1. Lewis Dot Structures
Developed by G. N. Lewis to serve as a way to describe bonding in polyatomic systems.
Central idea: the most stable arrangement of electrons is one in which all atoms have a “noble” gas configuration.
Example: NaCl versus Na+Cl-
Na: [Ne]3s1
Cl: [Ne]3s23p5
Na+: [Ne]
Cl-: [Ne]3s23p6 = [Ar]

2. LDS Mechanics
Atoms are represented by atomic symbols surrounded by valence electrons.
Lone Pair (6 x)
Electron pairs between atoms indicate bond formation.
Bonding Pair

3. LDS Mechanics (cont.) Three steps for “basic” Lewis structures:
Sum the valence electrons for all atoms to determine
total number of electrons.
Use pairs of electrons to form a bond between each
pair of atoms (bonding pairs).
Arrange remaining electrons around atoms (lone pairs)
to satisfy the “octet rule” (“duet” rule for hydrogen).

4. LDS Mechanics (cont.)
An example: Cl2O
20 e-
16 e- left

5. LDS Mechanics (cont.)
An example: CH4
8 e-
0 e- left
Done!

6. LDS Mechanics (cont.) An example: CO2 16 e- 12 e- left 0 e- left
Octet Violation
0 e- left
CO double bond more than one pair of electrons is shared

7. Lewis Diagram for CO2

8. LDS Mechanics (cont.)
An example: NO+ + +
10 e-
8 e- left +

9. Whiteboarding!
In a group of 3 try the following Lewis Structures!

10. CCl4
NF3
H2O
H2Se
NH3
OF2

11. Resonance Structures
We have assumed up to this point that there is one correct Lewis structure.
There are systems for which more than one Lewis structure is possible:
Different atomic linkages: Structural Isomers
Same atomic linkages, different bonding: Resonance

12. Resonance Structures (cont.)
The classic example: O3
When a molecule or ion contains double bonds next to single bonds, there are often several different and equally correct Lewis structures that we can draw
Both structures are correct!

13. Resonance Structures (cont.)
In this example, O3 has two resonance structures:
Conceptually, we think of the bonding being an average of these two structures.
Electrons are delocalized between the oxygens such that on average the bond strength is equivalent to 1.5 O-O bonds.

14. Resonance Structures for CO3-2
The double bond in each of the possible locations in a series of diagrams and connecting each diagram with a set of double arrows. It must be emphasized, however, that the pair of electrons does not move around between pairs of atoms as the diagrams might suggest.

15. Structure with less F. C. is more correct.
Formal Charge
Formal Charge: Compare the nuclear charge (+Z) to the number of electrons (dividing bonding electron pairs by 2). Difference is known as the “formal charge”. #e Z
Formal C
Structure with less F. C. is more correct.

16. Formal Charge Example: CO2 e- 6 4 6 6 4 6 7 4 5 Z+ 6 4 6 6 6 4 6 6 4FC
More Correct

17. Beyond the Octet Rule There are numerous exceptions to the octet rule.
We will deal with two classes of violation here:
Sub-octet systems
Valence shell expansion

18. Beyond the Octet Rule (cont.)
Some atoms (Be and B in particular) undergo bonding, but will form stable molecules that do not fulfill the octet rule.
Experiments demonstrate that the B-F bond strength is consistent with single bonds only.

19. Beyond the Octet Rule (cont.)
For third-row elements (“Period 3”), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding.
When this occurs, more than 8 electrons can surround a third-row element.
Example: ClF3 (a 28 e- system)
F obey octet rule
Cl has 10e-

21. In the end, you have to practice…..a lot!
Summary
Remember the following:
C, N, O, and F obey the octet rule
B and Be are often sub-octet
Second row (Period 2) elements never exceed the octet rule
Third Row elements and beyond can use valence shell expansion to exceed the octet rule.
In the end, you have to practice…..a lot!