Radiometric Dating Lab
Name:___________________________Date: ____________________________ Period: ___________________________
Background of Radiometric Dating: Most elements found in nature are stable; they do not change over
time. Some elements, however, are unstable—that is, they change into a different element over time.
Elements that go through this process of change are called radioactive, and the process of transformation
is called radioactive isotope decay. Because radioactive decay happens very steadily, scientists can use
radioactive elements like clocks to measure the passage of time. By looking at how much of a certain
element remains in an object and how much of it has decayed, scientists can determine an approximate
age for the object. So why are scientists interested in learning the ages of objects? By looking at very old
things, such as rocks and fossils, and determining when they were formed, scientists learn about the
history of the Earth and the plants and animals that have lived here. Radioactive dating makes this history
lesson possible!
Radioactive Decay: The starting form of the element is called the parent isotope and the form that
it changes into is called the daughter isotope. For example, U-238 is an unstable isotope of
uranium that has 92 protons and 156 neutrons in the nucleus of each atom. Through a series of
changes within the nucleus, it emits several particles, ending up with 82 protons and 124 neutrons.
Once it reaches this point, the nucleus becomes stable and there are no more changes that occur.
A nucleus with that number of protons is called lead (symbol Pb), and this particular isotope of
lead is called Pb-206 because it has 82 protons + 124 neutrons, which totals 206.
Half-life: One half-life is the time it takes for half of the atoms present within a sample to decay.
For instance, after conducting careful measurements on large numbers of U-238 atoms, scientists
determined that each U-238 atom has a 50% chance of decaying into Pb-206 during 4.5 billion
years. In other words, the half-life of U-238 is 4.5 billion years.
How Do They Do It?
a. Find a rock, mineral, or fossil for analysis
b. Determine what isotopes the sample was made of when it formed.
c. Put sample into a mass spectrometer to find the ratio of parent and daughter isotopes.
d. Calculate the number of half lives based on the ratio to determine the age of the sample.
Reliability of Radiometric Data: Radiometric dating using isotopes with predictable decay rates has
been found to be a highly accurate technique to date rocks and fossils. The average variation in
results from repeated radiometric dating from multiple sources is less than 1%. Scientists,
Part 1. Radiometric Dating Samples
Procedure:
1. Get into groups of 3.
2. Obtain a “rock/fossil sample” in a plastic bag.
3. Read the description of your sample on the half sheet of paper.
4. Record the number of the sample and the information about the type and half-life of the parent and daughter isotopes on your data table on the last page of this packet.
5. Dump the beads into the plastic bin and separate the two colors.
a. The beads will never leave the plastic bag or the bin during this activity; every bead must make it back into the bag.
6. Count the number of each bead and record it in the appropriate spot on your data table. 7. Use this equation below to determine the number of half-lives your sample has gone through:
Equation:
# of parent isotopes x 100 = percent of parent isotopes remaining
# of parents isotopes + # of daughter isotopes
a. Use the chart below to determine how many half-lives your sample has undergone. If your calculated percent doesn’t match exactly, round to the next closest half-life. Record this on your data table.
8. Then use this equation below to determine the age of your sample and record it on your data table: Equation:
# of half-lives x length of half-life = age of sample
9. Repeat this process until you have conducted radiometric dating on all nine samples.
Part 1 Analysis Questions (to be done after data collection):
1. What was the oldest sample you analyzed? How old is it? How do you know?
2. What was the youngest sample you analyzed? How old is it? How do you know? # of Half-Lives
1 2 3 4 5 6
Percent of Parent
3. Carbon-14 dating only works for fossils, bones, etc. under 60,000 years in age. Why can we not date items for Carbon-14 that are older than this?
a. Carbon-14 is useful to date bones and fossils since it is found in living organisms while Uranium is not. How do you think we are able to determine the age of bones of living organisms that lived millions of years ago then?
4. Uranium-238 decays until it finally becomes Lead-206, a stable isotope that never decays and is not radioactive. What sub-atomic particle (electrons or protons) besides neutrons must have also decayed from Uranium-238 to make it Lead-206?
Part 2. Geiger Counter Station
A Geiger Counter is a particle detector device that measures radioactive decay. Once you are done with Part 1, go to the Geiger Counter Lab Station in the front of the room. Use the Geiger counter to answer the following questions:
1. Before you pick up the Geiger Counter make some predictions. a. Which Rock Sample do you predict is the most radioactive?
b. Which Food Sample do you predict is the most radioactive?
2. Now, using the Geiger Counter, which rock sample is the most radioactive? How do you know?
3. Which rock sample is the least radioactive? How do you know?
4. Which food sample is the most radioactive?
Part 3. Geologic Scale Activity
A geologic timeline (or scale) shows, to scale, when key events in Earth’s history occurred. You will construct a geologic scale to show when the events that are associated with the 9 rock samples you evaluated happened during Earth’s history.
1. In your groups of 3, get two pieces of paper from your instructor and tape them together. On the side you taped it together on, write your NAMES, DATE, and PERIOD.
2. On the front side of the paper do the following:
a. The Earth is a little less than 4.6 Billion years old. You need to make a straight line near the top of your papers that is the length of your two papers. Your line needs to have etch marks that show every 200,000,000 years TO SCALE (you figure out the scale ratio).
b. Label the left side of your timeline as “4.6 billion years ago: Earth Forms”. Label the right side of you timeline as “Present Day: Modern Times.”
c. Also label every billion years on your timeline TO SCALE.
d. Now draw a separate timeline beneath your original timeline that is a close-up of the last 100,000,000 years. See the example on the board if you are confused about what this would look like.
e. Now place the 9 events on your Geologic timeline that you calculated during this lab onto your timeline. DO THIS IN REDCOLORED PENCIL!!
Units of time during the history of the Earth are classified using a system of terminology. The largest unit of time is an Eon. Eons are subdivided into Eras. Eras are further subdivided into Periods.
3.
Open the Prentice hall textbook to page 365 and use colored pencils and a key to show where the following units of geologic time occur on your time scale:Precambrian Eon; Paleozoic, Mesozoic, Cenozoic Eras
4. Now analyze your timeline. Answer the following questions:a. What is your overall impression about when these key events in Earth’s history occurred?
b. Were you surprised by when an individual event occurred?
Data Table:
Rock
Sample #
Parent
Isotope
Type
Daughter
Isotope
Type
Half-Life Time
(years)# of
Parent
Isotopes
# of
Daughter
Isotopes
# of
Half-lives
Calculated Age
(years)Order of
Occurrence
Practice Problem n/a 1 2 3 4 5 6 7 8 9Sample
Major Geologic Event
1 End of The Last Ice Age in North America 2 Land-based Plants First Appear 3 Oxygen Begins to Fill Atmosphere 4 Uplift of Rocky Mountains Begins
5 The Dinosaurs Go Extinct
6 Metamorphic Rocks Begin to Form on Earth
7 Mammoth Go Extinct
8 Dinosaurs First Appear
