Dating Rocks and Fossils Using Radioactive Isotopes
Name _________________________________
Introduction: Certain isotopes of some elements are radioactive. This means that
they slowly lose some of their particles (protons, neutrons, electrons) over long
periods of time. Some of these “decay” processes are well understood, and we know
exactly how long it takes for some of the original element to be transformed into
another element. If we can count the atoms of the original element (the “parent
isotope”) in an object and the atoms of the newly formed element (the “daughter
isotope”) in the same object, we can tell how long ago the object was formed.
Half-Life of radioactive isotopes: Radioactive isotopes decay at a very predictable
rate. The rate of decay is described in terms of a “half-life” of the isotope. The halflife of an isotope is the time it takes for exactly ½ of the original isotope to decay and
form the new isotope. Half-lives for a number of radioactive isotopes are known. The
graph below can be used to tell how many half-lives have passed.
Percentage of Parent Isotope Remaining
Predicted Decay Rate of an Isotope over Time
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Number of Half-lives
7
8
9
10
For example – If 50% of the original isotope is left in the sample, exactly one halflife has passed. If 25% of the original isotope is left, exactly two half-lives have
passed. How many half-lives have passed if 10% of the original isotope remains in
the object? ____________________
The half-lives of some isotopes are very, very long – in the hundreds of millions or
even billions of years. This means we can tell the age of very old rocks and fossils.
Procedure: Choose one of the 5 bags of “atoms” (A,B,C,D,or E). There is a card
with each bag explaining what isotopes are represented by the “atoms” in the bag. In
each bag, there are three types of objects: the parent isotope, the daughter isotope,
and objects representing all of the other atoms in the sample (you do not need to do
anything with these extra atoms).
1. Count the number of parent isotopes in the bag. Enter this number in the data table.
2. Count the number of daughter isotopes in the bag. Enter this number in the data
table.
[NOTE: PLEASE DO NOT LOSE ANY OF THE OBJECTS.]
3. Calculate the % of the parent isotope remaining in the object:
% parent isotope remaining =
(# of parent isotopes)
(# of parent isotopes + # of daughter isotopes)
X 100
Enter the % in the data table.
4. Use the graph on page 1 to determine how many half-lives have passed since the
object was formed. Be very exact when reading the graph – estimate carefully. Enter
this number in the data table.
5. Multiply the # of half-lives by the half-life of the isotope in your bag (printed on
the card with the bag). This is the age of your sample in years. Enter the age in the
data table.
6. Put ALL of the “atoms” back into the bag and seal the bag. Choose another bag
and repeat the procedure. Do all 5 bags if possible.
Data Table
Rock
Isotope Half- # of
# of
% of
Number
or
used
life
parent
daughter parent
of halfFossil
isotopes isotopes isotope
lives
Letter
A
B
C
D
E
Isotopes used:
Uranium 235; half-life of 704 million years.
Uranium 238; half-life of 4.5 billion years.
Thorium 232; half-life of 14 billion years.
Age in
years
Questions:
1. Which sample was the oldest? ______ How old was it? ____________________
Knowing what you know about the early earth, do you think any real rocks or fossils
could be this old? ____________ Explain your answer. ________________________
____________________________________________________________________
2. If you have a YOUNG sample and a very OLD sample of a certain rock type,
which has more of the parent isotope? _________ Explain why, in your own words.
____________________________________________________________________
____________________________________________________________________
3. If you have a fossil that you think is about 200 million years old, which isotope
would you use to date it – Thorium 232, Uranium 235, or Uranium 238? __________
Explain why, in terms of the graph we used. ________________________________
____________________________________________________________________
4. Suppose a geologist dated a rock using U-235, and only the daughter isotope Pb207 was found (no atoms of U-235 were detected). What could be said about the age
of the rock? __________________________________________________________
[Think – use the graph on page 1 in thinking about this question.]
If you really needed to know a more exact age for the rock, what could you do?
____________________________________________________________________
5. PLEASE READ: Carbon 14 (C-14) is an isotope of carbon used to date more
recent objects that were once living, like bones and hair of early Humans, fabric,
charcoal, wood, leather, seeds, pollen, etc. For example, the “Dead Sea Scrolls” and
the mummies in Egyptian pyramids were dated using C-14. Carbon 14 decays to
nitrogen 14, with a half-life of 5730 years.
a. Use the graph to see how much C-14 would be left in a sample after 2 half-lives
(11,460 years). (What % of the original C-14 is left?) _________________________
b. How about after 3 half-lives (17,190 years)? _________________
c. ABOUT how many half-lives could pass before the C-14 in the sample would be
such a small amount that it couldn’t be detected? _____________ Calculate how
many years this would be (multiply # of half-lives x 5730 years). ________________
(This is about the maximum age that can be dated using C-14.)
d. Could C-14 be used to date any of the “samples” we used in this activity? _______
Why not? ____________________________________________________________