GISAT 112
MOLECULAR MODELS: CHEMICAL BONDING AND SHAPE
INTRODUCTION
The properties of chemical compounds are directly related to the ways in which atoms are
bonded together into molecules. Chapter 2 in Chemistry in Context presents the basic ideas
of chemical bonding, while Chapter 3 shows how the 3-dimensional shapes of molecules
are related to the bonding. In this exercise, you will have the opportunity to apply your
knowledge from those two chapters by constructing simple ball-and stick models for some
common molecules. The models should help your understanding of electron arrangements
in molecules and the resulting shapes of the molecules. You will investigate a number of
small molecules containing carbon, nitrogen, oxygen and hydrogen, as well as a few
molecules containing fluorine, chlorine, or sulfur. These are mostly substances that are
important in the atmosphere and in polluted air, as discussed in Chapters 1, 2, and 3 of
Chemistry in Context. In the process of doing this exercise you will see how “models”
become very useful to chemists in understanding and predicting chemical properties.
BACKGROUND INFORMATION: ELECTRONS AND MOLECULES
The existence of chemical compounds with fixed composition implies that the atoms in
compounds must be connected in characteristic patterns. Early models showed the atoms
hooked together like links on a chain. Modern representations are a good deal more
abstract and often mathematical in nature. Nevertheless, it is possible to represent
molecular structures with reasonable accuracy by using relatively simple models. The
models serve as a three-dimensional representation of an abstract idea. Molecular model
building has proven so useful that it is rare to find a chemist who does not have a model kit
close at hand.
The chemical bonds that hold atoms together in molecules generally consist of pairs of
electrons shared between two atoms. Atoms tend to share outer electrons in such a way
that each atom in the union has a share in an octet of electrons in its outermost shell. This
generalization has come to be known as the octet rule. (You should review the discussion
of Lewis structures and the octet rule in Chapter 2 of the text before coming to lab.) The
location of each element in the Periodic Table provides information about the number of
electrons in the outermost level of the atoms. Carbon, for example, is in Group 4A and has
four outer electrons; thus, it must share four additional electrons from other atoms to
achieve a share in eight outer electrons (an octet). Oxygen, in Group 6A, has six outer
electrons and shares two electrons from other atoms to achieve an octet. Hydrogen is a
special case, needing to share is one electron with only one electron from another atom to
achieve the stable outer electron configuration of the non-reactive element helium (He).
A single bond consists of one shared pair of electrons; a double bond is two shared pairs
(i.e., 4 electrons), and a triple bond is three shared pairs (6 electrons). On paper the bonds
are represented by single, double, or triple lines, respectively (-, =, ). In model kits,
straight sticks represent single bonds, while pairs or triplets of curved sticks or springs
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GISAT 112
represent double and triple bonds. Electrons not involved in bonding are termed unshared
electrons.
An important part of this exercise involves identifying the 3-dimensional shapes of
molecules. (Molecular shapes are discussed in the text in Chapter 3.) Molecules have
certain shapes depending on their component atoms and the ways in which they are bonded
to each other. The important shapes encountered in this exercise are linear, bent,
triangular, pyramidal, or tetrahedral. Several factors contribute to determining molecular
shape: (1) Electron pairs (both shared and unshared) try to keep as far away from each
other as possible, while still remaining “attached” to atoms. (After all, they are all
negatively charged and electrical charges of the same type will repel each other.) (2)
Electron pairs tend to be symmetrically arranged around each atom in a 3-dimensiional
manner. (3) Electron pairs not involved in the bonding (“unshared pairs” or “lone pairs”)
are as important as bonding electron pairs (shared pairs) in determining the overall
molecular shape and arrangement of atoms.
MODEL BUILDING BASICS
Molecular model kits vary; therefore, your instructor will explain the particular models that
you will use. The kit probably contains balls (used for atoms), sticks (for single bonds and
unshared electron pairs), and springs or curved sticks (for double and triple bonds). Each
stick or spring represents two electrons. Hydrogen atoms are usually represented by small
light-colored balls (yellow, white, or pale blue) that have only one hole. The color code for
other atoms will vary. A common set of colors is shown in the table on the next page.
Note: there is one disadvantage to using the colored balls provided by most model sets.
They usually have only enough holes for the correct number of bond pairs and therefore
you will not be able to see the unshared electron pairs. An alternative approach is to use
balls with 4 holes for all atoms other than hydrogen, so that the octet (4 pairs of electrons)
will always be visible.
Typical Color Code for Molecular Model Sets
Atom
Color
Hydrogen
Carbon
Nitrogen
Oxygen
Fluorine
Chlorine
Sulfur
White
Black
Blue
Red
Green
Purple or Orange
Blue
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GISAT 112
THE ASSIGNMENT
1. Each pair of students should have a model kit. First, get acquainted with the
components of the kit. Note the holes in the various colored balls and their positions. If
there are two lengths of sticks, the shorter gray ones are for single bonds. The longer,
flexible gray ones are for double and triple bonds. (Don’t use the short white ones.)
2. Before building your models, complete the columns “Total Outer Electrons” and
“Lewis Structure” in the data sheet. (HINT: You’ll need to refer to a Periodic Table.)
3. Build models for each of the molecules listed on the data sheet.
a. Gather the kind and number of atoms required. (For example, to make the
CH4 molecule you will need one black ball and four white ones.)
b. To determine how many sticks (pairs of electrons) you will need, refer to
your Lewis structure and notice if the bonds are single, double, or triple.
c. Assemble the model by connecting the balls and sticks to match the
arrangement in your Lewis structure.
4. Use the models to fill in the information in the last column “Geometry”. You should
take time to think about (and write down in words and a diagram) the shape of each
molecule before proceeding to the next one. If you have questions, ask your instructor
or teaching assistant for help.
5. Save each model for reference (until you’ve completed the assignment.) When you’re
done with them, dissemble the models and return the pieces to the kit.
Questions to be Answered After Completing the Models
1.
2.
3.
4.
5.
6.
The tetrahedral shape is one of the most fundamental shapes in chemical
compounds. How would you describe it in words to someone who has not seen it?
The “octet” rule appears to be a very important rule governing the structures of
molecules. In light of your work with models, provide a simple explanation for the
importance of 8 electrons.
Explain in your own words why non-bonded electron pairs help determine the
shapes of molecules.
Do all of the assigned molecules obey the octet rule? If not, why (or in what way)
did the octet rule fail?
As a test of what you have learned, predict the shapes of (a) NF3 (b) H2S (c) Cl2O.
Models do not necessarily have to be physical objects. They can be 2-dimensional
drawings or even mental constructs. Cite one or more examples of such models
encountered outside of chemistry. Can you think of models that are used in your
own field of study or that you will use in your future career?
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GISAT 112
Name ____________________________________________________
Partner ___________________________________________________
Date___________
Section ______
Data Sheet- Molecular Models
Molecule
Total Outer
Electrons
Lewis Structure
Methane
CH4
Ammonia
NH3
Water
H2O
Oxygen
O2
Nitrogen
N2
Carbon Dioxide
CO2
Ozone
O3
* Linear, bent, triangular, pyramidal, or tetrahedral
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Geometry of the Atoms*
(Sketch structural formula)
GISAT 112
Name ____________________________________________________
Partner ___________________________________________________
Date ____________
Section ______
Data Sheet- Molecular Models (cont.)
Molecule
Total Outer
Electrons
Lewis Structure
CFC-12
CF2Cl2
CFC-22
CHF2Cl
Sulfur Dioxide
SO2
Carbon
Monoxide
CO
Nitric Oxide
NO
Nitrogen
Dioxide
NO2
Formaldehyde
CH2O
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Geometry of the Atoms
(Sketch structural formula)