Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2013

Lecture Notes 27: 10 April

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Covalent Bonds, cont.

Note Lewis Structure homework set in Discussion Manual.

Lewis Structures for Covalent Molecules

Covalent Bonds occur with the sharing of electrons by two atoms with similar tendencies to gain and loose electrons.

Formal Charge

This is another mode of "electron bookkeeping." Like oxidation numbers it uses a very simple set of rules to enable us to make realistic guesses about how atoms behave in molecules without having to have a pocket supercomputer to do that "quick" quantum mechanics calculation.

Formal charge helps us to determine how charges are distributed on atoms in a molecule or molecular ion. It is not always terribly accurate, but is very useful for approximating how molecules will behave in some situations. It is particularly useful in choosing among resonance structures in organic chemistry to determine which are likely to make the greatest contribution to the "real" structure.

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.

Examples:

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:

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© R A Paselk

Last modified 10 April 2013