How to calculate formal charge on resonance structure

  In this post,I am going to show you how to calculate formal charge of each atom from a resonance structure.

A molecule or ion can have more than one Lewis structure.These structures are called resonance structures of that molecule or ion.The difference among these Lewis structures is only the arrangement of electrons.

To find the formal charge of a atom from resonance structure,you need to apply the following formal charge formula or equation:

●Formal charge = Valence electrons in neutral atom - 1/2 (Bonding Electrons) - Nonbonding electrons

To use the above formal charge formula to a resonance structure,at first you need to calculate the number of valence electrons in a free atom,then you need to calculate the number of nonbonding electrons and at last you need to consider the half of the bonding electrons.

Here is an example of resonance structure of CO2 that will help you to understand how to calculate the formal charge of an an using formal charge formula.

These are the Lew is structures of CO2.These structures are called resonance structures of CO2.

Consider the best resonance structure of CO2 molecule.Here you need to calculate the formal charge of oxygen atom and carbon atom.

One of the resonance structure of CO2

Formal charge of O atom in CO2 resonance structure:

●The number of valence electrons in a neutral oxygen atom is 6 as it is the element of group VIA.

●Nonbonding electrons of O is 4.

●Bonding electrons of O is 4.

●Formal charge of O = Valence electrons in neutral O atom - 1/2 (Bonding Electrons of O) -Nonbonding elctrons of O

               =6  - 1/2(4) - 4

                =6 -2 -4

                  =6-6

                   =0

So,the formal charge of O atom in CO2 best Lewis structure is 0.Since the second O atom is in the same electronic environment,it's formal charge also will be zero. 

Formal charge of C atom in CO2 resonance structure:

●The number of valence electrons in a neutral carbon atom is 4 as it is the element of group IVA.

●Nonbonding electrons of C is 0.

●Bonding electrons of C is 8.

●Formal charge of C = Valence electrons in neutral C atom - 1/2 (Bonding Electrons of C) - Nonbonding elctrons of C

               =4 - 1/2(8) - 0

                =4-4

                  =0

So,the formal charge of C atom in CO2 best Lewis structure is 0.

Now if you add  the formal charge of each atom,you will get (0+0+0=0) zero.This means that CO2'overall charge is zero.

Here is the another formula for calculating the formal charge of atoms from a Lewis structure or resonance structure..

Formal Charge=Valence Electrons -Dots - Lines

Formal charge of O atom in CO2:

●The number of valence electrons in a neutral oxygen atom is 6 as it is the element of group VIA.

●Dots electrons of O is 4.

●Lines or bonds of O is 2.

●Formal charge of O = Valence electrons in neutral O atom -Dots - Lines

= 6 - 4 - 2

= 6 -6

=0

So,the formal charge of O atom in CO2 best Lewis structure is 0.Since the second O atom is in the same electronic environment,it's formal charge also will be zero. 

Formal charge of C atom in CO2:

●The number of valence electrons in a neutral carbon atom is 4 as it is the element of group IVA.

●Dots electrons of C is 0.

●Lines or bonds of C is 4.

●Formal charge of C = Valence electrons in neutral C atom -Dots - Lines

= 4 - 0 - 4

= 4 -4

=0