1. Radioactive Decay
Please open up your journal to your chemistry unit and write a definition of an isotope at the beginning of these notes.

2. How can anyone know the age of something millions of years ago?
Scientists for many years did not have a way to determine the age of fossils, rocks or even our planet.

3. Isotopes
Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes.
Remember – the number of protons determines the element’s name and place on the Periodic Table!

4. Sticking Together inside the Atom
Electrons, neutrons and protons need to have balanced forces to hold then together
Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together.
Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable and may emit energy in the form of radioactivity.

5. Radioactive Decay
Some isotopes that are unbalanced will “decay” or fall apart a little by losing some neutrons and/or protons.
This process will keep happening at a predictable rate until it reaches a more stable form that is balanced.
While it is losing particles, it also loses some energy from the breaking bonds in the form of electromagnetic energy -> radioactivity
click on atom for short clip – click on the “Uses” at the bottom of the video screen

6. Predictable Pattern
Atom decay, like many things in nature, follows a predictable path.
Notice that no matter is lost – it just changes form and gives off energy in the process

7. Uranium 238 Decay Chain
There are several forms of Uranium isotopes in nature and several pathways that the decay chain can take.
Uranium 238 (red line) starts with the blue arrow and ends as more stable Lead (Pb) with the red arrow
Notice that it also forms several other isotopes in the process

8. Radiometric Dating
After many years of research done by many scientists, the patterns of many isotope’s decay chains were recorded.
This information can be used to determine the geological age of land formations or fossils.
Click on atom for short video clip

9. Practicing Decay Charts
To accomplish this very complicated mathematical process, were will use an imaginary new element, Twizzlarium 290 or Tw-290
It’s half life has been found to be 1 year.
Remember, this new element should be considered dangerous until it has reached a stable form, so don’t eat your sample until we’re done!

10. Create this graph: Tw-290 Decay Chain
Spacing on the Graph Paper:
For the y-axis, use 4 spaces for each unit – number up to 9
For the x- axis, use 3 spaces for each year
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

11. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Place your sample of Tw-290 as shown and mark a dot at the top of the sample
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

12. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Cut your sample in half to show the decaying of half of the isotopes
Place it on the “1” of the x-axis to show what is expected after one “half-life”
Add a dot to the top of it.
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

13. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Cut your sample in half to show the decaying of half of the isotopes
Place it on the “2” of the x-axis to show what is expected after one “half-life”
Add a dot to the top of it.
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

14. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Cut your sample in half to show the decaying of half of the isotopes
Place it on the “3” of the x-axis to show what is expected after one “half-life”
Add a dot to the top of it.
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

15. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Cut your sample in half to show the decaying of half of the isotopes
Place it on the “4” of the x-axis to show what is expected after one “half-life”
Add a dot to the top of it.
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

16. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Cut your sample in half to show the decaying of half of the isotopes
Place it on the “5” of the x-axis to show what is expected after one “half-life”
Add a dot to the top of it.
Units of Tw-290 (Responding/Dependent Variable)
*Keep going as long as you can – until you can no longer cut your sample
Years (Manipulated /Independent Variable)

17. Years (Manipulated /Independent Variable)
Tw-290 Decay Chain
Connect the dots to show the trend of decay of this isotope
This pattern is predictable, but since the length of time for each isotope’s half-life may be different, the slope will vary.
The line approaches zero, but would never actually be zero.
Units of Tw-290 (Responding/Dependent Variable)
Years (Manipulated /Independent Variable)

18. Typical Half-life Graph

19. Exit Questions Include the question in your full sentence answers!
How many cuts did it take until you could not cut each licorice anymore? How many half-lives does this represent for the element?
What would happen to the material no longer in the sample? Did it disappear?
Some substances have very long half-lives. Uranium has a half-life of 4.5 billion years. Explain how dating using uranium could provide evidence for the age of the earth.