Radiometric Dating - Google Docs

Name __________________________________________
Before beginning the activity answer the following questions.
1. Which type of decay is the diagram showing? ______________________
2. In your own words, what is an isotope?
Read the following and watch the video, “How Does Radiometric Dating Work?”, on Classroom
before continuing.
Radiometric Dating is the process of using known decay rates of certain elements to
determine the age of samples. Recall that atoms are made of protons, neutrons and electrons.
The atomic number of an element is the number of protons in the nucleus. By adding the
element’s protons and neutrons, the atomic mass is determined. For example, uranium
contains 92 protons so it’s atomic number is 92. The number of neutrons, however, can vary in
a uranium atom, so it’s atomic mass can vary from uranium 238 to uranium 235, depending on
the number of neutrons present. Each of these varia ons of uranium is called an isotope.
An unstable isotope of an element is called the parent, while the isotope that is the result of
decay is called the daughter. Sometimes you’ll see the terms, parent material and daughter
The me it takes one half of the nuclei to decay is called the half-life of the isotope.
Different isotopes decay at different rates. If we know the decay rate of an isotope, and we
know the parent-daughter ra o, we can calculate the age of the sample. For example, if an
isotope has a half life of 1000 years, and we examine a sample that contains ½ parent material
and ½ daughter material, that means it has gone through one half life, and is therefore 1000
years old. If the sample contains ¼ parent material and ¾ daughter material, that means it has
gone through two half-lives and the sample is therefore 2000 years old.
Activity - Determining the Age of a Sample
The bags of beads you have been provided with represent samples of rocks or minerals, with each bag
reflecting a different ratio of parent material to daughter material.
Your Task: Given the following decay rates of known substances, calculate the ages that each of the bags
of beads represents.
1. Uranium-235 decays into Lead-207 and it has a half-life of 713 million years.
2. Carbon-14 decays into Nitrogen-14 and it has a half-life of 5730 years.
3. Potassium-40 decays into Argon-40 and it has a half-life of 1.3 billion years.
Checking for Understanding
1. Explain what the half-life of an isotope is.
2. Oxygen 16 and oxygen 18 are isotopes of oxygen. How are they different from each other?
3. Which isotopes are referred to as parent material and which isotopes are referred to as daughter
material? Explain.
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