1. Radioactive Isotopes and Half Life

2. What is a Radioactive Isotope?
What is Radioactive Decay?
What is Half Life?

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
Each element decays into a new element
- 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.

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
Time elapsed
100% 0%
0 years
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
Time Elapsed
100% 0%
0 years 1
50%
5,730 years
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
Time Elapsed
100% 0%
0 years 1
50%
5,730 years 2
25%
75%
11,460 years
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
Time Elapsed
100% 0%
0 years 1
50%
5,730 years 2
25%
75%
11,460 years 3
12.5%
87.5%
17,190 years
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. Band of Stability Stable, naturally occurring isotopes
For low atomic numbers the stable nuclei are those with a ratio of protons to neutrons approx. 1:1
As the atomic # increases, the stable ratio increases to about 1.5: 1. 1

12. Band of Stability Stable, naturally occurring isotopes
If an isotope falls above the band, then it needs to lose neutrons and gain more protons to become stable. How would it achieve that?
If an isotope had too many protons and too many neutrons, how would it become stable?
Alpha
Decay
Beta Decay
Positron Emission 1