2 peripheral Lewis’ bonded to a central Lewis
Skeletal structures
Because there are exceptions to the octet rule, we need a set of rules to determine how many electrons surround atoms
The first step is to determine how the atoms are bonded in a molecule
Generally, if there is only one of one element and multiple copies of another element, the unique element is central
Commonly, H is peripheral, bonded to O
Read 7.6 (pg. 236) up to PE5. Do PE5.

Counting total electrons
Once we have determined the basic structure of the molecule we can start placing electrons around atoms
The first step is to determine the total number of electrons that are available
We use the group number of an element to indicate the number of valence electrons that it contributes to the molecule.
E.g. O in group VIA (6A), contributes 6 e–s
Read 7.6 (pg. 237) up to PE6 (including example). Do PE6.

Lewis Structures
Once we have determined the number of total valence electrons we can start distributing them throughout the molecule
The rules for this are outlined in fig. 7.4 (pg. 241). These also appear on study notes (with additional details added)
When we represent electrons they will be in pairs (since an orbital holds 2 electrons)
Electron pairs can be represented with 2 dots or a solid line …

Placing electrons around atoms
ClO2–
7x1 + 6x2 +1 = 20
20 - 4 = 16
16 - 12 = 4
4 - 4 = 0
No need
or
CO
4x1 + 6x1 = 10
10 - 2 = 8
8 - 6 = 2
2 - 2 = 0
or
Try PE7 (pg. 241) (answer for HClO4 is wrong in book - missing an O)
Resonance structures
Lewis structures for certain atoms do not match experimental observations
For example, the bond lengths of CHO2– predicted by the Lewis structure are incorrect
The double CO bond should be shorter, and possess a greater bond energy (due to the higher concentration of e–s in a double bond)
Yet, experimentally, both bonds are the same
The reason is due to “resonance”
Resonance structures
A resonance structure can be drawn for any molecule in which a double bond can be formed from two or more identical choices
Resonance structures can be drawn 2 ways…
Resonance implies that the bond flips back and forth. Really, it lies between extremes
Read 7.7 (pg. 242). Do PE 8.
1
2
H
C
–
[
]
H
C
O
–
[
]
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