Lesson Plan for 9th grade Physical Science based on “Engaging Students in Scientific Practices: What does constructing and revising models look like
in the science classroom?” article and Next Generation Science Standards by Twaila Wiley
Modeling Radioactive Decay
HS-PS1-8 Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy
released during the processes of fission, fusion, and radioactive decay.
Prior knowledge / discussion:
Carbon is an element found in all living (organic) things. Only about one carbon atom in a trillion carbon atoms is C-14.
Most are C-12. The C-14 isotope is made in the atmosphere from nitrogen-14 (N-14). Cosmic radiation bombardment
changes N-14 into C-14. Nitrogen has an atomic number of 7 which means that it has 7 protons. A neutron from the
cosmic radiation displaces a proton, leaving the element with only 6 protons. Carbon’s atomic number is 6, so this
becomes a Carbon isotope (C-14) called radiocarbon because it is radioactive, but it is not dangerous. It is unstable so it
will spontaneously decay back into N-14 after a time. It takes about 5,730 years for half of a sample of radiocarbon (C14) to decay back into nitrogen. It takes another 5,730 for half of the remainder to decay, etc. This is called its half-life.
Radiocarbon combines with oxygen in the air and enters plants and animals when they eat and breathe just like the
much more common C-12 isotope. When something dies, it does not take in any more carbon isotopes and the C-14 in
it continues to decay back into nitrogen. So, if we find the remains of a dead creature whose C-12 to C-14 ratio is half of
what it's supposed to be (that is, one C-14 atom for every two trillion C-12 atoms instead of one in every trillion) we can
assume the creature has been dead for about 5,730 years (since half of the radiocarbon is missing). If the ratio is a
quarter of what it should be (one in every four trillion) we can assume the creature has been dead for 11,460 years (two
half-lives). After about 10 half-lives, the amount of radiocarbon left becomes too small to measure.
Discuss the radioactive decay of Uranium-238 to (Lead-206) Pb-206. U-238’s half-life is 4.5 billion years. Discuss parent
and daughter elements in radioactive decay as well as half-lives.
Objectives:
Students will be able to explain the term half life.
Students will create a model to show how parent material changes to daughter material.
Students will be able to calculate percentages, fractions, and mass of material remaining and formed.
Materials:
Yellow & Pink post it notes of the same size, Clock with a second hand, Scissors.
Procedure:
Stick the post it notes together back to back. The front color (let’s say yellow) represents Uranium-238. Keep the yellow
side up on your table.
The half-life of U-238 is actually 4.5 billion years, but we will use 1 minute to represent this time frame. Watch the clock
and in one minute, cut the post it note in half and turn the other side up. (Let’s say it’s pink.) This pink color represents
the daughter material, Lead-206.
Wait another minute and cut the yellow half of the post it note in half, again turning up the pink side to represent the
change.
Continue cutting the yellow paper in half and recording the results on a data chart.
Also shade in the portions of change as you go along on the diagram below, drawing the lines darker to represent each
half-life you cut and label your drawing.
Model diagram of Half-lives:
Results Chart: (Assume we started with 1 g of U-238 to calculate the mass of parent and daughter materials.)
Time
(minutes)
Time that is
represented
0
1
2
3
4
5
0 yrs
4.5 billion yr
Number
of half
lives
0
1
Percent of
Parent
material
100
Percent of
Daughter
material
0
Fraction
of Parent
material
1
Fraction of
Daughter
material
0
Mass of
Parent
material
1g
Mass of
Daughter
material
0g
Conclusion Questions:
1. In terms of Radioactive Decay, what does the original yellow whole piece of paper represent?
___________________________________
2. In terms of Radioactive Decay, what does the pink paper that you turned over in your model represent?
____________________________________
3. Explain the term half-life. _____________________________________________________________
__________________________________________________________________________________
4. What is the half-life of your paper element? ___________________ (5.)In our model, how much time would this
represent in actual years? __________________________
6. What fraction of your yellow paper element was left after the first three half-lives? ______________
7. What percent of your yellow paper element would be left after 6 half-lives? ___________________
8. What two factors in your model must remain constant for your model to be accurate?
___________________________________________________________________________
9. How does your drawing model represent radioactive decay? ______________________________
__________________________________________________________________________________
10. What is Carbon Dating and how is the idea of Radioactive Decay used in that process? __________
___________________________________________________________________________________
___________________________________________________________________________________
11. Why can’t Carbon Dating be used on bones that are older than 60,000 years? _________________
___________________________________________________________________________________
12. Why can’t we date ROCKS with Carbon dating? _________________________________________
____________________________________________________________________________________
13. What is an isotope? _________________________________________________________________