Intro to Half-Life PhET Lab (Radioactive Dating Game)
Introduction: Dead things decay into simpler molecules. Radioactive particles decay. Is it the same kind of decay? What does it mean when a substance is radioactive? In this simulation, you will investigate the concept of half-life.
Neutron: a subatomic particle of about the same mass as a proton but without an electric charge, present in all atomic nuclei except those of ordinary hydrogen.
Isotope: each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
Radioactive dating:a method of dating geological or archeological specimens by determining the relative proportions of particular radioactive isotopes present in a sample.
Decay: decay the half-life is the length of time after which there is a 50% chance that an atom will have undergone nuclear decay. It varies depending on the atom type and isotope, and is usually determined experimentally.
Procedure: PhET–Simulations — Chemistry – Radioactive Dating Game
∙ Take some time and play with the simulation. Those atoms are radioactive! How cool is that?!
∙ How many protons does Carbon-14 have? 6 (hint…what is its atomic number?)
∙ How many neutrons does Carbon-14 have? 8 (hint: the mass – the atomic number)
∙ Add a Carbon-14 atom to the play area. What happens to that Carbon-14 atom?
∙ Add more Carbon-14 atoms one at a time. Do all the Carbon-14’s decay at the same time? No
∙ Add 50 Carbon-14s. (click five times.) What happens? The carbons adds up
Does Carbon-14 ever get to zero? No
∙ and Start with 20 Carbon-14s atoms each time. Pause after the time period indicated. Record the number of C-14 and N-12 atoms. Calculate the percentage of each. Reset before the next
5000 Years 10000 years 15000 years
C: 12 (12/20)= 60%
N: 8 = 40%
C: 4 (4/20) = 20%
N: 16= 80%
C: 6 (6/20)
N: 14= 70%
∙ Redo the above with 100 Carbon-14 atoms and fill in the three boxes below.
∙ How do the calculated percentages starting with 20 atoms compare to those that started with 100 atoms?
The calculations starting with 20 atoms and the numbers had to
∙ Generally, does the size of a radioactive sample affect half-life? __________ Why/Why not? _________________________________________________________________________
∙ Carbon-14’s half-life was measured in thousands (5730) of years. How long is Uranium-238’s half-life? ______________________________________________________
∙ How many protons does U-238 have? 92 How many neutrons? __________________
∙ Into what atom does Uranium-238 decay? Lead 206
∙ Does the size of the sample of Uranium-238 affect its half-life? ______________________________
About that Unknown Element….
∙ How would you determine the half-life of this unknown element? Write up a little plan here:
∙ Estimate the half-life of this element. _________ seconds.
Decay Rates: Observe the decay curves (% remaining vs time) for Carbon-14 and Uranium-238. Sketch the decay curve for those isotopes here:
5000 Years 10000 years 15000 years
C: 58 atoms = 58
N: 41 atoms = 41
C: 32 atoms = 32
N: 68 atoms = 68
C: 14 atoms = 14
N: 86 atoms = 86
Measurement: You will use the Geiger counter to detect radioactive decay. Plant the tree. How much C-14 does the tree have while it’s alive? _________________________ What happens to the C-14 when the tree dies? _____________________________________________________ Choose the rock. Where does the rock come from? _______________________________________________________ Which radioactive element can be found in the rock? ________________________________________ Dating Game: Estimate the age of each of the following objects in the list below:
1. Click on the Dating Game tab. There are objects on the surface and in the five layers beneath the surface. There are both rocks and fossils in each layer.
2. Select the Carbon-14 detector. Move the Geiger counter to each fossil to see the % of original atoms remaining.
3. On the ½ life graph, move the green arrow right or left until the % of original matches the reading on the detector. Record your estimated age for each fossil in the table below (after you check with the simulation to make sure it’s correct!).
Item Estimated Age Item Estimated Age
Why was finding the age of the tilted skull more difficult? ____________________________________________ What did you do to come up with the correct answer? ______________________________________________ How much C-14 is in the rocks? ______________ Why is that the case? _______________________________