Prescriptive Activities: Facet Cluster 1.4: PF#60
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Prescriptive #60: The student believes that electrons reside in "shells" made up of some
other material.
Common conditions where facet occurs: Students are exposed to many different models of
the atom and exposed to many ways of describing where the electrons are and what they
are doing. They are told about electron shells, energy levels and they see many diagrams of
how to do electron configurations. Oftentimes, they believe that the models they see are
how the electrons are arranged or organized in the real world. A common misconception is
that of the electrons residing on shells in the atoms or that they are affixed to some
structure. This may be caused by the language that we use when describing electron
energy levels, orbitals and sometimes shells. Some students believe that electrons reside
on shells, like resting on an egg shell. A way to ameliorate this problem is to discuss the
model of the atom with students as presented here.
Materials:
 Student handout
Directions: Read and complete the activities/questions below
The model of the atom has changed over time as more experiments have been performed
and more evidence has been collected. Consider the pictures below that show how the
model has changed over time. The models have changed as we have gotten more
information about the nature of the atom.
An early model of the atom was the indivisible ball “atoms” . This idea was promoted
Ancient Greek philosophers such as Democritus. The belief then was that matter was
composed of small indivisible particles.
Much later came the plum pudding model promoted by Thomson. In this model the
negative charge of the electrons and the positive charge of the nucleus were spread out in
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Prescriptive Activities: Facet Cluster 1.4: PF#60
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the atoms. It was an improvement over the early atom motel because it could explain how
atoms might be different from one another and explained the charges.
The Rutherford model of the atom designed after the gold foil experiment came next. In
this model, there is a nucleus with a positive charge and a large area outside of the nucleus
that made up the rest of the atom and the electrons could be found in the large area. This
model explained why most particles would go straight through a sheet of gold foil and only
some would bounce back. The atom must be mostly empty space and the nucleus must be
positively charged and in the center.
The Bohr model of the atom followed. In the Bohr model, nucleus was the center but now
the electrons “lived” in energy levels. This addition to the Rutherford model helps explain
why only certain energies of light where absorbed or released by atoms (e.g., the hydrogen
spectrum or why certain materials are used to make particular colors of fireworks).
However people started believing that these energy levels or shells were like real life
things. Like there is an egg shell in the atom where the electron lives. This is not true,
because we know now that the electrons do not rest on shells or other structures in the
atom. The electron is there because it is attracted to the nucleus and is repelled by other
electrons. There are no shells or rings structures in the atom only places where we might
find the electron.
The idea about shells is useful because when we organize electrons in an atom, they are
grouped together by the amount of energy they have. These amounts of energy are we call
shells. So they are not structures but an amount of energy an electron might have.
We now have an idea about where the electrons can be found in the space around the
nucleus and we can say that we are most likely to find then one place and not another, but
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Prescriptive Activities: Facet Cluster 1.4: PF#60
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we cannot know for sure where the electron is. This is called the cloud model of the
electron or the quantum model. The cloud shows where you are likely to find the electron.
The darker the cloud, the more likely you are to find the electron there. If the cloud is
lighter, then you are less likely to find the electron there. A wrong idea is to think that the
electrons are like rain in the clouds in the sky. This is wrong because the electron clouds
do not exist, it is just a model showing you where you MIGHT find the electrons. This is
where we are now in our model.
Discussion Questions:
1. Draw each of the models above and label the nucleus and electrons. Note that in the
cloud model you can only say that they might be found in a certain place.
2. Find some examples of the Bohr model of the atom that are seen on internet. Collect
some samples of the Bohr model of the atom you find and state where you found it.
3. The shell model of the atom looks convincing. How is the cloud model of the atom
different than the shell model of the atom (Bohr model)?
4. When is the shell model useful and when is the shell model a problem?
Resources
You can check out some sample models of the atom at
http://phet.colorado.edu/simulations/sims.php?sim=Models_of_the_Hydrogen_Atom
While this activity is about light interacting with the atom, it shows you different ways to
visualize the model of the atom. The last two models are quantum models.
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