1. Alix Arungah Sarah Helman Michael Busam Michael Zuzak
Formal Charges
Alix Arungah
Sarah Helman
Michael Busam
Michael Zuzak

2. Definition
Formal charge of any atom in a molecule is the charge the atom would have if all the atoms in the molecule had the same electronegativity
So basically, that means: the charges of the atoms if the molecule was non polar
Why is it important? Because it helps us decide what Lewis structure should be preferred

3. Process Draw the Lewis structure(s)
Determine the valence electrons that each atom in the molecule has
Determine the number of electrons you assign to each atom
All the non-bonding electrons (lone pairs) + half of the shared electrons (so one electron per bond)
Subtract the assigned electrons from the valence electrons for each atom

4. FC = V - (L + 1/2 S) FC= formal charge V= valence electrons
L= lone pairs
S= shared electrons

5. What it tells you
When you have multiple options for a Lewis structure, the one with the lowest formal charge numbers is more accurate
If there’s a situation in which the results are not all zero, the more electronegative atoms should have the more negative charges
Remember: in most cases, the octet rule trumps formal charge.
Finally, the sum of the charges will always be the overall charge of the molecule

6. Example Problem Set Up Electron Table
Pic:

7. Example Problem
Fill out Valence Electrons

8. Example Problem Fill out the Assigned Electrons (each
electron in the lone pairs plus one for
each bond)

9. Example Problem Subtract Assigned electrons from Valence
Electrons to get Formal Charge
Note: This showed process, not the differentiation between structures

10. Example Problem Or you could use the formula
FC = Val e- - (all lone pair e- + half of shared e-)
Nitrogen: FC = 5 – (4 + 2) = -1
Nitrogen: FC = 5 – (0 + 4) = +1
Nitrogen: FC = 5 – (4 + 2) = -1

11. Another Example (8.11)

12. Another Example (8.11)

13. Another Example (8.11)

14. Another Example (8.11) Winner!!!
Structure 2 has the least numbers, and the negative over the more electronegative atom.