If you are looking for guidelines on calculating the formal charge of an atom in a molecule, this article has what you are looking for. Here, you will find a clear explanation and an illustrative example of the same.
What I like about chemistry is the amazing amount of detective work that is involved in determining the actual structure of a molecule. You need to be adept at experimenting and theorizing to be a chemist. A large part of chemistry study is about learning to identify and visualize molecules. Molecular nomenclatures are developed to accurately describe the structural details and expanse of any molecule. Another pictorial technique for depicting the electronic distribution and bonding of a molecule is a Lewis structure.
Atoms combine together to form molecules by sharing electrons (forming a covalent bond) or by exchanging electrons (through an ionic bond). Nature prefers low energy states which lead to stable molecular structures. When determining the exact molecular structure, one needs to know how the molecules have bonded together.
This necessitates that a chemist knows the low energy electronic configuration for every one of the bonding atoms. Formal charge calculation enables you to determine this. It is the difference between the valence electron number of the atoms and the combined sum of bonded and non-bonded electrons in an atom.
For calculating the formal charge of an atom in any compound, you need to know what is the bonding structure of the compound. You can picture the chemical bond using a Lewis structure diagram of the compound. Here is the formula:
Formal Charge = [V – N – (B/2)]
In this formula, V stands for the number of valence electrons of that atom (these are the electrons that revolve in the outermost orbit of the atom), N stands for the number of non-bonded electrons, and B stands for the number of electrons that are a part of the covalent bonds made by the atom.
To be able to decide which is the most stable Lewis structure for any compound, one must calculate the formal charge of every atom involved in the compound and sum up all the values. The right Lewis structure will be the one for which the sum of formal charges has a minimum value.
Let me now illustrate the calculation for a carbon atom in methane molecule (CH4). Here, carbon is bonded with four hydrogen atoms. The number of valence electrons of carbon are 4. The number of electrons in non-bonded state are 2, while the ones in the covalent bond are 8. All that you have to do is substitute these values in the above formula.
Formal Charge of carbon Atom in CH4 = 6 – 2 – (8/2) = 0
Thus, the carbon atom in methane has zero formal charge. Not surprisingly the value for hydrogen atoms too, turns out to be zero in methane molecule, which makes this compound structure to be highly stable.
As explained before, calculation of this parameter for all the atoms in a molecule will let you select the lowest energy configuration and also let you choose the best Lewis structure in the process. It is an important tool in the theoretical chemist’s arsenal when studying the structure of molecules.