1. Octet Exceptions & VSEPR Model
March 5, 2007

2. More Octet Exceptions
Some molecules have too many electrons to hold to the octet rules.
SF6 is an example
Only elements in period 3 and higher can exceed the octet rule.
If there is a question as to where the extra electrons go, put them on the central atom.
XeCl2

3. VSEPR Model Predicting the 3-D molecular structure of an atom.
The model attempts to minimize electron-pair repulsions.
Begin with the Lewis Structure!
Predict the arrangement of the electron pairs to minimize repulsions.
Determine the positions of the bonded atoms.
Determine the molecular structure name.

4. 4 Electron Groups
Tetrahedral arrangement
NOT 90º
109.5º angle

5. 4 Electron Groups (1 lone pair)
NH3 (ammonia)

6. 4 Electron Groups (2 Lone Pairs)
H2O (water)

7. Lone Pairs and Bond Angles
The more lone pairs, the smaller the bond angle.

8. Exam Practice
Which of the following molecules or ions has the smallest bond angle? or the largest?
a. H2O
b. HCN
c. O3
d. CH4
e. NO3
smallest
largest

9. 5 Electron Groups PCl5 – trigonal bipyramid
bond angles of 90, 120 (and 180)

10. 6 Electron Groups