Formal Charge

Chemistry: IB Diploma Higher Level, topic 2.3.6

Formal Charge

To accurately choose the correct Lewis structure for a molecule, formal charge is calculated and the structure with the least amount of overall charge is chosen.

$Formal charge equation$

Formal charge equation

When drawing Lewis structures, it is possible that a variety of structures can be drawn based on the particular method you choose to use.
The equation for calculating the formal charge is:
Formal charge = no. valence electrons - ( $\frac {1}{2}$ bonded electrons + lone electrons)

Formal charge variables

Valence electrons can be determined by looking at the location on the periodic table (i.e. group 7 has 7 valence electrons).
½ bonded electrons refers to the dash or shared pair of electrons in the bond.
Use only one electron for each dash or shared pair of electrons since only one electron is contributed from that atom.
Lone electrons refer to the non-bonded electrons on the atom where formal charge is being calculated.

Calculation

A formal charge calculation must be done on each atom in the structure.
For neutral molecules the total charge will add up to zero.
For ions, the charge will add up to the total charge of the ion.

Choosing a structure

The most stable structure is the one with the least amount of charge.
In other words, choose the structure whose formal charge on each atom is zero.
If charge must exist (because it’s an ion) then choose the structure whose negative charge resides on the most electronegative atom or most positive charge resides on the least electronegative atom.

Formal Charge - Example Calculation

Now we have covered formal charge, let's go through an example so you can feel super confident with these questions!

Remember me?

The example we are going to go through is for the famous thiocyanate ion.
The formal charges of the different thiocyanate resonance are shown on the subsequent slides.
Remember the equation for formal charge is:
Formal charge = no. valence electrons - ( $\frac {1}{2}$ bonded electrons + lone electrons)

So, which one's right?

The correct resonance structure is the middle because the formal charge on nitrogen is -1.
Nitrogen is the most electronegative atom in the structure.
Remember we want the structure with the least amount of charge, so it can't be the bottom structure (both the Nitrogen and Sulfur atom have formal charges).

Test yourself on Formal Charge

What's the equation for formal charge?
What would be the total formal charge of a SO₄^2- ion?
For a negative ion, the correct resonance structure will have the formal charge on which atom?
What is the formal charge of the nitrogen atom in the adjacent resonance structure?
What is the formal charge of the carbon atom in the adjacent resonance structure?

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