1. Chapter 6-2
Lewis Structures

2. Drawing Lewis structures for molecules
1. Determine the total number of valence electrons in the molecule.
2. The atom with the lowest electronegativity is often the central atom (usually written first in the chemical formula). Arrange the other atoms’ symbols around the central atom.
3. Distribute eight electrons around each atom except hydrogen. If there aren’t enough e- to go around, form double or triple bonds.
4. Compare the number of valence electrons used in the structure with the number available from step 1.
5. Change each pair of dots that represents a shared pair of electrons to a single dash.

3. Octet Rule
Remember…
Most atoms form bonds in order to have 8 valence electrons.

4. Exceptions to Octet rule
For some molecules, it is impossible to satisfy the octet rule

5. F B F F H O H N O F F F S F Very unstable!! Octet Rule Exceptions:
Hydrogen  2 valence e-
Be gets 4 valence e-; B gets 6
Expanded octet  more than 8 valence e- (e.g. S, P, Xe)
Radicals  odd # of valence e-

6. Drawing Lewis Diagrams
BeCl2
1 Be × 2e- = 2e-
2 Cl × 7e- = 14e-
16e-
Cl Be Cl
- 4e-
12e-

7. Exceptions to Octet rule
NO2 has 17 valence electrons, because the N has 5, and each O contributes 6
impossible to satisfy octet, yet the stable molecule does exist