1. Radioactivity provides a method to determine the age of a material

2. Carbon is use to determine age of a material
most living organisms are based on organic molecules (i.e. contain carbon).
When organisms die carbon-14 stops being created and slowly decays
C-14 has a Half Life of 5730 years
- the time it takes for a radioactive isotope to be halved is called Half-life
So by measuring the amount of carbon-14 remaining in a sample and then comparing it to the amount of carbon-12 in the sample we can determine the age of the material
Measuring the relative amounts of carbon-12 : carbon-14 is called radiocarbon dating

3. The grid below represents a quantity of C14. Each click is
one half-life (5730 years) .
C14 – blue N14 – red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio 1
50%
1:1 2
25%
75%
1:3 3
12.5%
87.5%
1:7
After 3 half-lives (17,100 years) only
12.5% of the original C14 remains. For
each half-life period half of the material
present decays. And again, notice the
ratio, 1:7
Age = 3 half lives (5730 x 3 = 17,190 yrs)

4. Table of radioisotopes:
Parent isotope = the original, radioactive material.
Daughter isotope = the stable product of the radioactive decay.
The rate of decay remains constant, but some elements require one step to decay, while others decay over many steps before reaching a stable daughter isotope.
Carbon-14 decays into nitrogen-14 in one step
Uranium-235 decays into lead-207 in fifteen steps.
Thorium-235 decays into lead-208 in ten steps.

5. Decay of a 50.0 g radioactive sample of C-14
After 2nd half-life
After 1st half-life
12.5 g
After 5th half-life
After 4th half-life
6.25 g
1.625 g
3.125 g
After 3rd half-life
Half-life 1 2 3 4 5
Time
5730
11460
17190
22920
28650
Amount (g)
Percentage (%)
50.0
100%
25.0
50%
12.5
6.25
3.125
1.625
25%
12.5%
6.25%
3.125%

6. Decay curves
The curve shows the relationship between half- life and percentage of original substance remaining.

7. The Potassium-40 Clock
Radioisotopes with very long half-lives can help determine the age of very old things. Potassium-40 is a key element.
The potassium-40 to argon-40 has a half-life of 1.3 billion years.
Potassium-40 can exist in hot molten rock, while argon-40, the daughter isotope, escapes from the molten rock because it is a gas.
Argon-40 produced by the decay of
potassium-40 becomes trapped in rock.
Ratio of potassium-40 : argon-40
shows age of rock.