Title: Modeling the electron Location in a Hydrogen atom
Purpose:
To determine where an electron is most likely when in its lowest energy state.
Introduction:
In the atomic models of the early 20th century, electrons were said to move around
the nucleus along specific paths, much as the planets move around the sun.
According to the Bohr model of an atom, the electron is located in a precise orbit
or energy level outside of the nucleus. Bohr labeled each energy level, and
consequently each orbit, by a quantum number, n. For the lowest energy level, or
the ground state, n = 1. This energy level corresponds to the orbit closest to the
nucleus. When the electron absorbs the appropriate amount of energy, it “jumps”
to a level of higher energy called an excited state. The excited states represent
larger orbits with the electron farther from the nucleus.
Experimental evidence has indicated that the precise position of an electron in an
atom cannot be known or predicted. Scientists can speak only of the probability of
finding electrons at various locations, not of their exact positions. Probability is a
measure of how often a certain event will occur out of a total number of events.
An electron cloud model provides a visual model of the probable behavior of an
electron in an atom. In the current electron – cloud model of the atom, it is
impossible to describe the exact positions of electrons; the model does describe the
probability or chance that electrons will be found in certain locations around the
nucleus.
The probability of finding an electron in various locations around the nucleus can
be pictured in terms of a blurry cloud of negative charge. The cloud is most dense
where the probability of finding the electron is highest. The cloud is least dense
where the probability of finding the electron is lowest. The density of an electron
cloud is referred to as electron density. Those regions or volumes of space of high
probability are said to have high electron density. Conversely, regions of low
probability are said to have low electron density. In summary, the electron - cloud
model represents the most probable location of an electron at a single instant.
The probability of finding electrons in certain regions of an atom is described by
orbitals. An atomic orbital is a region in space around the nucleus of an atom
where an electron with a given energy is likely to be found. Rather than drawing
electron clouds to represent orbitals, it is more convenient to draw the surface
within which an electron is found 90 percent of the time. A sphere can be drawn
that encloses 90 percent of the electron density. The probability of finding the
electron is the same for all points on the surface, and there is a 90 percent chance
of finding the electron within the sphere. In this investigation, you will use
probability to describe the location on an electron in an atom.
Procedure:
1. Roll the die, note which number appears, and use a single crayon to color in
a square according to the following instructions:
If the number 1 appears face up on the die, color in any square that is in ring 1
If the number 2 appears face up on the die, color in any square that is in ring 2
If the number 3 appears face up on the die, color in any square that is in ring 3
2. Repeat step 1, tossing the die and marking the rings for a total of 50 tosses.
3. Make sure to color the squares in a “scattered” pattern within that particular
ring. (Not clustered together).
Processing Data:
1. Count the number of colored blocks in each ring and record the number in
data table – I
2. Using a metric ruler, measure and record the diameter of each circle to 0.1
centimeters in data table – II
3. Calculate and record the radius and area of each circle in data table – II
[Area(circle) = π r 2 ]
4. Calculate and record the area of each ring in square centimeters ( cm2 ) in
data table – III. ( see equations )
5. Calculate and record the number of blocks in a given ring per Area of that
ring ( # blocks / Area (ring)) in data table – III
6. Determine the percent probability or chance of finding a block in each of the
rings using the following equation:
% = # colored blocks in ring / Total # of colored blocks x 100
Electron – Cloud Activity Report
Name: _________________
Period: ____
Questions:
1. Before you rolled or tossed the die, could you predict exactly what number
would appear?
2. Scientist use probability to predict the behavior of electrons in an atom.
What is probability?
3. What does each colored-in block on your diagram represent?
4. How does the number of blocks change as the distance from the center
increases?
5. Based on your results, which ring has the highest probability of having a
colored-in block?
6. If this was an atom, will there always be an electron in this ring? Why?
7. Which ring has the lowest probability of having a colored-in block?
8. Compare your diagram to a classmate’s. Are they identical? In what ways
are they alike or different?
9. How is this model similar to the structure of the Bohr Atomic Model?
10.How is this model different from the Bohr Atomic model?
11.Can the exact position of an electron in an atom be determined?
12.What can you know about the location of an electron?
13.Supposed you tossed the die 100 times. How do you think your results
would have compared with the results you obtained by tossing the die 50
times?
An atomic orbital is defined as the region within which an electron is found 90
percent of the time. On your plot, draw a circle within which 90 % of the
blocks are found.
14. 90% of (50 positions) = ______________
15. What is the distance of this new circle from its center? (radius) __________
16. What is the area of this circle? _________________
17. Which orbital do the results of this experiment best approximate?
Redesign the procedure in this investigation to model the behavior of the two
electrons in a helium atom.
Briefly describe the new procedure:
Can both of the electrons be in the same location at different times? ___________
Describe the modified results you might expect?
Conclusion:
Explain what this activity taught you about electron positioning and the atom.
Sample Calculations: ( Show all work in significant figures)
Area of Circle
Area of ring
# blocks # / Area of ring
Percent probability in a ring