Name:
TOC#
Radioactive Decay Lab
Introduction:
Most elements have atoms that come in two or more forms
called isotopes. Isotopes are atoms of the same element, but with
different atomic masses. This occurs because different isotopes
have different numbers of neutrons. For example, hydrogen has
three isotopes that are listed in the table to the right.
Some isotopes are unstable or radioactive. For instance,
in the example above, tritium is an unstable isotope of hydrogen.
Radioactive isotopes slowly decompose by discarding part of
the nucleus. This nuclear decomposing process is called nuclear
decay. The length of time required for half of the isotope to
decay is the substance's half-life. Each radioactive isotope takes
its own particular amount of time to decay. However, when the
amount of remaining isotope is plotted against time, the
resulting curve for every radioisotope has the same general
appearance as seen in the graph to the right.
Isotope:
Hydrogen Deuterium Tritium
Atomic Number
1
1
1
Atomic Mass
1
2
3
# Protons
1
1
1
# Neutrons
0
1
2
Vocabulary
Isotope – atoms of the same element with different
number of neutrons in the nucleus of the atom
Radioactive – Isotope that is emitting particles as it decays due to discarding part of the nucleus
Half-Life – the amount of time it takes for half of an element to decay
Materials
50 M&Ms and 50 Skittles
Resealable bag
Stop watch or visible clock that displays seconds
Graph paper
Procedure
1. Place atoms (candy pieces) in the bag.
2. Seal the bag and gently shake for the specific amount of time that corresponds to the half-life of your
radioactive element.
- Half-life of M&Mium (M&Ms) is 10 seconds.
-Half-life of Skittlium (Skittles) is 20 seconds.
3. Gently pour out candy.
4. Count the number of pieces with the print side up. These atoms have "decayed."
5. Return only the pieces with the print side down to the bag. Reseal the bag.
6. Consume the "decayed" atoms.
7. Gently shake the sealed bag again for the prescribed amount of time.
8. Continue shaking, counting, and consuming until all the atoms have decayed.
9. Graph the number of un-decayed atoms vs. half-life.
Pre-lab questions.
1. What does each piece of candy represent?
2. What does the shaking time represent?
3. What happens after each half life?
4. What “element” has the longer half-life?
Data Sheet for M&Miums:
Number of Half-lives
(number of shake rounds)
0
1
2
Total Time Passed
# of Decayed Atoms of
M&Miums
0 seconds
10 seconds
First 10 s + Second 10 s =
20 seconds
3
4
5
6
7
8
9
10
Graph for M&Miums
Total Time
# of Un-decayed atoms of
M&Miums
Data Sheet for Skittliums
Number of Half-lives
(number of shake rounds)
0
1
2
Total Time Passed
# of Decayed Atoms of
Skittlium
0 seconds
10 seconds
First 20 s + Second 20 s =
20 seconds
3
4
5
6
7
8
9
10
Graph for Skittliums
Total Time
# of Un-decayed atoms of
Skittlium
Using your graph and data table, answer the following questions.
1. Define half-life in your own words.
2. How are half-life and radioactive decay related?
3. At the end of 2 half-lives, what fraction of the atoms had not decayed?
4. Describe the shape of the curve from the graph of your data and the reason for this shape.
5. You ate your “decayed” elements. In a real fossil or substance, do the elements disappear when they decay?
Why or why not.
6. Why might the half-lives of different elements differ?
7. Why do scientists analyze radioactive decay?
8. Why do scientists specifically use Carbon-14 radioactive dating?
9. If you have a fossil that is 100 grams and you use carbon-14 dating (carbon-14 has a half life of 5730 years)
and you find that 50% of the substance is still radioactive (has not decayed), how old is the fossil?