Name _________________________
CH-180
Scientific Models
We have seen how scientists apply the scientific method to draw conclusions about the
physical world. The hypotheses that scientists propose to explain their observations may take any
number of forms: hypotheses may be written sentences, mathematical formulas, or even threedimensional models. In this experiment, we see how a model can be constructed and used to
express a hypothesis about the arrangement of rods and washers inside a box that cannot be opened.
Now, before you say "This is silly; it has nothing to do with chemistry!” read on, and learn how this
experiment uses thinking not much different from that used by the scientists who discovered the
internal structure of atoms without ever actually seeing an atom’s insides!
Reflections on Models
From Chemistry, Industry, and the Environment, by James N. Lowe, University of the South, Wm.
C. Brown Pub. Dubuque IA, © 1995.
Which model is correct? Each model closely represents some features of a molecule
and is incorrect on others. Molecular models are designed to select and emphasize the
features of real compounds. By emphasizing some features and excluding others, a model
may be simple or complex. Scientific theories are models of reality. Consider Lewis
structures -- are electrons really particles that can be represented by dots? Lewis structures
ignore the wave character of electrons in atoms; however, they are very useful for an
understanding of covalent bonding. Is Dalton's atomic theory an abstraction? It is
formulated to account for combining weights and ratios? Properties of a compound such as
bonding, taste, and smell are totally excluded. The proper question might be, "Which
model is more useful for which given purpose?"
Scientists often choose to emphasize the utility of a model rather than its accuracy. A
model may or may not be useful for a given purpose. For example, either Dreiding or
spacefilling models would be preferred for estimating the dimensions of a complicated
molecule. However, for illustrating Lewis structures, the simple ball and stick models
prove superior.
It is apparent that a plastic peg is not an electron pair, or that a bent metal tube is not
water. Molecular models are clearly artificial. A model is an imperfect representation of a
part of reality. It simplifies. It can be used to gain insight, and it can be manipulated to
predict new relationships. A good model may have high predictive power, or it may serve
to relate many seemingly different phenomena in an intellectually satisfying way. The
ultimate test of any model is always the degree to which it agrees with experimental
observations of the properties of real substances. The greater the agreement, the more
useful the model.
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Models of Individual Atoms
Every atom consists of a nucleus surrounded by one or more electrons (negatively charged
particles). The nucleus consists of a number of protons (positively charged particles) and, usually,
neutrons (particles with no electrical charge). You may have seen a model of an atom that looked
something like this:
The "orbits” actually form spheres in which the electrons exist. To us, the electrons would appear
to be smeared over the sphere's entire volume. You've probably accepted this model of an atom as
being true, but how do you know it is true? A single atom is on the order of 1  10-8 cm in
diameter -- too small for any kind of microscope to detect -- so it is impossible for you, or anyone
else, to have actually seen inside a single atom. Nevertheless, educated people generally do not
question the statement that an atom is composed of electrons around a nucleus consisting of protons
and (usually) neutrons. Why?
Even though scientists have never seen inside an atom, they have devised experiments in
which the behavior of atoms were observed and measured. From the observations and
measurements made during these experiments, scientists were able to construct a model of an atom
that proved to be consistent with all the observations and measurements. It is this model of the
atom that is pictured above, and since it agrees with all the observations, we accept it as the truth.
Of course, it is possible that there are other, as yet unrealized, models that also agree with all
observations and measurements, but such models are not likely to be found, since there are many
thousands of observations and measurements with which the models would have to agree. We
therefore have great faith in the truth of our model of the atom. We firmly believe that an atom is
made up of electrons around a nucleus composed of protons and (usually) neutrons!
In this experiment, you will make a number of observations on a sealed box that contains three
rods and up to four washers. The washers may be skewered on one or more of the rods, or they
may be loose in the box. After you have made your observations, you will draw a model that
describes the internal structure of the sealed box. Your model should show the number of washers
in the box and the arrangement of the washers on the rods. Like the scientists who constructed a
model of the atom without ever seeing the inside of an atom, you will construct a model of the box
without ever seeing the inside of the box.
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Experimental Procedure
1. Work individually. However, other students may have boxes of the same type (letter
designation) as yours. Make use of reproducibility of results by comparing your conclusions with
those of your classmates.
2. Select two boxes labeled by a letter: A, B, C, etc. Both boxes should have the same label on
them: Don’t mix up the letters.
Write the letter designation of your boxes here:
One of the boxes will be used to make observations from which your model will be constructed.
The other box will be used to test your model. If the behavior of the second box is consistent with
the model, the model gains credibility. If the second box behaves differently than the model
predicts, then your model loses credibility, and you may wish to revise your model.
Note: Since you only have two boxes, you must be very careful not to waste your opportunities for
observation. After your two boxes are gone, you won't have another chance to make observations!
3. Make some mass measurements. At the balance caves, you will find EMPTY boxes (identical to
your boxes, except that they contain no washers, and the number of rods may be different) and
washers. Use these resources, as well as your team's box, to determine the number of washers that
must be in your team's box. Use the space provided to write down your data and any calculations
that you might want to make:
The number of washers is: _______________
4. Now determine the arrangement of the washers in the box. To do this, you will want to perform
several more operations on the box, making observations after each operation is performed.
Without opening or damaging the box, what kind of operations could you make on the box to help
you determine where the washers are? Can you tell anything by shaking the box? Can you tell
anything by listening to the box as the rods are slowly pulled out, one at a time? Provide your
comments below:
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5. Draw a model of the rods and washers that is consistent with your observations.
6. Test your model by making measurements and observations on your second box. List all
operations and observations that could help you construct or validate your model.
Is your new data consistent with your model? If you answered "No", draw a new model below,
based on your new data.
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7. Describe in sentence format how your model represents one or more inferences about the internal
structure of your black boxes.
8. Question. Models are very important to scientists of all kinds. Select the statement about
scientific models that is false. Be sure to analyze each choice, and provide a justification for
accepting or rejecting each response.
a. A major use of any scientific model is to predict future behavior.
b. A direct relationship exists between the accuracy of a model and the model's complexity.
c. A mathematical equation may be a model.
d. A model communicates our scientific observations to other people.
e. The most complicated model is always the best model to use when solving problems.
Analysis:
a.
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b.
c.
d.
e.
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9. Question. Provide one example of a model that has helped you to learn how something works.
Use no more than two sentences to describe the model, and no more than two additional sentences
to explain what the model was able to teach you.
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Turn in the entire handout as your report for this experiment. Be sure that your name is on the front
page.
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Why the Quantum Atom is Difficult to Model
Ultimately, the observations of Dalton, Crookes, Rutherford, Bohr, etc. led to the Quantum
Model of the atom, in which energy is quantized, and electrons exhibit wave nature. The model is
so far from our ordinary experience with matter, that nearly everyone has difficulty comprehending
it. Even our best analogies fall short, leaving us to imagine, science-fiction style, how the quantum
atom works.
Still, we try for understanding. We might think of energy quantization like moving from
the first to second floor in a building. Basically, we could complete the trip two ways (on foot): by
stairs or by a ramp. Think of our vertical position when we use each method:
Energy quantization corresponds to the stairs, in which only certain vertical positions are possible.
The wave nature of matter is tougher. Imagine an airplane propeller: with the engine off,
the propeller is easy to locate; start the plane, and the propeller’s location is no longer precisely
known: