Transcript VSEPR

VSEPR
Valence Shell Electron-pair
Repulsion model
VSEPR
• This model is useful in
predicting the geometry
of molecules.
• Remember: molecules are
nonmetal & nonmetal.
• The geometry around a
given atom in a
molecule is determined
by minimizing electron
pair repulsions.
Linear
• Electron Pair
Repulsion.
• Note there are two
electron pairs
surrounding Be.
• The charge of an
electron is negative.
• The position that
reduces that
repulsion is 180 ° or
linear.
Three pairs or Trigonal Planar
• Consider three
bonds such as BH3.
• What arrangement
will minimize
electron pair
repulsion?
• 120 ° or trigonal
planar.
What about 4 pairs?
• With 4 pairs in a
plane, the largest
angle available is
90°.
• Is that the largest
angle possible?
• What if you go to
three dimensions?
Tetrahedral
• By arranging the 4
bounds around the
central atom in
three dimensions
you can increase
the bond angle to
109.5° and reduce
the electron pair
repulsion.
Trigonal Bipyramidal
• 5 bonds
• This is the only
electron arrangement
in which there are two
different angles
between electron
pairs to reduce
repulsion.
• Example: PCl5
Octahedral
• 6 electron pairs.
• Each angle is 90°
• Examples include
PCl5-
Steps to apply VSEPR
1. Draw the Lewis Dot structure for the
molecule.
2. Count the electron pairs and arrange
them in the way that minimizes
repulsions.
3. Determine the positions of the atoms
from the way the electron pairs are
shared.
4. Determine the name of the shape from
the position of the atoms.
Lone pairs and Bond angles
• Lone pairs have
a greater
repulsion than
bound electrons.
• This repulsion
has the effect of
“squishing” the
bond angle in the
molecule.
# of
lone
pairs
CH4
NH3
H2O
0
1
2
Bond 109.5° 107°
angle
104.5°
Repulsion of lone pair electrons
Question
• What are the bond angles of CH3CO2CH3?
Answer
• How close were you?
• Is the angle between
the C-O-C really
109°?
• Why or why not?