
Students will
How fast does a radioactive isotope decay? How long will it take for a sample to completely transmutate (change) into a new element?
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 below.

Isotope: 






Atomic Number 



Atomic Mass 



# Protons 



# Neutrons 



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 halflife. 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.
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Teacher Preparation
Materials
Procedure
Data Sheet
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Using a whole sheet of graph paper and the data above, make a graph with time on the xaxis and number of atoms on the yaxis.

Extensions
Repeat the experiment starting with 50 atoms and 75 pieces of candy. Compare the resulting graphs. (The graphs can be plotted on the same paper used for the first graph.)
Students with special needs
Some students may have difficulty manipulating small objects.
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(Please write complete sentences):
1. Define halflife in your own words.
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2. In the experiment, what was the halflife of the isotope M&Mium? Skittlium?
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3. At the end of 2 halflives, what fraction of the atoms had not decayed?
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4. Describe the shape of the curve from the graph of your data?
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5. As a class, compare and contrast the graphs made by the different lab groups.
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Answers to Assessment
1. Halflife is the length of time required for one half of the isotope to decay.
2. The halflife of M&Mium in this activity was 10 seconds. The halflife of Skittlium in this activity was 20 seconds.
3. At the end of two halflives 1/4 of the original sample remained; 3/4 of the sample had decayed into a new element.
4. The graph is a decreasing logarithmic curve.
5. The graphs will be almost the same.