Formal Charge, cont.
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.
- Perchlorate ion (ClO4-)
- FCCl: 7 - 4 = +3
- FCO: 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
- FCO: 6 - 7 = -1
- FC = FCMn + 4 (FCO) = -1, the charge on the ion, thus
- FCMn = -1 - 4 (FCO) = -1 - 4 (-1) = +3
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:
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=
Hint: sulfur is immediately below oxygen in Periodic Table, so it should be similar to carbon dioxide.
- Carbonate ion - CO32-
- valence electrons = 4 + 3 (6) + 2 = 24
- 6y + 2 = 26, but e- = 24, therefore expect one multiple bond.
- LS =
- However, other equally symmetrical structures are possible, so:
Expanded Valence Shell Example:
(Expanded valence shells only occur when the element has unoccupied d-orbitals, so only for p-block elements in Periods 3 and greater. So here we have dropped the octet rule when nature requires it for the central atom.)
- valence electrons = 6 + 4(7) = 34
- 6y + 2 = 32, but e- = 34, therefore expect expanded valence shell with one extra electron pair.
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.
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
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© R A Paselk
Last modified 6 April 2015