What is formal charge?

A formal charge can be defined as the charge which is presenton an individual of apolyatomic molecule, presuming that the electrons in all chemicalbonds are equally distributed between atoms irrespective of relative electronegativity. Formal charge is there due to the deficiencies in the configuration of an atom which are involved in the compound formula. Formal charge is an important aspect of chemistry.

Formal charge formula

The formula of formal charge is:

Formal Charge= Valence Electrons – 0.5Bonding Electrons – Nonbonding Electrons

The value of bonding electrons is divided into half because the bond is present between two electrons.

Where

V is the number of valence electrons

N is the number of non-bonding valence electrons

B is the total number of electrons present in bonds with other atoms in the molecule.

Examples

Co2 is a neutral molecule with 16 total valence electrons. Now there are three different methods to draw the Lewis structure.

It has been noticed that all three structures gave a total charge of zero, but the final structure is the supreme of them all because charge is absolutely nil in the molecule.

Alternative method

The following is similar:

• Now draw a circle around the atom for which the formal charge is needed as with carbon dioxide below.
• Now count the number of electrons in the circle of atom. As the circle divides the covalent bond “in half’, so each covalent bond should be counted as one electron instead of two.
• Now the number of electrons in the circle has to be subtracted from the group number of the element (the Roman numeral and not the IUPAC 1-18 system)in order to find the formal charge

It should be kept in mind that formal charges are only formal meaning that it is only a method for tracking the valence electrons that each atom brings forth when the molecule if formed.

How to calculate Formal Charge of So2?

In order to calculate formal charge of S02, the Lewis structure has to be unlocked. Hence, the step is to draw the Lewis structure. The Lewis structure of S02 is given below.

The above example helped us to calculate formal charge of SO2. Therefore, it is a must to know the Lewis structure of the molecule or ion, and the formula has to be applied to calculate formal charge.

Importance of Formal Charge

The followingare the important points of formal charge.

• The concept of formal charge is theoretical which is useful while doing in-depth study of the molecules. It does not show any actual charge separation in the molecule. This concept is vital and also the knowledge of what formal charge is.
• Formal charge is very essential which decides the configuration of lowest energy among many Lewis structures for the given molecule. Therefore, to calculate formal charge is of utmost importance.
• To know about the lowest energy structure is crucial in finding out the primary product of a reaction. This knowledge is also helpful in describing several other terms.
• The structure of least energy is the one which has the least formal charge and most distributed real charge.

Formal charge compared to oxidation state

Formal charge is a tool to know the distribution of electric charge which is present in a molecule. Whereas oxidation state is a competing method to know the distribution of electrons present in molecules.

Difference between formal charge and oxidation state

People often gets puzzled between the concepts of formal charge and oxidation state. Although both these concepts deal with electron distribution, their views are different and hence, the results are also different. Please note it down to remember the minute difference between these concepts. In formal charge, it is assumed that the electrons which are present in a bond are equally distributed between both the atoms. When this concept and the formula are followed, a value is derived at which is known as formal charge. But for the oxidation state, the differences in electronegativity of the two atoms are considered. So, the atom which attracts more electrons has an advantage over the bond. These concepts are different and should not be jumbled up.

Solved Formal Charge Examples

1. Find out the formal charge of the following.

Answer:

Key points

• In organic chemistry, it is believed that formal charge is needed to show a correct Lewis-Kekule structure. But it does not apply to inorganic chemistry.
• The molecule which has the least amount of charge is the most valuable.
• The differences between formal charge and oxidation state paved the way to valence bond theory of Slater and the molecular orbital theory of Mulliken.