How To Find Formal Charge in Lewis Structure

In chemistry, the Lewis structure is a diagrammatic representation of electrons and atoms in a molecule. It is named after Gilbert N. Lewis, who introduced the concept in 1916. The valence electrons and the chemical bonding between atoms can be clearly depicted using Lewis structures.

Formal charge is the charge that an atom has after it is bonded in the molecule. It determines the distribution of charges and helps in the prediction of relative stability. Knowing how to calculate formal charge is vital in understanding chemical reactions and predicting their outcomes. In this article, we will discuss various methods of finding formal charge in Lewis structures.

Method 1: Counting electrons

The first method to determine formal charge is to count the valence electrons of an atom. For main group elements, the valence electrons are the outermost electrons on the atomic shell, which are involved in chemical bonding.

To calculate the formal charge of an atom using this method, use the following formula:

Formal Charge = Valence electrons – Non-bonded electrons – Half of the bonded electrons

Here, non-bonded electrons are the electrons that are not involved in forming bonds in the molecule. Bonded electrons are the electrons that are shared between atoms in a chemical bond.

For example, let us take the example of water molecule (H2O) to calculate the formal charge of oxygen.

The Lewis structure shows the two hydrogen atoms sharing their valence electrons with the oxygen atom through covalent bonds. The lone pairs on the oxygen atom comprise the non-bonded electrons. To calculate the formal charge of the oxygen atom, we will use the formula as follows:

Valence electrons of oxygen = 6
Non-bonded electrons of oxygen = 4
Bonded electrons of oxygen = 4 (two shared with each hydrogen atom)
Formal charge of Oxygen = 6 – 4 – 2 = 0

Therefore, the formal charge of oxygen is zero, which indicates that it is not carrying any charge in the water molecule.

Method 2: Drawing resonance structures

Resonance structures are different configurations of the same molecule, and they occur due to the delocalization of electrons. These structures play a significant role in determining the overall distribution of charge in the molecule.

To calculate the formal charge of atoms in resonance structures, follow these steps:

Step 1: Draw the resonance structure(s) of the molecule.

Step 2: Count the electrons of each atom in each resonance structure.

Step 3: Calculate the average of the formal charges of the atoms in all resonance structures.

Let us take the example of ozone (O3) molecule to illustrate this method.

The Lewis structure shows the ozone molecule with a central oxygen atom bonded to two other oxygen atoms. We can draw the following resonance structures for ozone:

In the resonance structures, the electrons are delocalized between the oxygen atoms. The atoms that are negatively charged in one structure become positively charged in the other, and vice versa. To find the formal charge of the central oxygen atom in each structure:

Resonance structure 1: Formal charge (central oxygen atom) = 6 – 4 – 2(2/3) = 0

Resonance structure 2: Formal charge (central oxygen atom) = 6 – 4 – 4(1/3) = 0

Step 3: Average of formal charges = (0 + 0)/2 = 0

Therefore, the formal charge of the central oxygen atom in ozone molecule is zero.

Method 3: Using electronegativity difference

The electronegativity of an atom is the measure of its ability to attract electrons towards itself in a chemical bond. The electronegativity difference between the bonded atoms helps to predict the polarity of the bond and the distribution of electrons.

To calculate the formal charge of an atom using electronegativity difference, follow these steps:

Step 1: Find the electronegativity difference between the atom and the bonded atoms.

Step 2: Divide the difference by two to find the shared electrons.

Step 3: Assign all non-bonded electrons to the atom.

Step 4: Calculate the formal charge using the formula:

Formal charge = Valence electrons – Shared electrons – Non-bonded electrons

Let us take the example of the carbon dioxide molecule (CO2) to illustrate this method.

The Lewis structure shows the carbon dioxide molecule with two oxygen atoms bonded to the central carbon atom. To find the formal charge of the carbon atom in CO2 using electronegativity difference:

Step 1: The electronegativity difference between carbon and oxygen in CO2 is 3.5 – 2.5 = 1.

Step 2: The shared electrons are half of the electronegativity difference = 0.5.

Step 3: The carbon atom has no non-bonded electrons.

Step 4: Formal charge of carbon = 4 – 0.5 – 0 = 3.5

Therefore, the formal charge of the carbon atom in CO2 is +1.5.

FAQs (Frequently Asked Questions)

Q1: What is formal charge in Lewis structures?

A1: Formal charge is the difference between the valence electrons and the electrons that an atom has in a molecule. It is used to determine the distribution of charges in a molecule and helps in predicting the relative stability.

Q2: What is the significance of formal charge in chemistry?

A2: Formal charge helps in predicting the relative stability of molecules, determining the distribution of charges in molecules, and understanding chemical reactions.

Q3: What are the different methods to find formal charge in Lewis structures?

A3: The different methods to find formal charge in Lewis structures are counting electrons, drawing resonance structures, and using electronegativity difference.

Q4: Why are resonance structures important in determining formal charge?

A4: Resonance structures represent the multiple configurations of a molecule, due to electron delocalization. Calculating the formal charge in all the resonance structures helps in determining the overall charge distribution in the molecule.

Q5: What is the role of electronegativity difference in finding formal charge?

A5: Electronegativity difference helps to determine the sharing of electrons in a chemical bond and assign formal charges to the atoms participating in the bond.