Formal Charge And Its Properties

What is a Formal Charge?

A Formal charge is also known as a Fake Charge. It’s a theoretical charge over an individual atom of an ion as the real charge over a polyatomic molecule or ion is distributed on an ion as a whole and not over a single atom. The formal charge of an atom of a polyatomic molecule or ion is defined below.

Table of Contents

• Formal Charge Definition:
• Formal Charge Formula:
• Formula Charge Calculation of SO₄^2-:
• Importance of Formal charge:

Formal Charge Definition:

“The formal charge over an atom of a polyatomic molecule or ion is the difference between the valence electron of that atom in the elemental state and the number of electrons assigned to that atom in Lewis structure.”

Formal Charge Formula:

Mathematically, it can be expressed by the following formula:

F.C. = [Total no. of valence e^– in the free state] – [total no. of e^– assigned in Lewis structure] F.C. = [Total no. of valence e^– in the free state] – [total no. of non-bonding pair e^– (lone pair)] – 1/2 [total no. of bonding e^–]

The factor of ½ is attached to the no. of bonding e^– because bonding e^– is shared between two atoms.

Formula Charge Calculation of SO₄^2-:

The Lewis structure of SO₄^2- is as follows:

1,2,3 and 4 are for the indexing of oxygen while calculating the formal charge.

 Importance of Formal charge:

1. The formal charge being a theoretical charge doesn’t indicate any real charge separation in the molecule.
2. Formal charges help in the selection of the lowest energy structure from a number of possible Lewis structures for a given species.
3. Knowledge of the lowest energy structure helps in predicting the major product of a reaction and also describes a lot of phenomena.
4. Generally, the lowest energy structure is the one with the smallest formal charges on the atoms and the most distributed charge.

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