Formal Charge

There are several rules that help us decide, when we are drawing Lewis Dot Structures, which element is at the center of a molecule. Most of these are not really rules as much as they are suggestions.

• Carbon goes in the middle
• Hydrogen is only the outside
• If you have only one of an element, it goes on the inside
• The element that makes the most bonds goes on the inside
• The least electronegative element goes in the inside

Of course, sometimes these “rules” conflict. For instance in the compound N2O, there is only one oxygen (which should put it in the middle) but nitrogen makes more bonds and is less electronegative. So, how do we decide? We use formal charge.

Formal Charge

Formal charge is a way of looking at a Lewis Dot Structure and determining how “normal” it is. 

Let’s try to make sense of that idea. If we look at a simple Lewis Dot Structure and “break” the bonds in half (so that each element gets ONE of the two electrons in the bond, we can then compare that to the “expected” number of electrons.

Let's look at the Lewis Dot Structures of methane and cyanide.

If we break all of the bonds in methane, as shown below, each hydrogen has one electron. That’s what we expect, so the formal charge is 0. The carbon has 4 electrons, which is also what we expect (counting only valence) electrons, the formal charge is also 0.

In cyanide, however, things are a little different. If we break all of the bonds, as shown below, the carbon has 5 electrons, rather than the normal 4, so the formal charge on the C atom is -1. The nitrogen also has 5 electrons, but that’s normal for nitrogen, so the formal charge is 0.

Note that the sum of the formal charges is the overall charge. That is ALWAYS true.

Now let's look at the two possible arrangements for the atoms in N₂O.

When we break the bonds in the structure on the left, both of the nitrogen atoms have 6 electrons - that’s one more than normal, so the formal charge on each of them is -1. At the same time, the oxygen has only 4 electrons - two less than normal, giving a formal charge of +2.

In the structure on the right, breaking the bonds leaves a different situation. The nitrogen on the left has 6 electrons (formal charge of -1), the nitrogen in the middle has 4 electrons (formal charge of +1) and the oxygen has 6 electrons (formal charge of 0)

We compare those two results by adding the absolute value of the formal charges. For the structure on the left this adds up to 4 (1+2+1), while for the structure on the right, the sum is 2 (1+1+0). Since formal charge is a measure of “how far from normal” something is, this sum tells us that the structure on the right is “more normal.” We interpret that to mean that it will be favored by nature.