Formal Charges and Resonance in Lewis Structures

For Students:
Today we are going to discuss two important features of Lewis dot structures.  The first is called the formal charge.  The formal charge is given to an individual atom in a molecule or polyatomic ion.

The sum of the formal charges for the atoms in a species equals the overall charge on the ion or is zero (for an uncharged molecule).  The formal charge for an atom is calculated based on the Lewis structure.
Formal charge = group number of atom - (LPE + 1/2(BE))
In this equation LPE stands for the number of lone pair electrons on an atom and BE is the number of bonding electrons around the atom.
So in the above example for the top oxygen on the left molecule, the formal charge = 6 (group number) - (2 (lone pair) + 1/2(6) (bonding electrons))----> which is 6 - (2+3) = 1.  That is why there is a + above the atom.

Next, what happens when you come up with two different ways to write a Lewis structure, and both ways are correct?

This is called resonance, and was first proposed by Linus Pauling.  Resonance structures are used to represent bonding in a molecule or ion when a single Lewis structure fails to describe accurately the actual electronic structure.  Resonance structures differ only in the assignment of electron-pair positions, never atom positions.  Also, there will always be at least one multiple bond in each resonance structure.

For Parents:
We will be having another exam in 3 weeks.  Be sure that your child understand all the material we will be going over.  This is the section of the course that gets difficult and I am more than happy to help anyone who needs it.  Thanks!