1. Radioactive Isotopes and Half Life

2. What is a Radioactive Isotope?
What is Radioactive Decay?
What is Half Life?
(Take notes as we discuss)

3. Radioactive Isotopes
Radioactive elements are unstable. They decay, and change into different elements over time.
Not all elements are radioactive. Those that are listed below are the most useful for geologic dating of fossils are:
U-238 Half-life = 4.5 Billion Years
K-40 Half-life = 1.25 Billion Years
C-14 Half-life = 5, 730 Years

4. Radioactive Decay and Half Life
Here are some facts to remember:
The half-life of an element is the time it takes for half of the material you started with to decay.
2. Each element has it’s own half-life

5. Radioactive Decay and Half Life
3. Each element decays into a new element
C14 decays into N14 while U238 decays into Pb206 (lead), etc.
4. The half-life of each element is constant. It’s like a clock keeping perfect time.
Now let’s see how we can use half-life to determine the age of a rock, fossil or other artifact.

6. The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and turns red.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio
As we begin notice that no time has gone by and that 100% of the material is C14

7. The grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio 1
50%
1:1
After 1 half-life (5730 years), 50% of
the C14 has decayed into N14. The ratio
of C14 to N14 is 1:1. There are equal
amounts of the 2 elements.

8. The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red .
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
Now 2 half-lives have gone by for a total
of 11,460 years. Half of the C14 that was
present at the end of half-life #1 has now
decayed to N14. Notice the C:N ratio. It
will be useful later.

9. The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
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 (17,190 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

11. What is the half life represented in this graph?

12. We will complete a Half Life laboratory using Twizzlers!!!!!
The End
We will complete a Half Life laboratory using Twizzlers!!!!!