Chem 109 - General Chemistry - Spring 2013

Lecture Notes 27: 10 April

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.

Covalent Compound Lewis Structure Examples:
- Water: first write formula = H₂O, then determine central atom, O, then add electrons to get octet and draw L.S.
- Ammonia = NH₃
- Ammonium ion = NH₄⁺
- Extra "practice" structures below:

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:

Draw a correct Lewis Structure.
Assign both electrons of a lone pair to its associated atom.
Divide all bonding pairs, giving one electron of each pair to each atom in the bond.
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

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:

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.
- Multiple bond examples:
  - Carbon monoxide, CO
    - valence electrons = 4 + 6 = 10
    - 6y + 2 = 14, thus 4 fewer electrons than required for all single bonds, 4/2 = 2 multi-bonds (2 double or 1 triple)
    - LS = :C:::O:
  - Carbon dioxide, 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=
  Practice example:
  - Carbon disulfide
    - Hint: sulfur is immediately below oxygen in Periodic Table, so it should be similar to carbon dioxide.