekulé structure in organic chemistry. Inorganic chemistry, however, does not follow the same rules.
  • The structure variation of a molecule with 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.

FAQs

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

Formal definitions of charge involve stating that it is a theoretical charge present on individual atoms of a polyatomic molecule. The formal charge on an atom exists because of an unfulfilled orbital configuration. Since the physical charge on a molecule or ion is distributed throughout its structure, it is only theoretical and considered fake.

However, its calculation is pretty essential as it leads us to make several predictions. The lower the charge on a specific 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 primary product of a reaction. We can also justify many phenomena using the concept of formal charges.

How to calculate formal charges?

Formal charge calculations utilize the principle that the electrons in a bond are equally distributed between the two atoms involved.

Therefore, for each atom, we find the valence electrons present in their free state, the number of electrons 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.

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