VSEPR THEORY:
How do we determine
the shapes of
molecules and ions?
VSEPR THEORY
What does VSEPR stand
for?
Valence Shell Electron Pair
Repulsion
VSEPR THEORY
Why is this important to
know?
It explains how molecules
and ions behave.
VSEPR THEORY
For example:
It explains why water
molecules are so good at
dissolving ionic substances
even though water does
not have an ionic bond.
VSEPR THEORY:
Basic procedure
1) Determine the central
atom (usually the atom
with the lowest subscript
and/or the atom capable of
forming the most bonds).
Ex: CH4
H2O
VSEPR THEORY:
Basic procedure
2) Draw the electron dot
structure
Ex:
VSEPR THEORY:
Basic procedure
3) Determine the molecular
geometry using ALL
electron pairs AND atoms
around the central atom.
VSEPR THEORY:
Basic procedure
 4) Modify the geometry to
determine the molecular shape if
non-bonding electron pairs exist
Even if the electrons have no atom
attached, these unbonded electron pairs
still affect the shape of the structure.
Molecule Shapes
Linear Molecules
 Linear
 2 regions of edensity
 Bond angle = 180°
Angular or Bent
 Angular or bent
 At least 3 regions of
e- density
 Only 2 regions
bonded
 Bond angle = 105 °
Triangular Planar (OR
Trigonal Planar)
 Triangular planar
 3 regions of edensity
 All regions bonded
 Bond angle = 120 °
Tetrahedral
 Tetrahedral
 4 regions of edensity
 All regions bonded
 Bond angle =109.5°
VSEPR THEORY:
Example: SH2
 1) Central Atom?
S (only 1 atom)
VSEPR THEORY:
Example: SH2
 2) Electron Dot?
H S
H
 2) Bar Diagram?
__
H— S —H
VSEPR THEORY:
Example: SH2
 3) Geometry?
VSEPR THEORY:
Example: SH2
 4) Shape?
BENT
VSEPR THEORY:
Example: BF3
 1) Central Atom?
B (only 1 atom)
VSEPR THEORY:
Example: BF3
 2) Electron Dot?
F B F
F
 2) Bar Diagram?
F—B—F
F
Note that B
violates the
octet rule—
this is an
exception!
VSEPR THEORY:
Example: BF3
 3) Geometry? Hint: What is
the furthest apart you can
spread three atoms attached
to a central atom?
F
F
B
F
VSEPR THEORY:
Example: BF3
 4) Shape?
TRIGONAL PLANAR
F
F
B
F
VSEPR THEORY:
Example: NH3
 1) Central Atom?
N (only 1 atom)
VSEPR THEORY:
Example: NH3
 2) Electron Dot?
H N H
H
 2) Bar Diagram?
H—N—H
H
VSEPR THEORY:
Example: NH3
 3) Geometry? Hint: What is the
furthest apart you can spread three
atoms plus one unbonded pair of
electrons attached to a central atom?
Think in 3D!
~109.5o
N
H
H
H
VSEPR THEORY:
Example: NH3
 4) Shape? Ignore any unbonded
pairs of electrons —it IS
necessary in this case.
 trigonal
pyramidal
~109.5o
N
H
H
H
VSEPR THEORY:
Example: CH4
 1) Central Atom?
C (only 1 atom)
VSEPR THEORY:
Example: CH4
 2) Electron Dot?
H
H C H
H
 2) Bar Diagram?
H
H—C—H
H
VSEPR THEORY:
Example: CH4
 3) Geometry? Hint: What is the
furthest apart you can spread four
atoms attached to a central atom?
H
Think in 3D!
C
H
H
H
VSEPR THEORY:
Example: CH4
 4) Shape? Ignore any unbonded
pairs of electrons —not necessary
in this case.
H
 tetrahedral
C
H
H
H
VSEPR THEORY:
Example: H2O
 1) Central Atom?
O (only 1 atom)
VSEPR THEORY:
Example: H2O
 2) Electron Dot?
O H
H
 2) Bar Diagram?
O—H
H
VSEPR THEORY:
Example: H2O
 3) Geometry?
~109.5o
O
H
H
VSEPR THEORY:
Example: H2O
 4) Shape? Ignore any unbonded
pairs of electrons —it IS
necessary in this case.
 bent
~109.5o
O
H
H