What Is Formal Charge

Q

What is Formal Charge of an atom in a molecule? And what is the total formal charge of a molecule?

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A

The formal charge of an atom in a molecule is defined by the formula below:

Fc (Formal charge) of atom = Ve - Ue - Be/2

where: 
Ve = number of valence electrons 
Ue = number of unbonded electrons
Be = number of bonded electrons

The formal charge of a molecule is defined by the formula below:

Formal charge of molecule = Sum of formal charges of all atom

The best way to calculate the formal charge of an atom in a molecule is to present the molecule in the Lewis structure format. The calculation can be divided into 4 steps:

Step 1: Look up "Ve (valence electrons)" from the periodic table for atom.
Step 2: Count dots around the atom as "Ue (unbonded electrons)".
Step 3: count bound lines connected to the atom as "Be (bonded electrons) / 2".
step 4: Calculate "Fc (Formal charge)" as Ve - Ue - Be/2.

Example 1 - Hydrogen Isocyanide: Both Lewis Structure and Skeletal Formula of Hydrogen Isocyanide molecule are shown below:

Lewis Structure and Skeletal Formula of Hydrogen Isocyanide
Lewis Structure and Skeletal Formula of Hydrogen Isocyanide

From the Lewis Structure in the above picture, you can easily calculate the formal charge of each atom in the molecule and the formal charge of the molecule itself:

Fc of H = 1 - 0 - 1 = 0
Fc of N = 5 - 0 - 4 = 1
Fc of C = 4 - 2 - 3 = -1 

Fc of molecule = 0 + 1 + -1 = 0

So for Hydrogen Isocyanide, the entire molecule is ectrically neutral with 0 formal charge. But the N atom has a formal charge of 1, losing 1 electron to the neighbor atom C; which has a formal charge of -1 because of this extra electron.

Atom's formal charge is usually included in the Skeletal Formula as shown in the above picture.

Example 2 - Carbonate Anion: Both Lewis Structure and Skeletal Formula of Carbonate Anion are shown below:

Lewis Structure and Skeletal Formula of Carbonate Anion
Lewis Structure and Skeletal Formula of Carbonate Anion

From the Lewis Structure in the above picture, you can easily calculate the formal charge of each atom in the molecule and the formal charge of the molecule itself:

Fc of C = 4 - 0 - 4 = 0 
Fc of O = 6 - 4 - 2 = 0
Fc of O = 6 - 6 - 1 = -1
Fc of O = 6 - 6 - 1 = -1

Fc of molecule = 0 + 0 + -1 + -1 = -2

So for Carbonate Anion, the entire molecule has a formal charge of -2, coming from two carbon atoms, each of them has a formal charge of -1. Those 2 extra electrons comes from outside of the anion.

 

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2020-05-29, 145👍, 0💬