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 tp 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:

• Phosphoric acid (H₃PO₄)
  • LS:
    • FC_P: 5 - 4 = +1
    • FC_H: 1 - 1 = 0
    • FC_O: 6 - 7 = -1
    • FC_{3 O's}: 6 - 6 = 0
  • FC = 1 + 3 (0) + (-1) = 0. Notice that the Formal charges add up to give zero, the charge on the molecule.
• Perchlorate ion (ClO₄^-)
  • LS:
    • FC_Cl: 7 - 4 = +3
    • FC_O: 6 - 7 = -1
  • FC = +3 + 4 (-1) = -1. Notice that the Formal charges add up to give -1, the charge on the molecular ion.
• Note can also determine FC's for compounds containing transition metals. Just don't worry about transition metal, use octet rule for representative elements, and find transition metal FC by difference. For example, permanganate ion MnO₄^-
  • Oxygen will have an octet in normal oxygen compounds, and will share one bonding pair, thus
    • FC_O: 6 - 7 = -1
  • FC = FC_Mn + 4 (FC_O) = -1, the charge on the ion, thus
    • FC_Mn = -1 - 4 (FC_O) = -1 - 4 (-1) = +3

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

• Sarin (nerve gas)
• "drugs"
• estrogen mimics ("feminization" of various animal populations - birth control complication)

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).

VSEPR (Valence Shell Electron Pair Repulsion) Theory is based on three assumptions (there are more advanced versions, but unnecessary for us):

• Electron pairs will orient around a central point to minimize repulsion.
• Lone-pairs of electrons will have greater repulsion than bonded pairs of electrons (note that the atoms are ignored in terms of repulsion).
• Repulsion is strong at 90° and weaker at 120° (weakest at 180°).

VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

1. Draw a correct Lewis Structure.
2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs.
3. Maximize the angles between electron pairs, placing the lone (unbonded) pairs at the extremes.

For central atoms with eight outer electrons (octets) there are three possible electron pair geometries:

1. Linear with angles of 180° ( a single pair and a triple bond, or two double bonds).
2. Trigonal planar with angles of 120° (one double bond and two single pairs).
   
   
   
3. Tetrahedral with angles of 109.5° (four single pairs). [model]
   
   

These three electron pair geometries can lead to five molecular geometries:

• Linear (carbon monoxide)
  • CO₂
    • valence electrons = 4 + 2x6 = 16
    • 6y + 2 = 20, thus 4 fewer electrons than required for all single bonds, 4/2 = 2 multi-bonds (2 double or 1 triple)
    • LS: from symmetry C will be central atom, therefore= :O::C::O:
    • Considering C as the central atom, have 2 bonded atoms and no lone-pairs, therefore
    • steric number = 2, so linear electronic geometry, and
    • linear molecular geometry

---

Syllabus / Schedule

C109 Home

C109 Lecture Notes

© R A Paselk

Last modified 1 April 2009