Download Molecular Geometry Experiment 1: Molecular Models of Neutral

Molecular Geometry
Experiment 1: Molecular Models of Neutral Molecules
In this experiment you will predict the three dimensional geometry of a series of neutral
molecules using the VSEPR theory.
Materials
Colored Pencils
35 Marshmallows (Miniature)
Permanent Marker
Protractor
35 Toothpicks
*Camera (camera phone is sufficient)
*You Must Provide
Procedures
Part 1: Periodic Table
1. Use your Periodic Table of Elements to determine the elemental symbol, group number
and valence electrons for the elements listed in Table 1. Record this data in Table 1.
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Molecular Geometry
2. Use your colored pencils and the data in Table 1 to sketch the Lewis Structures for each
of the elements in Table 1.
Procedures
Part 2: Construction of Molecules
1. Now, construct the three dimensional geometry for the molecules listed in Table 2. While
you are constructing your molecules it will be useful to keep the following points in mind:

Nature “loves” symmetry which means equal bond lengths and angles.

Electrons prefer to be as far apart from each other as possible without disrupting
the symmetry too drastically.

And, lone pairs take up more space because they are not confined by bonds.
2. Look at Table 2 and fill in the bond angles for each of the molecules that you will be
building.
Hint: Use the column in Table 2 labeled “Structure” to determine the molecular
geometry.
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Molecular Geometry
3. Using your colored pencils make a Lewis Dot Structure sketch for each of the molecules
in Table 2 that you will be building.
4. Gather as many marshmallows and toothpicks you will need for your first molecule
(carbon dioxide).
Hint: You will need two marshmallows for the oxygen (O) and one marshmallow
for the carbon (C).
5. Using your permanent marker label the miniature marshmallow with the elemental
symbol for each atom in your molecule (Figure 6).
Figure 6: To satisfy the octet rule the linear
carbon dioxide molecule has double bonds.
Note, the bond angles should be 180 degrees.
6. Refer to Table 2 to determine which atom will be the central atom in your molecule.
7. Connect the atoms together with toothpicks.
8. Compare your model with the diagram of the Linear molecular geometry in Table 2.
9. Use your protractor to verify that you have constructed your molecule with the correct
bond angles.
10. To determine the angle between your atoms place the center of the protractor on the
central atom, and a line the base with a bond (Figure 6). Then measure the angle. When
in doubt think "should this angle be bigger or smaller than 90°?"
11. If your bond angles are incorrect, remove the toothpicks from the marshmallows,
construct the molecule again, and use your protractor to verify that you have constructed
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Molecular Geometry
your molecule with the correct bond angles.
12. Follow the above procedures for each of the molecules in Table 2.
13. When you are finished take a picture of the molecules and send the images to your
instructor. Make sure you identify yourself, name of the lab, and any other necessary
identifying information.
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Molecular Geometry
Table 1: Lewis Dot Structures
Element
Symbol
Group
Number
Valence
Electrons
Lewis Dot Structure
Hydrogen
Carbon
Chlorine
Aluminum
Oxygen
Fluorine
Neon
Nitrogen
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Molecular Geometry
Table 2: Molecular Models
Molecule
(central atom is
underlined)
Sketch of
Lewis Dot
Structure
Shape Name
Bond Angle
O C
CO
Bond Angle
Structure
O
Linear
2
Cl
AlCl
3
Trigonal Planar
Cl
Al
Cl
..
HO
2
Bent
H
O
:
H
..
NH
3
Trigonal
pyramidal
H
N
H
H
Cl
CCl
4
Tetrahedral
Cl
C
Cl
PCl
5
SF
6
Trigonal
bipyramidal
Octahedral
Cl
Cl
F
F
Cl
Cl
P Cl
Cl
F
S
F
F
F
Post-Lab Questions:
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Molecular Geometry
1. Should all of the angles in methane (CH4) be equal? Why or why not?
2. What additional information does the VSEPR theory give you beyond electron dot structures,
in terms of molecular structure?
3. Sketch the molecular shape of the following compounds. Be sure to label the bond angles.
Compound
H
Sketch of Molecular Shape
2
Cl
2
SO
2
NH
3
MoF
6
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