1. Radioactive Isotopes and Half Life

2. What is a Radioactive Isotope?
What is Radioactive Decay?
What is Half Life?
Most elements are stable which means we can count on what the number of protons, neutrons, and electrons will be. When we know the number of protons, we know what element it is. But an unstable element or radioactive isotope begins to lose particles according to either alpha, beta, or gamma decay (more info on this later). When we say decay, we mean the nucleus changes and because of that, a new isotope of an element or even a new element is formed. This change through radioactive decay is called transmutation. P 416 in your book.

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
These elements have long half lives. But some radioactive isotopes have a short decay time, like days, hours, or minutes.

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.
Each element has it’s own half-life

5. Radioactive Decay and Half Life
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.

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
So we start with some Carbon 14. It’s radioactive so it will decay. It decays and becomes Nitrogen ( which means it has a new number of protons right?) So on the next slide, we will see when half of what we start with has turned into Nitrogen.

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.
In real life it takes 5730 years for half of the Carbon 14 to turn into Nitrogen 14. Mrs. Taylor will explain more about carbon dating of things when she gets back. How many more years will need to go by for half of the blue to become red? ( 5730) So that would equal 2 half lives for a total of??? 11460

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.
This graph shows that. Note that the 25% in the chart refers to 25% of the original blue remains. But to get to that, 50% of the blue we just had decayed. Get it? So, how many years before half of the 4 blue blocks would turn to red? (5730) How many blue blocks will remain (2)?

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
So now we re down to 12.5% of the original amount. But we lost 50% each time.

10. This graph shows percent and grams.

11. What is the half life represented in this graph?
1 million. So at one million50 percent of the original remains. After another million, 50% of that goes away, leaving 25% of the original. Now as this one is decreasing, whatever it decays to is increasing, but this graph does not show that. What percent will be left after 3 million years?

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