Formal Charge

If you are taking up one of your advanced chemistry courses that focus on molecular bonding, you will need to know what is formal charge. It is a concept that is extensively used to describe the electronic structure of atoms bonded in molecules.

Understanding molecular structure is all about grasping the electronic distribution within the molecule. For a student of chemistry, it is important that he can visualize the molecular structure. For this purpose, the chemist Gilbert Newton Lewis introduced what are known as Lewis structures, that are pictorial representations of molecular structure.

It is a pictorial representation of bonds and lone pairs in a molecule, drawn using dots and lines. I am sure that you are already familiar with Lewis structures, if you have reached to this point in your course work. Formal charge as a concept, is used to understand Lewis structures and is used to determine the right Lewis structure for any molecule.

What is Formal Charge?

It is the number that determines how stable a Lewis structure will be. Formal charge of any atom is a number obtained by subtracting half of the covalently bonded or shared electrons and non-bonded electrons, from the total number of valence electrons (these are electrons in the outermost orbit of any atom). There are more than one possible Lewis structures for any molecule and it is only the formal charge that enables you to select the best Lewis structure.

It is defined in such a way, that it enables you to determine the lowest energy configuration for every atom in a molecule and hence, the lowest energy configuration for an entire molecule. While calculating formal charge, the difference in electronegativity of atoms, which results into unequal electron pair sharing is ignored. It is assumed that electrons in covalent bonds are shared equally by each of the constituent atoms. For the right Lewis structure, the sum of formal charges for each atom is the least possible one.

Equation

Formal Charge of an Atom = [V - N - (B/2)]

Here V is the number of valence electrons of an atom, N is the number of electrons that are not part of a bond and B is the number of shared electrons of the atom (that are part of covalent bonds formed by the atom). Formal charge helps you to choose the most stable Lewis structure

Calculation
Consider the NO₂ molecule. Let us calculate the formal charge for the lone Nitrogen atom in this molecule. For Nitrogen atom, the atomic number is known to be 7. Out of these electrons, the number of valence electrons is 5, number of non-bonded electrons is 2 and the number of electrons that have been shared with Oxygen to form covalent bonds with are 6.

Formal Charge of Nitrogen in NO₂ = 5 - 2 - (6/2) = 0

A molecular configuration, for which the sum of formal charges of each atom is the lowest, is the most stable structure. Work out some examples, to understand it better. It is an essential tool in understanding Lewis structures of molecules, as it identifies the electron rich and electron deficient areas in the molecule. Of all the Lewis structures that can be drawn, the best fit is the one which has the least formal charge, indicating a low energy configuration, which is preferred by every molecule.