Lewis structures are diagrams of molecules that represent electron pairs by dots, crosses or lines. They show all of the valence electrons, including lone, or non-bonding pairs.
Occasionally it is possible to draw more than one valid Lewis structure for a particular molecule. The concept of Formal Charge, FC, may be used to decide which structure is the most likely.
Occasionally it may be possible to represent the electronic arrangement of a molecule in more than one way while still fulfilling all of the requirements. The question is, "how do we know which representation is most likely to be correct?"
The easy answer is that we don't!
However, we can use a concept called 'formal charge' (FC) to give us the best chance of making a more likely decision.
The formal charge of an atom in a molecule is the charge that that atom would have if all of the atoms in the molecule had the same electronegativity.
It can be calculated by:
The number of valence electrons - 1/2(the number of bonding electrons) - the number of non-bonding electrons.
The preferred Lewis structure of a molecule is that in which the formal charges are closest to zero.
Formal charge can also be applied to simple molecules and ions, by considering all of the atoms involved.
For example, NH4.
Here the nitrogen atom is bonded to four hydrogen atoms. It has a formal charge of 5-(8/2) = +1. This is (of course) also the actual charge on the ammonium ion, NH4+.
Each hydrogen atom has a formal charge of 1 - (2/2) = 0.
It should be appreciated that the sum of all of the formal charges on the atoms in a species must give the actual charge on the species.
Note that the formal charge gives no actual information about where any charges within a species actually reside. In most cases they are spread out over the species with greater density about atoms of higher electronegativity.
Example: What are the formal charges of the atoms in the carbon monoxide molecule?
The formal charge of the carbon atom is 4 - (6/3) - 2 = -1
The formal charge on the oxygen atom is 6 - (6/3) - 2 = +1
The overall charge on the carbon monoxide molecule is (of course) zero