DRAFT DOCUMENT
LAB 11
ELECTRON DISTRIBUTION USING PEAS
Could you determine the exact position and momentum of a baseball as it soared through
the air? Of course, you could—by taking timed series of snapshots of the baseball as it
moved. Why then can’t scientists follow a similar procedure to determine the position
and momentum of an electron?
You can see a moving baseball or its image because of the light bouncing off the
baseball. The effect of light on either the position or the momentum of the baseball is
negligible. By contrast, an electron has such an extremely small mass that light disturbs
it in an unpredictable way. How then can the position and momentum of an electron be
determined?
Knowledge of the behavior of electrons in the atom comes from theoretical work done in
the 1920s by the German physicist Werner Heisenberg (1901-1976) and the Austrian
physicist Erwin Schrodinger (1887-1961). Heisenberg postulated that it was impossible
to determine exactly both the position and momentum of an electron at the same instant.
Heisenberg deduced that the more precisely you know the position of an electron, the less
certain you are about its momentum, and vice versa. Because its exact position and
momentum can never be established at any given time, the exact path of an electron
around the nucleus cannot be determined. Instead the quantum-mechanical model of the
atom gives the probabilities of finding an electron in a particular region around the
nucleus.
In this investigation, you will model the probable locations of electrons around the
nucleus of an atom. You will use peas to represent electrons to help you visualize
regions of high and low electron density.
OBJECTIVES
When you have completed this activity, you should be able to:
1. Compare population density of a region and relate to the quantum mechanical
model of the atom.
2. Observe the effects of probability on electron distribution of an atom.
3. Describe Heisenberg’s Uncertainty Principle.
in order to explain the arrangement of sub-atomic particles.
Adapted and edited from Chemistry Connections to Our Changing World, Prentice Hall, 1996
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LAB 11
MATERIALS
goggles
15 metric rulers
15 pairs scissors
15 compasses
1-lb bag of dried whole peas
15 plastic cups (100 peas each)
PROCEDURE
15 pieces of circular filter paper, 15-25 cm in diameter
15 marking pens
15 tripod ring stands
15 sheets of butcher paper/art paper
15 beakers, 150-Ml
CAUTION: Wear your goggles and lab apron at all times.
1. Put on your goggles. Fold the filter paper in half and then fold it again into
quarters. Using the ruler, measure up 1.5 cm from the closed point of the paper
and make a mark. Make a small hole in the bottom of the folded filter paper by
cutting at the 1.5 –cm mark with the scissors. Insert the cut filter paper into the
ring of the tripod ring stand to create a funnel with the small hole at the bottom.
2. Use the compass to draw a circle with a radius of 3 cm in the center of a large
sheet of paper. Then draw four more concentric circles 3 cm apart, around the
first circle. Number the rings 1-5, starting from the center.
3. Mark the center of the innermost circle with a large dot. Let this dot represent the
nucleus of the atom. Place the ring stand so that the hole in the filter paper is
exactly above the large dot, or nucleus, as shown in Figure 1.
Figure 1
Adapted and edited from Chemistry Connections to Our Changing World, Prentice Hall, 1996
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LAB 11
4. Count out 100 dried peas and place them in a plastic cup. Pinch closed the hole at
the base of the filter paper and add the 100 dried peas, or “electrons,” to the filter.
Let go of the filter, allowing the peas to fall through the small hole onto the target
beneath the ring stand. If the peas jam up in the filter, push them gently to keep
them moving.
5. Count the number of peas in each ring around the nucleus, as well as any that fall
outside the rings. Record the data in the table, beginning with the innermost ring
number 1.
6. Gather up the peas and place them in the plastic cup. Return all equipment to the
supply area. Clean up your work area and wash your hands before leaving the
lab.
DATA
DATA TABLE: Distribution of Peas
Ring
1
2
3
4
5
Outside of rings
Distance from Nucleus
Number of Peas in Ring
3 cm
6 cm
9 cm
12 cm
15 cm
Beyond 15 cm
Adapted and edited from Chemistry Connections to Our Changing World, Prentice Hall, 1996
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DRAFT DOCUMENT
LAB 11
Using the data in the table, graph the number of peas vs. the distance from the nucleus.
This can be a “best fit” curved line graph.
ANALYSIS AND CONCLUSIONS
1. Judging from your graph, in which region would you be most likely to find
electrons? In which region would you be least likely to find electrons?
2. Were you able to determine the exact path by which each pea (electron) arrived at
its position on the target? How does this finding relate to the quantum theory?
3. Which orbital do the results of this experiment best approximate?
Adapted and edited from Chemistry Connections to Our Changing World, Prentice Hall, 1996
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LAB 11
4. What would your graph look like if you had used 200 peas instead of 100?
5. Write a brief paragraph explaining how the quantum-mechanical model of
electron distribution in an atom differs from the Bohr model.
6. Why do you think that many people persist in visualizing the atom according to
the out-dated Bohr model as opposed to accepting the quantum-mechanical
model?
EXTENSION
Complete a one-page summary to compare and contrast the electron distribution
probability graph to the galaxy graphic (See attached).
Adapted and edited from Chemistry Connections to Our Changing World, Prentice Hall, 1996
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DRAFT DOCUMENT
LAB 11
PRE-LAB: ELECTRON DISTRIBUTION USING PEAS
Read the entire laboratory investigation and the relevant pages of your text book. Then
answer the questions that follow.
1. Why isn’t it possible to determine the exact path of an electron in an atom?
2. What does the quantum-mechanical model of the atom tell you about the location
of an electron in an atom?
3. What do the peas represent in this investigation? What setup do you use to
distribute the peas around the nucleus?
4. How will you process your data to illustrate the probabilities of finding electrons
in particular region around the nucleus?
Adapted and edited from Chemistry Connections to Our Changing World, Prentice Hall, 1996
74