Finding Formal Charge ( To Determine Major and Minor Contributors)
So now that you know what major and minor contributors are, let's show how to calculate which one of them any given resonance structure is. The first part of doing this is determining the formal charge.
Formal charge = group number of the atom - (lone pair electrons + (1/2) Bonding electrons)
What this basically means is that you go to the periodic table and find whatever atom you are trying to find the formal charge of in the molecule. How many valence electrons does it have? This is your group number. Carbon would be 4. Nitrogen would be 5. Etc.
For the second number in the equation you find it by taking the number of lone pair electrons and adding them to half the number of bonding electrons. First, look at the molecule you have been given. How many things is the atom bonded to? Each bond is two electrons, so (1/2) bonding electrons would essentially give you the same number as the number of bonds the atom has. Because of this, I generally like to just count the bonds and add them to the lone pair electrons.
The lone pair electrons will be drawn on the atom (black dots around it). Just add these up.
Let's calculate the Formal Charge of the Nitrogen in the molecule Pyrrole:
On the periodic table, Nitrogen has 5 valence electrons. This is our group number. Now, let's count the bonds. The Nitrogen here is bonded to the Hydrogen and single-bonded to the Carbons on either side of it. That's three bonds. This means six electrons total, so (1/2) of the six would be three. It also has a lone pair of electrons above it. That's 2 more electrons.
This brings the total for the second number in the equation to 5.
Remembering that the group number is 5, we fill in the numbers of our equation:
Formal Charge = (group number of 5) - (combined bonding electrons and lone pairs adding up to 5)
Formal Charge = (5) - (5)
Formal Charge = 0
In this molecule, the Formal Charge of the Nitrogen is 0, so the Nitrogen does not have a formal charge.