Review
Draw the Lewis structure for CH3OH.
Include polarity of the bonds.
How many unpaired electrons in the
center atom of oxygen?
Chapter 8 “Molecular Shape”
Are we living in a two dimensional
world?
Lewis structures show 2-dimensional
pictures of molecules. In real life
molecules are 3-dimensional.
We are learning to…
Identify the shapes of small molecules;
Describe and apply the VSEPR theory;
8-1 The Shape of Small Molecules
 The shape of a molecule (molecular geometry) is
described by the geometric figure formed when
the atomic nuclei are imagined to be joined in
straight lines.
 The bond angle is the geometric angle between
two adjacent bonds.
8-1 The Shape of Small Molecules
Why do we use models?
Just as architects use models to see how a
building will look when finished, chemists use
models to help visualize how atoms of
molecules are arranged in space.
EXAMPLES
Ball & stick
Space-filling
Wire
VSEPR Theory
The VSEPR theory helps explain the
shapes of simple molecules.
Valence Shell Electron Pair Repulsion
(VSEPR) theory states that, in small
molecules, the pairs of valence electrons
are arranged as far apart from each other as
possible (due to repulsion of electrons).
This results in geometries that minimize the
energy of molecules.
VSEPR Theory: Electron Group
Geometry & Molecular Geometry
This leads to the Molecular Geometry,
which shows how bonded atoms are
arranged around that same central atom.
-This is due to charge repulsion of the
electrons that surround the atoms.
Common Molecular Shapes
LINEAR
TRIGONAL PLANAR
TETRAHEDRAL
TRIGONAL PYRAMIDAL
BENT
LINEAR
180º bond angle
CO2 or O2
2 lone pairs of electrons
TRIGONAL PLANAR
120º bond angle
1 central atom surrounded by 3 other
atoms.
No loan pair of electrons in the central
atom.
BCl3
3 electron domains
TRIGONAL PYRAMIDAL
107º bond angle
1 lone pair of electrons in the central atom.
NH3
4 electron domains
TETRAHEDRAL
109.5º bond angle
1 central atom surrounded by 4 other
atoms.
No unshared electrons
CH4
4 electron domains
BENT
105º bond angle
2 lone pairs of electrons in the central
atom.
OF2
4 electron domains
Exit Task
Draw the structure of NBr3.
How many lone pairs of electrons in the
center atom?
What is the geometry of NBr3?
What is one factor that determines the
geometric shape of a molecule?
PROPERTIES OF BONDS
 In our models the bonds are all the same length,
but this is NOT true in reality.
 As one moves down a Group of the Periodic Table,
the atoms form longer bonds.
Atoms get larger moving down a Group.
 Multiple bonds are shorter and stronger than single
bonds.
The more electrons in a bond, the greater the
attraction to the positive nuclei of a bond.
Electrons act as the ‘electrical glue’ between the two
nuclei.
Characteristics of Various Bonds)
BOND TYPE
DISTANCE (nm)
STRENGTH (kJ/mol)
H-H
Single
0.075
436
C-H
Single
0.109
413
F-F
Single
0.128
155
Cl-Cl
Single
0.198
242
Br-Br
Single
0.228
193
I-I
Single
0.266
151
O-O
Single
0.132
146
O=O
Double
0.121
498
S=S
Double
0.189
C-O
Single
0.143
358
C=O
Double
0.121
745
CΞO
Triple
0.113
1046
C-N
Single
0.147
305
C=N
Double
0.138
615
CΞN
Triple
0.116
887
C-C
Single
0.154
347
C=C
Double
0.134
614
CΞC
Triple
0.120
839
BOND
(Source: Textbook and Teaching resources; also Kotz & Treichel, “Chemistry & Chemical Reactivity,”
5th Ed., Thomson, Brooks/Cole, 2003, pp 354-356.)
8-2 POLARITY
 Recall what we learned about polar & non-polar bonds.
 Are electrons shared equally in all bonds?
 What happens if they are not shared equally?
 Because of the polarity of bonds and how they are
arranged around a central atom, molecules may also be
polar or non-polar.
 Dipole: a polar molecule, one that has a positive end
and a negative end.
 What determines the polarity of a molecule?
Together the shape of a molecule and the
polarity of its bonds determine if a molecule is
polar or non-polar.
Properties of Polar Molecules
 Formaldehyde
 A dipole because of the imbalance of polar bonds.
 A gas, but very soluble in water.
 Carbon Dioxide
 Not polar because the effect of the two polar bonds cancel.
 Gas at room temperature due to lack of attraction between
molecules; also soluble in water.
 Water
 A dipole because the bent shape of the molecule does not
cancel the polar bonds.
 The shape of the water molecule has a major impact on it’s
properties, such as melting, boiling points, solubility and physical
states at room temperature.
 Large Molecules
 Dipoles are critical to the functions of life.
 Proteins, DNA/RNA, carbohydrates, lipids, etc.