1. Uses radioactive elements to determine the absolute age (age in years) of a rock or fossil
Radiometric Dating
Only some materials can be dated using this method
Volcanic Ash
Organic material
Igneous Rocks
Some sedimentary rocks

2. Structure of the Nucleus - Review
Which elements are these?
(protons are shown in red and neutrons in white.)
They are both carbon. Both have 6 protons. i.e. they both have an atomic number of 6.
These are two isotopes (varieties) of carbon.
- same chemical properties, but different physical properties (e.g. how they behaving in nuclear reactions)
- different number of neutrons, therefore different atomic masses
Some isotopes are unstable and therefore radioactive.

3. Mass number = #p+ + #no Atomic number = #p+ Two isotopes of carbon:
common
and stable
rare &
radioactive
Carbon-12
Carbon-14
Mass number = 12
p+ = 6
n0 = 6
Mass number = 14
p+ = 6
n0 = 8
Mass number = #p+ + #no
Atomic number = #p+

4. Energy is released (in the form of radiation)
Because radioactive isotopes are unstable, they undergo radioactive decay:
Energy is released (in the form of radiation)
The atom transforms into a completely different element
Parent Material
(the original unstable
atom)
Daughter Material
(the stable product left
after decay happens)
Radioactive
Decay

5. “When I was young I used to feel so alive, so dangerous
“When I was young I used to feel so alive, so dangerous! In fact, would you believe I started life as Uranium-238? Then one day I accidentally ejected an alpha particle … now look at me, a spent old atom of lead-206. Seem that all my life since than has been nothing but decay, decay, decay…”

7. Radioactive Decay Curve
100%
Half life: the time it takes for half of the parent radioactive element to decay to a daughter product.
% of Radioactive Material
Each isotope has a particular Half-life. This allows us to date the sample containing radioactive isotopes.
50%
25%
12.5%
Time

8. Different radioactive isotopes have different half lives
Good for dating really old volcanic layers and meteorites
Carbon -14 is good for dating actual remains. To a limit of about years

9. Time elapsed since death (years)
Example 1:
A baby mammoth found frozen in a glacier is found to contain one quarter of its original carbon-14. Determine its age.
Elements for Radioactive Dating: Page 8 of Data Booklet
Time elapsed since death (years)
Amount Remaining
5.73 x 103 (5730)
1/2
11460
1/4
The baby mammoth is 1.15 x 104 years old.

10. The orthoclase is 2.9 x 109 years old.
Example 2:
A sample of orthoclase is dated using potassium–40. A mass spectrometer analysis shows that 20% of the potassium-40 is present in the sample. Use this data to determine the age of the orthoclase sample.
Elements for Radioactive Dating: Page 8 of Data Booklet
Determine if potassium-40 is the parent nuclei
Use decay curve to determine # of half lives elapsed
(2.3 half-lives)(1.26 x 109 a/half-life) = 2.9 x 109 a
The orthoclase is 2.9 x 109 years old.

11.
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