Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 28: 6 April


Formal Charge, cont.

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.


Lewis Structures for Covalent Molecules-Octet Variations

Multiple Bonds & Resonance

Recall we must show an octet (or duet for Period I) in the outer-most shell (valence electrons). When this does not occur with single electron pairs (bonds) between atoms can sometimes make it happen with multiple bonds. You might find "Clark's Method" useful for determining the bonding patterns of various molecules:

Additional exercises on Lewis Structures are available in the Lewis Structure Module and your laboratory manual

For a modern view of bonding illustrated with QuickTime movies based on quantum calculations you may enjoy the Supplement.

Molecular Geometry

The importance of molecular shape: recognition at the molecular level in organisms. Shape and electron density are extraordinarily important to the interaction of biomolecules - Examples

Lewis Structures enable us to predict bonding patterns for compounds of the representative elements, but how can we predict their shapes? We will add another tool, VSEPR Theory, to our chemical toolbox - a simple way to predict the geometry of bonds around a central atom (for larger molecules predict one center at a time).


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© R A Paselk

Last modified 6 April 2015