Honors Chemistry
Name: ______________________________
Date: __________
Molecular Models Lab
Covalent Compounds
Introduction
The purpose of this lab is to evaluate the geometries and polarities of simple covalent
molecules. Many physical and chemical properties of compounds depend upon the shape
or geometry of the molecule and its polarity. A hemoglobin molecule, for example,
becomes ineffective as an oxygen transport vehicle if its shape is altered. Molecular
shape determines a compound’s boiling point, freezing point, and viscosity. A
molecule’s polarity determines whether it dissolves in water or not.
Molecular Geometries
The geometry of a small molecule can be predicted by examining the central atom. The
valence electron pairs that surround the central atom will repel each other. The electron
pairs will position themselves so that there is a maximum distance between them. This
rule is called the VSEPR theory (Valence Shell Electron Pair Repulsion). The electron
pairs include the covalent bonds (shared pairs) and the unshared pairs of electrons. The 5
simplest geometries and their characteristics are found in the following table:
No. of Electron
Pairs on Central
Atom
No central atom
Geometry
Name
Central Atom Characteristics Comments
Linear
XX
2 shared, 0 unshared
Linear
3 shared, 0 unshared
Trigonal
planar


 X  or  X 
Bond angles are
120
4 shared, 0 unshared
Tetrahedral
Since the molecule is 3dimensional, the bond
angles are 109, not 90.
2 shared, 2 unshared
Bent
3 shared, 1 unshared
Pyramidal

X


:X:


X:

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Revised: 04 Dec 2007
 X or = X =
If a molecule only
contains 2 atoms it is
linear.
Double and triple bonds
only count as a shared
pair for geometry
purposes.
The bond angle is
slightly < 109, not 90.
The bond angle is
slightly < 109, not
90.
Name: ________________________
Date: _____________
Period: ______________
Molecular Polarity:
Covalent molecules can be classified as polar or nonpolar. A polar molecule contains
polar covalent bonds that are asymmetrically arranged. This lack of symmetry causes an
uneven electron distribution around the central atom that leaves part of the molecule with
a more negative region, and part of the molecule with a more positive region. Remember
that polar bonds are covalent bonds between 2 atoms that have different
electronegativities. (Between 0.4 to 2.0 difference.)
Model Kits:
The model kits are used to demonstrate the shapes of small molecules. They are useful
because they represent the molecules in three dimensions, as opposed to two-dimensional
drawings. The different colored balls represent certain atoms, according to the chart:
Color
Black
White
Red
Blue
Yellow
Green
Purple
Grey
Element
Carbon
Hydrogen
Oxygen
Nitrogen
Sulfur
Halogen
Phosphorous
Metal
# of holes
4
1
2
3
6
1
3
6
The Medium Grey links are used to represent single covalent bonds. Two Long Grey
Links are used to represent a double bond, and Three Long Grey Links are needed for a
triple bond.
Prelab Assignment:
Answer the following questions:
1. List the five molecular geometries that you will be studying in this lab.
2. What is a covalent bond?
3. What is a polar molecule (or dipole)?
4. Which geometries contain central atoms with unshared pairs of electrons?
5. Which two factors are required for a molecule to be polar?
6. Calculate the electronegativity difference for the following pairs of atoms. Then
indicate if the bond between them is ionic, polar covalent, or nonpolar covalent.
Atom Pair
Electronegativity
Difference
Bond Type
Na --- Cl
C --- H
S --- O
N --- N
H ---Cl
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Revised: 04 Dec 2007
Name: ________________________
Date: _____________
Period: ______________
7. Complete the dot diagrams for all the molecules in the data table. The first 7
molecules contain all single bonds, but the last 6 molecules have at least one double
or triple bond.
Lab Instructions:
Build each molecular model using the model kit code and your dot diagram. Draw a
representation of the molecule, determine its shape and polarity and record the data in the
table below. To build some of the molecules, you may have to obtain additional atoms
or additional bonds from the bulk atom boxes.
Data Table
Formula
Dot Diagram
3D
Representation
Molecular
Geometry
Molecular
Polarity
Example: H2
H-H
H-H
Linear
Nonpolar
1. H2O
2. NH3
3. CH4
4. HBrO
5. CH3Cl
6. CH3NH2
7. H2O2
8. C2Cl2
9. N2
10. HCOOH
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Revised: 04 Dec 2007
Name: ________________________
Date: _____________
Period: ______________
Formula
Dot Diagram
3D
Representation
Molecular
Geometry
Molecular
Polarity
11. HCN
12. H2CO
13. CO2
Conclusion Questions:
1. List the molecules that are nonpolar due to nonpolar bonds.
2. List the molecules that are nonpolar because symmetry cancels out the bond polarity.
3. Which two shapes are always asymmetric?
4. Explain how molecular shape affects molecular polarity.
5. What are the advantages and disadvantages of using the model kits to represent
molecules?
6. What is the relationship between the number of holes in each ball to the number of
valence electrons of the atom that the ball represents? (Hint: It has to do with the
octet rule.)
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Revised: 04 Dec 2007
Name: ________________________
Date: _____________
Period: ______________
Critical Thinking Questions:
1. Using the results of this lab, predict the molecular geometry and molecular polarity of
the following substances.
Substance
Geometry
Polarity
HI
SH2
PH3
CCl4
2. Molecular polarity is the determining factor in determining whether 2 substances are
soluble or not. The general rule is “like dissolves like.” Knowing this, predict the
following:
a. What is the polarity of alcohol, formula CH3OH? Explain your reasoning.
b. Why are special solvents needed to remove dirty grease stains from fabrics?
Explain your reasoning.
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Revised: 04 Dec 2007