Octet Exceptions &
VSEPR Model
March 5, 2007
More Octet Exceptions
• Some molecules have too many electrons
to hold to the octet rules.
F
F
• SF6 is an example
F
F
F
F
S
S
F
F
F
F
F
F
• 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.
Cl
Xe
Cl
• XeCl2
VSEPR Model
•
•
1.
2.
3.
4.
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 Electron Groups
• Tetrahedral
arrangement
• NOT 90º
• 109.5º angle
4 Electron Groups (1 lone pair)
• NH3 (ammonia)
4 Electron Groups (2 Lone Pairs)
• H2O (water)
Lone Pairs and Bond Angles
• The more lone pairs, the smaller the bond
angle.
Exam Practice
• Which of the following molecules or ions
has the smallest bond angle? or the
largest?
smallest
• a. H2O
largest
• b. HCN
• c. O3
• d. CH4
• e. NO3
5 Electron Groups
• PCl5 – trigonal bipyramid
• bond angles of 90, 120
(and 180)
http://www.elmhurst.edu/~chm/vchembook/220trigbipy.html
6 Electron Groups