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.

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 following formula:

F.C. = [Total no. of valence e in free state] – [total no. of e assigned in Lewis structure]
F.C. = [Total no. of valence e in 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 SO42-:

The Lewis structure of SO42- is as follows:

Formal Charge

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

Atom

Valence ein free state No. of non-bonding e in Lewis structure No. of bonding pairs in Lewis structure Formal Charge
Sulphur (S) 6 0 12 = 6 – 0 – 12/2

=6-6

=0

Oxygen (O) – 1 6 4 4 = 6 – 4 – 4/2

= 6 -6

= 0

Oxygen (O) – 2 6 6 2 = 6 -6 -2/2

= 6 -7

= -1

Oxygen(O) – 3 6 4 4 = 6 – 4 – 4/2

= 6 -6

= 0

Oxygen(O) – 4 6 6 2 = 6 -6 -2/2

= 6 -7

= -1

 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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Practise This Question

What is the formal charge on the starred oxygen in the carbonate ion? [O  O||C  O]2