Valence
Shell
Electron
Pair
Repulsion
Planar
triangular
Tetrahedral
Trigonal
pyramidal
Bent
VSEPR Theory
 Based on Electron Dot (Lewis structures)
 Theory predicts shapes of compounds
 abbreviated VSEPR
 VSEPR (pronounced “vesper”) stands for:
Valence Shell Electron Pair Repulsion
 VSEPR predicts shapes based on electron pairs
 repelling (in bonds or by themselves)
 Electrons around central nucleus
repel each other. So, structures
have atoms maximally spread out
Molecular Shape
VSEPR theory assumes that the shape of a
molecule is determined by the repulsion of
electron pairs.
Vocabulary:


“domain”
= any electron pair, or any double or triple bond is considered one domain.


“lone pair” = “non-bonding pair” = “unshared pair”
= any electron pair that is not involved in bonding


“bonding pair” = “shared pair”
= any electron pair that is involved in bonding
 Need help:
 VSEPR introduction video
 VSEPR practice problems
How to determine shapes of molecules
1. Determine the central atom (usually the atom with the lowest
subscript and/or the atom capable of forming the most bonds).
2. Draw the electron dot structure and bar diagram
3. Determine the molecular geometry using ALL electron pairs
AND atoms around the central atom
4. Modify the geometry to determine the molecular shape if nonbonding electron pairs exist by ignoring them, BUT LEAVE THE
ATOMS OF BONDED PAIRS WHERE THEY ARE. This is
done because even if the electrons have no atom attached,
these unbonded electron pairs still affect the shape of the
structure
NOTE- when counting the total number of electron pairs around
the central atom, include both the bonding pairs and lone pairs.
Double and triple bonds are counted as a bonding pair.
6. Use the VSEPR geometry table to predict the geometry of the
molecule (it can only be one of 5 shapes!!).
VSEPR THEORY:
Example: BeH2
1) Central Atom?
Be (only 1 atom)
VSEPR THEORY:
Example: BeH2
2) Electron Dot?
H Be H
2) Bar Diagram?
H—Be—H
Note that
Be violates
the octet
rule—this is
an
exception!
VSEPR THEORY:
Example: BeH2
3) Geometry? Hint: What is
the furthest apart you can
spread two atoms attached
to a central atom?
H
Be
H
VSEPR THEORY:
Example: BeH2
4) Shape? Ignore any
unbonded pairs of electrons —
not necessary in this case.
 LINEAR
H
Be
H
LINEAR
 They push each other to
opposite sides of center
(180 apart).
BeH2
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? Ignore any
unbonded pairs of electrons —
not necessary in this case.
 trigonal planar
F
F
B
F
TRIGONAL PLANAR
 They push each other apart
equally at 120 degrees.
BF3
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? Think in 3D!
H
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
TETRAHEDRAL
Each repels the other equally
- 109.5 - not the expected
90.
 Think in 3D.
CH4
VSEPR THEORY:
Example: NH3
1) Central Atom?
N (only 1 atom)
VSEPR THEORY:
Example: NH3
2) Electron Dot?
2) Bar Diagram?
H N H
H
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
TRIGONAL PYRAMIDAL
 The thicker, lone pair forces
the others a little bit closer
together (~107.3)
NH3
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? Hint: What is the furthest
apart you can spread two atoms plus two
unbonded pairs of electrons attached to a
central atom? Think in 3D!
~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
BENT
The 2 bonding pairs are
pushed apart by 3rd and 4th
pair (not seen)
H20
NOTE:
 The geometry around the
central atom of a water
molecule is tetrahedral
 The molecular shape is
bent.
H20
What do you mean by the molecular
shape vs the geometry???
 The geometry takes into account ALL the
bonds and unbonded electrons
 The molecular shape “ignores” the unbonded
pairs of electrons
Comparing the 2 “bents”…
Both bent molecules are affected by unshared
pairs – 1 pair on the left, 2 on the right.
Tetrahedral vs. Trigonal pyramidal
Tetrahedral geometry
Tetrahedral geometry
around the central atom
around the central atom
Tetrahedral
Molecular Shape
Trigonal Pyramidal
Molecular Shape
Tetrahedral vs. Trigonal pyramidal
On the right, the 4th lone pair, is not seen as part
of the actual molecule, yet affects shape.
If another one of the bonding pairs on “trigonal
pyramidal” were a lone pair, what is the result?
VSEPR overview
 Each shape has a name
(you will have to know these)
 Names of Shapes:

tetrahedral

trigonal pyramidal

Bent

Linear

trigonal planar
Models
Tetrahedral
Triangular Planar
Trigonal pyramidal
Bent or V
Linear
H
Another way to draw structures in “3D”
C H
H
H
N H
H
H
H
O
H
Molecule
CH4
Lewis Structure
Number of
electron pairs
4
Tetrahedral
NH3
Trigonal
Pyramidal
4
(3 shared
1 lone pair)
SHAPE
Molecule
Lewis Structure
Number of
electron pairs
Bent
H2O
4
(2 shared
2 lone pairs)
Linear
CO2
2
MOLECULAR
SHAPE
Molecule
BeCl2
BF3
Lewis Structure
Number of
electron pairs
Linear
2
Trigonal
Planar
3
SHAPE
Back to Electronegativy
 You must also figure our if there are polar or
nonpolar bonds between the atoms AND if
the overall molecule is polar or nonpolar
 What????????
What is the difference between a polar
bond and a polar molecule?
 A polar bond is a charged bond. There is a
positive and negative end that do not cancel
each other out
 A polar molecule contains polar bonds that do
not cancel each other out.
Determine if OF2 is polar or non polar
 The electronegativities of oxygen and
fluorine, 3.44 and 3.98, respectively, produce
a 0.54 difference that leads us to predict that
the O-F bonds are polar. The molecular
geometry of OF2 is bent. Such an
asymmetrical distribution of polar bonds
would produce a polar molecule.
Determine if BF3 is polar or non polar
 B-F bonds are polar but they are
symmetrically arranged around the central
fluorine atom. No side of the molecule has
more negative or positive charge than
another side, and so the molecule is
nonpolar.
Is CCl4, a polar molecule? Or
nonpolar?
 In CCl4, there are 4 bonds arranged
symmetrically around the central carbon atom.
 Each bond is polar, you can tell due to the
electronegativity differences between carbon and
chlorine.
 Because the 4 atoms of chlorine are arranged
symmetrically, the polarity of the bonds cancel
each other out, making a non-polar molecule.
Predict whether the following molecules are
polar or nonpolar:
(a) BrCl; (b) SO2;
 SOLUTION (a) Chlorine is more electronegative than
bromine. Consequently, BrCl will be polar with chlorine
carrying the partial negative charge:
 (b) Because oxygen is more electronegative than sulfur, the
molecule has polar bonds.
 For each of these, the VSEPR model predicts a bent
geometry. Because the molecule is bent, the bond dipoles
do not cancel and the molecule is polar
Determine if OF2 is polar or non polar
 The electronegativities of oxygen and
fluorine, 3.44 and 3.98, respectively, produce
a 0.54 difference that leads us to predict that
the O-F bonds are polar. The molecular
geometry of OF2 is bent. Such an
asymmetrical distribution of polar bonds
would produce a polar molecule.
QUIZ
Which of the following molecules contains
polar bonds but is nonpolar?
a) H2O
b) NH3
c) F2
d) CCl4
e) CH2Cl2
Answer
 In CCl4, there are 4 bonds arranged
symmetrically around the central carbon atom.
 Each bond is polar, you can tell due to the
electronegativity differences between carbon and
chlorine.
 Because the 4 atoms of chlorine are arranged
symmetrically, the polarity of the bonds cancel
each other out, making a non-polar molecule.