Formal Charge (FC) = the charge an atom would have if all bonding pairs were shared equally (polar bonds don't exist in this model).

To assign Formal Charges:

1. Draw a correct Lewis Structure.

2. Assign both electrons of a lone pair to its associated atom.

3. Divide all bonding pairs, giving one electron of each pair to each atom in the bond.

4. Calculate FC = # electrons on the unbonded (elemental) atom - # electrons assigned to the bonded atom.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Clark's Method (abbreviated) for determining bonding in covalent Lewis Structures:

• Add up all of the valence electrons in the structure (remember to add one electron for each negative charge, or subtract one for each positive charge)

  • If e^- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.

  • If e^- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = /2 (remember a triple bond is 2 multiple bonds!).

  • If e^- > 6y + 2 then have an expanded valence shell.

  • If you can draw more than one structure, then chose the most symmetrical.

  • If two or more structures are equally symmetrical, then you probably have resonance and should show all structures connected by double arrows.

© R A Paselk

Last modified 3 April 2015