Radioactive Decay – Grade 11
Ohio Standards
Connection:
Life Sciences
Benchmark D
Relate how biotic and
abiotic global changes have
occurred in the past and
will continue to do so in
the future.
Lesson Summary:
Students will understand how geologic time can be
estimated by observing rock sequences and using fossils to
correlate these sequences. They will examine relative
dating and then model radioactive decay in fossils. Student
learning will be assessed by writing a news article on the
determination of the age of a “new” fossil that has been
discovered.
Estimated Duration: Two hours
Indicator 14
Describe how geologic
time can be estimated by
observing rock sequences
and using fossils to
correlate the sequences at
various locations.
Recognize that current
methods include using the
known decay rates of
radioactive isotopes
present in rocks to measure
the time since the rock was
formed.
Commentary:
In this lesson decay rates of radioactive isotopes are
modeled to illustrate how the age of fossils are determined.
Students are to use the information from the lab exercise
and blend it with the scenario that is given to write a
cohesive article on relative dating and radioactive decay.
Radioactive decay is modeled in this activity using colorchange markers instead of the traditional candy pieces. This
is done to show students that the fossil doesn’t disappear
(like the candy does), but it does change. The lesson should
follow a lesson on relative dating, as it presumes that
students are already familiar with dating fossils using rock
strata.
This lesson was field tested by Ohio teachers. Some of the
comments about this lesson were:
“Excellent post-assessment – uses many skills and much
knowledge to write a readable (by the average person)
article.”
“I like the idea of an alternate assessment.”
Pre-Assessment:
1. Organize the students into groups of three or four.
2. Distribute Attachment A, Pre-Assessment, to students.
This worksheet contains two graphics with index fossils
that students must analyze.
3. Have students work in small groups (no more than four
students) to complete the analysis.
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Radioactive Decay – Grade 11
Scoring Guideline:
 Assess student understanding with Attachment B, Pre-Assessment Key. Use student
responses to determine prior knowledge. Adjust instruction to meet student needs.
 Students need to exhibit understanding that placement of fossils is key to determining
their age, and that similar index fossils found in different areas are the same age,
regardless of the depth at which they are found.
Post-Assessment:
Students will be given a scenario for which they will have to write a news article. They will
act as science reporters for their local paper covering a story on the discovery of a new fossil.
In their article, they will explain how scientists will determine the age of the fossil. See
Attachment C, Post-Assessment.
Alternate post assessment: Students can act as newscasters broadcasting their story about the
new fossil.
Scoring Guidelines:
Assess student understanding with Attachment D, Post-Assessment Scoring Guideline.
Instructional Procedures:
1. Encourage students to use their critical thinking skills to analyze the pre-assessment
activity about relative dating.
2. Clarify any misunderstandings that students have about relative dating. At this point
introduce the concept of radioactive dating (also called absolute dating, see Attachment
G, Teacher Notes on Radioactive Dating).
3. Have students complete a hands-on activity that models the decay of the radioactive
isotopes uranium-235 and potassium/argon to reinforce their understanding of how
scientists use isotopes to determine the age of fossils. See Attachment E, Radioactive
Decay Activity.
Upon completion of the activity on radioactive dating, give students the opportunity to
ask clarifying questions about anything that they don’t understand. The answer key for
radioactive dating activity is Attachment F, Radioactive Decay Activity Key.
4. Question the students about relative and radioactive dating to determine a basic level of
understanding. This is to be used as a formative assessment. If the students understand
radioactive decay and relative dating, the post-assessment is to be given at this time. If
not, re-teach.
5. Assign the post-assessment.
Differentiated Instructional Support:
Instruction is differentiated according to learner needs to help all learners either meet the
intent of the specified indicator(s) or, if the indicator is already met, to advance beyond the
specified indicator(s).
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Radioactive Decay – Grade 11
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Kinesthetic and visual learners will benefit from the lab activity involving coloring the
shell and graphing the data. Auditory learners will benefit from oral instruction and small
group work.
If students have trouble with relative dating according to position of fossils, have them
make a three-dimensional model in class. One student can keep track of time to help
students see that the organisms at the bottom are fossilized first.
Have students work in pairs on the news article and document the contribution of each
partner.
Assign students to work in mixed ability groups on the radioactive decay lab, with each
group member having an assigned role. Students who are visual learners can color.
Students with logical thinking abilities can keep time or set up the data table.
Extensions:
 Have students research the half-lives of several other radioactive isotopes, such as
uranium.
 Investigate several other non-isotopic methods of dating fossils (paleo-magnetism or
tree-rings).
Homework Options and Home Connections:
 Make observations of rock strata in the community (e.g., along highways), and determine
where the oldest and youngest rock will be found.
Interdisciplinary Connections:
English Language Arts
 Writing Process
Benchmark A: Formulate writing ideas, and identify a topic appropriate to the purpose
and audience.
Indicator 1: Generate writing ideas through discussions with others and from printed
material, and keep a list of writing ideas.
Indicator 4: Determine a purpose and audience and plan strategies (e.g., adapting focus,
content, structure and point of view) to address purpose and audience.
Materials and Resources:
The inclusion of a specific resource in any lesson formulated by the Ohio Department of
Education should not be interpreted as an endorsement of that particular resource, or any of
its contents, by the Ohio Department of Education. The Ohio Department of Education does
not endorse any particular resource. The Web addresses listed are for a given site’s main
page, therefore, it may be necessary to search within that site to find the specific information
required for a given lesson. Please note that information published on the Internet changes
over time, therefore the links provided may no longer contain the specific information related
to a given lesson. Teachers are advised to preview all sites before using them with students.
For the teacher:
Stopwatches, color-change markers, graph paper, shell/grid handouts.
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Radioactive Decay – Grade 11
For the students:
Stopwatches, color-change markers, graph paper, shell/grid handouts.
Vocabulary:
 half-life
 radioactive decay
 radioactive isotope
 fossil
 relative dating
 uranium-235
 potassium/argon dating
Technology Connections:
 Have students do Internet research on index fossils in Ohio.
 Allow students to use spreadsheet software to graph their data from the lab exercise.
Research Connections:
Marzano, R., Pickering, D., Pollock, J. Classroom Instruction that Works: Research-based
Strategies for Increasing Student Achievement, Alexandria, Va.,: Association for Supervision
and Curriculum Development. 2001
Nonlinguistic representations help students think about and recall knowledge. This
includes the following:
 Creating graphic representations (organizers);
 Making physical models;
 Generating mental pictures;
 Drawing pictures and pictographs;
 Engaging in kinesthetic activity.
Cooperative learning groups have a powerful effect on student learning. This type of
grouping includes the following elements:
 Positive interdependence;
 Face-to-face promotive interaction;
 Individual and group accountability;
 Interpersonal and small group skills;
 Group processing.
General Tips:
 Students may have trouble with drawing a data table. If so, provide one for them. See
example below.
 Allow students to work in pairs or groups with a maximum of four for the coloring
activity, but require that each student complete his or her own graph of the data. They
will all look similar, but each student will receive practice in graphing.
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Radioactive Decay – Grade 11
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Show students layers of rock strata, if possible, so that they have a concrete example of
the layering and relative dating.
Color change markers work similar to pH paper. When one color marker is used, and the
“magic wand” is colored directly on top of the original color, a new color is produced,
similar to the way that pH paper changes color in acids or bases.
Color change in the markers can be done using a cotton swab and vinegar solution,
instead of the “magic wand.” This will make one packet of markers last longer and be
used by more students.
Example of a data table:
Time in seconds
“Radioactivity”
Attachments:
Attachment A, Pre-Assessment
Attachment B, Pre-Assessment Key
Attachment C, Post-Assessment
Attachment D, Post -Assessment Key
Attachment E, Radioactive Decay Activity
Attachment F, Radioactive Decay Activity Key
Attachment G, Teacher Notes on Radioactive decay
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Radioactive Decay – Grade 11
Attachment A
Pre Assessment
Name _____________________
Relative Dating
Directions: The two drawings below represent rock strata from two different areas in Ohio.
Study them and answer the questions that follow:
1. Which layers in Figure 2 are the same age as those in Figure 1?
_____________________
2. Which fossil layer in Figure 1 is the OLDEST? _________________________
3. Which fossil layer in Figure 2 is the youngest? __________________
4. Which fossil layer is older? Those in layer C or in layer I? ___________________
5. Which fossil layer is younger? Layer F or layer K? _____________________
6. Based on this exercise, what does “superposition” mean?
7. If a trilobite (layers D and I) is considered an index fossil for 240 million years ago,
what fossil layers are more than 240 million years old? Explain your answer.
8. Which layers are less than 240 million years old? Explain your answer.
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Radioactive Decay – Grade 11
Attachment B
Pre-Assessment Key
1. Which layers in Figure 2 are the same age as those in Figure 1?
A and H are the same age.
D and I are the same age.
E and K are the same age.
2. Which fossil layer in Figure 1 is the OLDEST?
Layer A
3. Which fossil layer in Figure 2 is the youngest?
Layer K
4. Which fossil layer is older? Those in layer C or in layer I?
Layer C
5. Which fossil layer is younger? Layer F or layer K?
Layer F
6. Based on this exercise, what does superposition mean?
Answers will vary, but students should mention something about younger fossil layers
being above the older layers.
7. If a trilobite (layers D and I) is considered an index fossil for 240 million years ago, what
fossil layers are more than 240 million years old? Explain your answer.
In figure 1, layers A, B and C are older than 240 million years because the trilobite fossil
is found above them. In figure 2, layers G and H are older because the trilobite is found
above them. Anything lower than the index fossil will be older, and anything above the
index fossil will be younger.
8. Which layers are less than 240 million years old? Explain your answer.
In figure 1, layers E and F are younger, and in figure 2, layers J and K are younger.
These layers are found above the trilobite fossil, which is the index fossil for 240 million
years ago.
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Radioactive Decay – Grade 11
Attachment C
Post-Assessment
Name ___________________________
“Extra, Extra, Read All About It”
NEW FOSSIL DISCOVERED
A new fossil was recently discovered in northeast Ohio. As the science reporter for your local
paper, you are asked to visit the site of the discovery and write an article on determining the
age of the fossil. Upon visiting the site, you are provided with the following information
about the fossil:
 Scientists think the organism is some type of mollusk.
 It precedes trilobites (an index fossil in Ohio).
 It is similar to a crinoid in shape.
Your editor informs you that the focus of your article should be on determining the age of the
fossil. It needs to be written in terms that all people (not just scientists) can understand. It
needs to be approximately 500 words.
In writing a good news story, remember the following items should be in the first two
paragraphs: Who? What? Where? When? Only after these points have been determined
should you go into the “meat” of your story – determining the age of the fossil.
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Radioactive Decay – Grade 11
Attachment D
Post –Assessment Scoring Guideline
Students work will be scored according to the following rubric:
Parameter
Level 4
Level 3
Level 2
Scientific ideas
Scientific ideas Scientific ideas
Depth of
about rock
about rock
Understanding about rock
sequences, relative
sequences,
sequencing,
dating and
relative dating
relative dating
radioactive isotopes and radioactive and radioactive
are accurate,
isotopes are
isotopes have
thoughtfully
accurate and
occasional
explained .
linked to major inaccuracies or
Patterns and trends
scientific
is simplified.
in dating methods
themes or
Patterns or
are discussed and
concepts.
trends in
extended.
Patterns and
dating methods
Scientific
trends in dating are implied.
connections are
methods are
Scientific
correctly discussed. identified.
connections
Scientific
may be
connections are implied.
identified.
Communication Information about
rock sequencing,
relative dating and
radioactive isotopes
in communicated
clearly and
precisely but may
also include
inventive/expressive
dimensions.
Presentation is
effectively focused
on content material.
Information
about rock
sequencing,
relative dating
and radioactive
isotopes is
communicated
clearly.
Presentation is
focused on
content
material.
Level 1
Scientific
information
about rock
sequencing,
relative dating
and radioactive
isotopes has
major
inaccuracies or
is overly
simplified.
Patterns and
trends in
dating methods
are unclear or
inaccurate.
Scientific
connections
are unclear or
absent.
Information
Information
about rock
about rock
sequencing,
sequencing,
relative dating relative dating
and radioactive and radioactive
isotopes has
isotopes is
some clarity.
unclear.
Presentation
Presentation
has some focus lacks focus on
on content
content
material.
material.
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Radioactive Decay – Grade 11
Attachment E
Radioactive Decay Activity
Name_______________________
Half-life is the term used to describe the amount of time it takes for half of an element to
decay. It is commonly used to determine the age of rock that houses fossils, which will in
turn indicate the age of the fossils themselves. Scientists know the half-life of known
chemicals such as uranium-235 or potassium-argon, and they can use this to help determine
the age of fossils. In this activity you will model radioactive decay in a “fossil.”
Materials: Two “fossil” diagrams, color change markers, graph paper, stopwatch
Procedure:
1. Create a data table on a separate sheet of paper. Label the first column “time in
seconds” and the second column “uranium-235 decay.”
2. Using one of your color change markers, color the entire numbered grid on your first
shell diagram.
3. Label your first fossil shell diagram “uranium-235 dating.”
4. Ask one group member to act as timekeeper. Using your “magic wand” marker, color
half of your grid every 30 seconds. You should start coloring at square #1 and color
left to right from the upper left hand corner. Record the time in the “seconds”
column, and the number of squares that you color in the “uranium-235 decay”
column. Repeat every 30 seconds, coloring half of the “unchanged” portion of the
grid until you have colored as much of the grid as you can color. USE THE 30
SECONDS TO COLOR EACH HALF of the grid.
5. Create a second data table next to the first one that you made earlier. Label the first
column “Seconds” and the second column “potassium-argon decay.”
6. Using a different color change marker, color the entire numbered grid on your
SECOND shell diagram.
7. Label this diagram “Potassium-Argon Dating.”
8. Select a different group member to be timekeeper. Using your “magic wand” again,
color half of the grid every 45 seconds. Record the time in the “seconds” column,
and the number of squares that you color in the “potassium-argon decay” column.
Repeat every 45 seconds, coloring half of the “unchanged” portion of the grid, until
you have colored as much of the grid as you can color. USE THE 45 SECONDS TO
COLOR EACH HALF of the grid.
9. Make ONE graph with both sets of data on it.
10. Answer the analysis questions that follow.
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Radioactive Decay – Grade 11
Attachment E (continued)
Radioactive Decay Activity
Data analysis questions:
1. When you used the magic wand, what happened to the original color of each grid?
How does this model radioactive decay?
2. Why did we do two trials with two different chemicals (Uranium-235 and potassium
argon)?
3. When we used the magic wand to color the grid, did the numbers on the grid go
away? What do you think this represents if the grid were a real fossil?
4. Look at your graph. Compare/contrast the two curves on it. How are they alike?
How are they different?
5. What do the differences in the curves on the graph represent?
6. What does the color change represent?
7. Does the entire grid ever change to the second color? Explain.
8. In your own words, define half-life.
9. The half-life of uranium-235 is 704 million years, not really 30 seconds like it was in
your lab activity. Using your graph, determine the age of a fossil with “49 squares”
remaining in both seconds AND years, using the half-life.
10. The half-life of potassium-40 is 1.25 billion years, not really 45 seconds like it was in
your lab activity. Using your graph, determine the age of a fossil with “36 squares”
remaining in both seconds AND years, using the half-life.
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Radioactive Decay – Grade 11
Attachment E (continued)
Fossil Diagram
(Make 2 copies)
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Radioactive Decay – Grade 11
Attachment F
Data Analysis Questions Key
1. When you used the magic wand, what happened to the original color of each grid?
How does this model radioactive decay?
It changed to another color. In radioactive decay, energy is lost, the fossil remains, it
is just different than it was before.
2. Why did we do two trials with two different elements (uranium-235 and potassium
argon)?
There is more than one type of element that has measurable half-life, and can be used
to date fossils.
3. When we used the magic wand to color the grid, did the numbers on the grid go
away? What do you think this represents if the grid were a real fossil?
No, the numbers are there in the fossil shell. They represent the fossilized shell.
4. Look at your graph. Compare/contrast the two curves on it. How are they alike?
How are they different?
They have different half-lives. Potassium-argon’s half-life is longer than that of
uranium-235. Each curve is similar in shape because the amount of the chemical is
reduced by half over each time interval.
5. What do the differences in the curves on the graph represent?
The differences in the curves represent the differences in half-lives of uranium-235
and potassium-argon.
6. What does the color change represent?
It represents the change or decay of radioactive isotopes
7. Does the entire grid ever change to the second color? Explain.
No, there will always be a tiny bit of the original radioactive substance left.
8. In your own words, define half-life.
Student answers may vary, but should mention reduction of radioactive isotopes by
half over a specific time interval.
9. The half-life of uranium-235 is 704 million years, not really 30 seconds like it was in
your lab activity. Using your graph, determine the age of a fossil with “49 squares”
remaining in both seconds AND years (using the half-life).
The fossil is approximately “40 seconds” old according to the lab data, and 1170
million years old, according to half-life.
10. The half-life of potassium-40 is 1.25 billion years, not really 30 seconds like it was in
your lab activity. Using your graph, determine the age of a fossil with “36 squares”
remaining in both seconds AND years, using the half-life.
The fossil is approximately “82 seconds” old according to the lab data, and 22,922
million years old, according to half-life.
Instructional Tip:
The figures for questions nine and 10 should be extrapolated from the graph and will not be
exact.
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Radioactive Decay – Grade 11
Attachment G
Teacher Notes on Radioactive Decay
Relative dating – the use of position in the rock layers to determine the approximate age of
fossils
 Rock strata – the layers of rock in an area; oldest layers are at the bottom, youngest layers
are at the surface.
 Requires the use of index fossils.
 Index fossils – common fossils of a particular time period whose approximate ages are
known.
Radioactive dating – the method of obtaining a geological age by measuring the relative
abundance of radioactive parent and daughter isotopes in geological materials.
 An isotope is one of a set of chemically identical species of atom which have the same
atomic number but different mass numbers.
 Half life is the amount of time that it takes for half of a radioactive sample to decay.
Radioactive dating can be done using a variety of elements
 Uranium-235 can be used to find the age of very old fossils. Uranium’s half-life is 704
million years and it breaks down into lead-207.
 Potassium-argon dating can be used to find the age of a fossil if it’s thought to be much
older; the half-life is 1.25 million years. Potassium-40 breaks down over time into argon.
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