A.8 MODELING MATTER: PICTURES IN THE MIND
You live in a macroscopic world—a world filled with large-scale (“macro”), readily observed
things. As you experience the properties and behavior of bulk materials, you probably give
little thought to the particulate world of atoms and molecules. If you wrap leftover cake in
aluminum foil, it is unlikely that you think about how the individual aluminum atoms are
arranged in the wrapping material. It is also unlikely that you consider what the mixture of
molecules making up air looks like as you breathe. And you probably seldom wonder about
atomic and molecular behavior when you observe water boiling or a rusted iron nail.
Nevertheless, having a sense of how atoms and molecules might look in elements and
compounds and behave in reactions is a useful tool for understanding the chemistry that is
occurring. To develop this sense, it is useful to construct and evaluate models, or
representations, of atoms and molecules. This activity will give you practice in observing,
interpreting, evaluating, and creating visual models of matter, “pictures in the mind,” at the
particulate (atomic and molecular) level. You will also practice using and interpreting
chemical symbols and formulas that make up the language of chemistry.
Use the following key to draw and interpret models of elements and compounds containing
hydrogen, carbon, nitrogen, oxygen, and chlorine:
Sample Problem 1: Draw a particulate level model of carbon dioxide (CO2).
(Hint: Carbon is in the center of the molecule.)
The chemical formula CO2 shows that carbon dioxide contains one carbon atom and two
oxygen atoms. We’re told that carbon is in the middle, so we’ll draw a model that looks like
this:
Sample Problem 2: Write the chemical formula for the element represented by this
particulate level model:
Using the key above, we identify the atoms as nitrogen. Two nitrogen atoms make up the
molecule, so the chemical formula for elemental nitrogen must be N2.
Now it’s your turn to create and interpret particulate-level models of elements and
compounds.
1. In Investigating Matter A.1, you mixed ammonia solution and vinegar solution.
a. Draw a molecular-level model of ammonia, which has chemical formula NH3.
b. The substance in vinegar solution that reacted with the ammonia is called acetic acid.
Below is a molecular level model of acetic acid.
Write its chemical formula. (Hint: Write the symbol for carbon first.)
2. Draw molecular-level models of these compounds:
a. Methane (CH4): The primary component of natural gas.
(Hint: Carbon is in the center of the molecule)
b. Water (H2O): The most abundant substance on Earth. (Hint:
Oxygen is in the center of the molecule.)
c. Elemental chlorine (Cl2): Poisonous gas used to make plastics and disinfect water.
d. Hydrogen chloride (HCl): Important industrial chemical also sometimes used in pure
form to etch semiconductor crystals.
e. Carbon monoxide (CO): Toxic gas used in many industrial processes, including
purifying nickel.
3. You melted and burned paraffin wax in Investigating Matter A.1. Write the chemical
formula of paraffin wax given its model below.
So far, none of the models you have drawn or interpreted are of metals. How can you
visualize solid metals? Figure 1.12 (page 39 in your textbook) contains a photograph of
aluminum foil. Although we use the chemical symbol “Al” to represent aluminum, what we
visualize is actually a large collection of aluminum atoms, like this:
This model, like all models, has some limitations. It is really only showing one small
segment of the aluminum foil; the atoms extend outward from what is shown. The main
point, though, is that we still only use a single symbol, Al, without any subscripts, to
represent an entire collection of aluminum atoms. Think about how that is different from the
molecular compounds you worked with in Questions 1-3.
4. In Section A.4, you learned about the composition of current pennies. Re-examine
Figure 1.9 (page 35 in your textbook), which shows a cross section of a penny.
a. What chemical symbols represent copper and zinc?
b. Zero in on a small area of the cross-section in Figure 1.9. Draw a particulate- level model
of what you think the copper and zinc atoms look like in that small area.
c. Describe in words how you decided to draw your model.
5. In chemistry, we often try to relate the atomic and molecular structure of materials to their
properties. One property of metals is that they can be pounded into thin sheets.
a. Consider the particulate-level model of aluminum foil and other metals. Does this model
help you to explain why metals can be pounded into thin sheets?
b. Describe your reasoning.
Investigating Matter A.1 provided you with opportunities to observe matter and its changes
on a large scale. In fact, you observe every phenomenon you encounter using your senses
of sight, touch, smell, and hearing. One of the most important ways of thinking like a chemist
is to take in what you observe and visualize what is happening at a level you cannot see—
the level of atoms and molecules. Another skill is using chemical symbols and formulas to
represent matter and its changes. As you continue to read, challenge yourself
to visualize what the atoms and molecules might look like when you encounter a symbol or
formula. Do the same when you look at a photograph. While these ways of thinking are often
taken for granted by experienced chemists, they are difficult to develop. You will have
opportunities to practice these skills throughout this course, but you will become even more
skilled if you challenge yourself to think this way any time you are making an observation in
everyday life.