Radioactive Isotopes
and Half Life
1
What is a Radioactive
Isotope?
What is Radioactive Decay?
What is Half Life?
2
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
K-40
C-14
Half-life = 4.5 Billion Years
Half-life = 1.25 Billion Years
Half-life = 5, 730 Years
3
Radioactive Decay and Half Life
Here are some facts to remember:
1. 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
4
Radioactive Decay and Half Life
2. Decay happens in pairs. One element
decays to another, more stable element.
As one decreases, the other increases
Ex:
C14 decays into N14
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.
5
The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and turns red.
Ratio of
Half
% C14
%N14
14
14
C to N
C – blue
N - red
lives
14
0
100%
0%
14
no ratio
As we begin notice that no
time has gone by and that
100% of the material is C14
6
The grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
Ratio of
Half
% C14
%N14
14
14
C to N
C – blue
N - red
lives
14
14
0
100%
0%
no ratio
1
50%
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.
7
The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red .
Ratio of
Half
% C14
%N14
14
14
C to N
C – blue
N - red
lives
14
14
0
100%
0%
no ratio
1
50%
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.
8
The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
Ratio of
Half
% C14
%N14
14
14
C to N
C – blue
N - red
lives
14
14
0
100%
0%
no ratio
1
50%
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
9
10
What is the half life
represented in this
graph?
11
The End
We will complete a Half Life
laboratory using
Twizzlers!!!!!
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