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CHEM 108
LABORATORY 6
MOLECULAR MODELING:
AN INTRODUCTION TO 3-DIMENSIONAL GEOMETRY AND
POLARITY OF MOLECULES
Materials Needed
Molecular Structure Models
Introduction
The purpose of this experiment is to introduce the basic concepts of three-dimensional
reasoning as applied to VSEPR (Valence Shell Electron Pair Repulsion) theory.
Molecular model sets will be employed to give students hands-on experience with
understanding and manipulating three-dimensional models. This relatively simple theory
can be used to predict the shapes of molecules, and then to predict molecular properties
such as polarity.
Procedure
Using the model kits and the help of a partner, build models of the molecules indicated
on the report sheet. Start by drawing a Lewis structure for each of the species before
attempting to build a model of it. Count the total numbers of electron groups around the
central atom. Lone pairs, single bonds, double bonds, and triple bonds each count as one
group. Count the number of bonding and nonbonding electron groups around the central
atom.
Table 1: VSEPR shape and bond angles
Total # # of
# of
Electron
of
bonding
nonbonding
Geometry
electron (shared)
(unshared)
groups electron
electron
around groups
groups or
the
lone pairs
central
atom
2
2
0
Linear
3
3
0
Trigonal
planar
(triangular)
3
2
1
Trigonal
planar
(triangular)
4
4
0
Tetrahedral
4
3
1
Tetrahedral
4
2
2
Tetrahedral
1
VSEPR Shape
(Molecular
Geometry)
Bond
angles
Example
Linear
Trigonal
Planar
(triangular)
Bent
180o
120o
BeF2
BH3
120o
SO2
Tetrahedral
Triangular
pyramidal
Bent
109.5o
109.5o
CH4
NH3
109.5o
H2O
Use the table 1 to determine the electron geometry and molecular geometry of the
molecule. Make sure you understand the concepts of VSEPR shape and polarity as you
proceed. Complete the report sheet as you build the models.
The molecular geometry is used to predict the polarity of a molecule. A molecule is
polar if the polar bonds in the molecule are arranged to form a net dipole moment. A
polar molecule must contain polar covalent bonds along with the right molecular
geometry to add together the polarity of bonds. Some molecules contain polar bonds but
dipole moments cancel each other and therefore, the molecule itself is nonpolar.
To predict the polarity of bonds, we use the electronegativity difference between
elements. Electronegativity is the ability of an element to attract electrons within a
covalent bond. Table 2 lists the electronegativity of the elements and table 3 lists the
effect of electronegativity difference on bond type.
Table 2: Electronegativity of the Elements
H
2.1
Li
Be
B
1.0
1.5
2.0
Na
Mg
Al
0.9
1.2
1.5
K
Ca
Ga
0.8
1.0
1.6
Rb
Sr
In
0.8
1.0
1.7
Cs
Ba
Tl
0.7
0.9
1.8
He
C
2.5
Si
1.8
Ge
1.8
Sn
1.8
Pb
1.9
N
3.0
P
2.1
As
2.0
Sb
1.9
Bi
1.9
O
3.5
S
2.5
Se
2.4
Te
2.1
Po
2.0
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.1
Ne
Ar
Kr
Xe
Rn
For example, the electronegativity difference between O and H in water molecule is 3.52.1 = 1.4 and therefore, the O-H bond is polar covalent. Since water has a bent structure,
and the two dipole moments do not cancel, water is a polar molecule. On the other hand,
CO2 is a nonpolar molecule even though it contains two polar covalent bonds. The
electronegativity difference between C and O is 3.5-2.5 = 1.0 which is polar covalent.
CO2 has a linear geometry and the two dipole moments cancel each other out.
Table 3: The Effect of Electronegativity Difference on Bond Type
Electronegativity Difference
Bond Type
Example
Zero (0-0.4)
Nonpolar covalent H-H bond
Intermediate (0.4-2.0)
Polar covalent
H-O bond, C-O bond
Large (greater than 2.0)
Ionic
Na-Cl bond
2
REPORT SHEET (Turn in one per team)
LABORATORY 6: MOLECULAR MODELING
Names:___________________________________
1.
Date:___________
Draw Lewis structures and use table 1 to predict the VSEPR shapes and bond
angles for the following:
Molecule
Lewis
structure
Total # of
electron
groups
around the
central
atom
# of
bonding
(shared)
electron
groups
# of
nonbonding
(unshared)
electron
groups
BF3
(B is
central)
N2O
(N is
central)
CCl4
(C is
central)
CH2Cl2
(C is
central)
H2Te
(Te is
central)
PF3
(P is
central)
3
Electron
Geometry
VSEPR
shape
(Molecular
Geometry)
Bond
Angles
2.
Use Table 2 to find the electronegativity difference between the elements in each
compound. Use table 3 to classify the bonds as nonpolar covalent or polar
covalent. Sketch the molecule and classify the molecule as polar or nonpolar.
Molecule
Electronegativity
Difference
between elements
Polar or
nonpolar
covalent
bond?
Sketch of
molecule
BF3
(B is
central)
N2O
(N is
central)
CCl4
(C is
central)
CH2Cl2
(C is
central)
H2Te
(Te is
central)
PF3
(P is
central)
4
Is there a net
dipole moment?
Polar or
nonpolar
molecule?