Formal Charge

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Formal charge is a way of counting electrons involved in bonding. There are some simple rules:

1) Any nonbonding electrons associated with an atom are counted as "belonging" to that atom.
2) The electrons in a bond are assigned half and half to the two atoms in the bond.

That's it for the rules. Now, how do you determine if a given atom has formal charge?

Add the two values (from above) together. Compare that to the number of valence electrons the atom has in its neutral, unbonded state.

For example, nitrogen has five electrons in its neutral state. Suppose we applied the rules to a nitrogen in a molecule and we came up with 4. That means that the formal charge on the nitrogen is +1.

Suppose we had an oxygen and it had seven electrons associated with it. Then the oxygen would have a formal charge of -1.

Resonance structures often have formal charges associated with them. Here is NO2 and ozone, O3:

In the image just below, examine the right hand oxygen. It has three nonbonding pairs, so this counts for six electrons. Then oxygen gets one of the two electrons in the bond with nitrogen. That's electron #7, giving the O a minus one formal charge.

The other O has two nonbonding pairs for 4 electrons and it also gets 2 of the four electrons in the double bond. That's six electrons, the the left hand O has no formal charge.

The nitrogen gets two electrons for the double bond, one from the single bond and one from the half-filled orbital associated with it. That's a total of 4 and gives N a +1 formal charge since itneeds 5 to be neutral.

The Rules for Writing Lewis Structures has some discussion of formal charge in it.

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