Molecular Modeling Activity
Overview: Molecules and polyatomic ions are not all flat structures. Each has a three-dimensional shape that helps
account for its various chemical and physical properties. This Activity aims to reinforce the conept of molecular
geometry through the use of traditional molecular modeling kits, balloons, and computer generated images downloaded
from the internet.
I. Cooperative Learning Activity:
Electron dot structures and structureal formulas usually show a given molecule in only two dimensions. In reality, many
molecules and polyatomic ions exist in three dimensions. The VSEPR (valence shell electron-pair repulsion) theory
provides an explanation for the shape of many molecules and ions. According to VSEPR theory, valence shell electron
pairs will stay as far apart as possible so that the repulsions between them are minimized.
Time Frame: Two 40 minute periods.
Objectives:
 Define the VSEPR theory and explain its relationship to the shape of molecules.
 Differentiate electron pair geometry from molecular geometry.
 Name and describe the five electron pair geometries which can surround the central atom.
 Show how molecular geometry is a function of electron pair geometry.
 State the two factors that determine the polarity of a molecule.
 Explain how the geometry of a molecule helps to determine its properties.
 Organize your information in the form of a chart.
Materials:
 Pens & pencils
 Textbook or other reference books
 Pear-shaped balloons (6 of one color, 2 of another)
 Molecule model building kit
Instructions:
1. Build the following molecules using a molecule model building kit.
 Use wooden sticks for single bonds
 Use metal springs for multiple bonds
 NOTE: You may not be able to build ALL molecules with the kit. If not, rely on the balloon model.
2. Complete the table as instructed below.
3. Draw the Lewis-dot diagram for each molecule.
4. Sketch a 3-d diagram of each molecule.
5. Using balloons, show the geometry of the molecule.
 Use one color to represent bonding pairs.
 Use the other color to represent nonbonding pairs.
6. Identify the electron pair geometry by focusing on all the electron domains (ALL the balloons).
7. Identify the molecular geometry by focusing on only the bonded electron domains.
8. Complete the table, with the exception of “Polarity”. (We will address this later.)
9. Table 8.6 on page 394 of your Zumdahl, 6th ed. Chemistry textbook is helpful for determining shapes of
molecules.
NOTE: there is often a difference between the electron pair geometry and the molecular geometry. Electron pair
geometry takes into account ALL of the electron domains. For example, NH3 has 4 electron domains, giving it a
tetrahedral electron domain geometry. However, since only 3 of those domains have bonding electrons (atoms
attached to them) the MOLECULAR geometry is pyramidal.
Data Table: Draw table in notebook or continue on back of paper, if space is not sufficient.
Molecule
BeH2
Sketch of molecule
Lewis dot diagram
# of
bonding
pairs (edomains)
# of
nonbonding
pairs (edomains)
Electron
pair
geometry
Molecular
geometry
Molecula
r shape
Polarity
2
0
Linear
AX2
Linear
nonpolar
BH3
SO2
CH4
NH3
AX3E
H2O
PCl5
SF6
Molecular Geometry symbols: A= central atom
X = non-central atoms
E= non-bonding electron domains
Drawing Sketches of Molecules:
 Dashed bonds are used to represent bonds that project backward (behind the drawing plane).

Wedged bonds are used to represent bonds that project outward (in front of the drawing plane).

The three common types of bonds used in drawing chemical structures:

See CH4, above, for an example of how to represent the molecular shape.