1. Radioactive Decay
L.O: SWBAT model how Carbon-14 is used to determine the age of recent fossils.

2. L.O: SWBAT carbon-14 is used to determine absolute age of fossils
How do you think geologists know these absolute ages?
Lived exactly 251,000,000 mya.
Neanderthals lived exactly 130,000 yrs old
Earth is exactly 4.6 billion years old
L.O: SWBAT carbon-14 is used to determine absolute age of fossils
this trilobite lived exactly 544,000,000 m.y.a

3. L.O: SWBAT carbon-14 is used to determine absolute age of fossils
Radioactive decay of radioactive elements is used to determine the absolute age of fossils! Occurs when the nuclei of unstable atoms break down or decay. The original atoms are changed to a stable atom of another element.
All elements have a certain number of protons and neutrons.
An isotope has the same amount of protons but a different amount of neutrons, making the atom unstable. This unstable atom decays and changes into a NEW stable element over many years.
L.O: SWBAT carbon-14 is used to determine absolute age of fossils

4. Half-life
Radioactive isotopes decay at a predictable rate called a half- life.
Half-life: The time it takes for one-half (1/2) of the unstable isotope to change into a stable decay product.
Each radioactive element has its own half-life.

5. THE HALF-LIFE IS A CONSTANT AND DOES NOT CHANGE
THE HALF-LIFE IS A CONSTANT AND DOES NOT CHANGE!!!!!!!!!!! Temperature and pressure changes DO NOT affect half-life.

6. The half-life of an isotope remains constant – this is why it is so useful!
The cover page of your ESRTs lists the common radioactive isotopes and their half-lives. According to your ESRTs, what is the half-life of the following isotopes?
Carbon-14 :
Potassium-40 :
Uranium-238 :
Rubidium-87 :
It means that after 5,700 years a sample once containing 100% of C-14 now has 50% C-14 and 50% of N-14.
After 11,400 years, that same sample has 25% C-14 and 75% of N-14 … and so on …

8. Determining the amount of half-lives that have elapsed:
At time 0 (the initial time) there is 100% of the radioactive material.
After 1 half-life, only 50% of the radioactive material remains (50% has become stable)
After 2 half-lives, only 25% of the radioactive material remains (75% has become stable)

9. Determining the amount of half-lives that have elapsed:
After 3 half-lives, only 12.5% of the radioactive material remains (87.5% has become stable)
After 4 half-lives, only 6.25% of the radioactive material remains. (93.75% has become stable)

10. Carbon-14 Example:
Carbon-14 is used to date the remains of once living things.
Carbon-14 Half-life = 5,700 years

11. Carbon-14 Example: Original amount of Carbon-14
Carbon 14 remaining after 1 half-life (5,700 years) – ½ remains
Carbon 14 remaining after 2 half-lives (11,400 years) – ¼ remains
Carbon 14 remaining after 3 half-lives (17,100 years) – 1/8 remains

12. material is C14 As we begin notice that no time
The grid below represents a quantity of C14.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
Age
100% 0%
no ratio
O years
As we begin notice that no time
has gone by and that 100% of the
material is C14

13. The grid below represents a quantity of C14.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
Age
100% 0%
no ratio 1
50%
1:1
5700 x 1 =
5700
After 1 half-life (5700 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.

14. The grid below represents a quantity of C14 C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
Age
100% 0%
no ratio 1
50%
1:1
5700 2
25%
75%
1:3
5700 x 2 = 11,400 yrs
Now 2 half-lives have gone by for a total of 11,400 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.

15. The grid below represents a quantity of C14. C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
Age
100% 0%
no ratio 1
50%
1:1
5700 2
25%
75%
1:3
11,400 yrs 3
12.5%
87.5%
1:7
5700 x 3 = 17,100 yrs
The grid below represents a quantity of C14.
C14 – blue N14 - red
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

16. How do we know which radioactive element to use?
We use Carbon-14 for RECENT and ORGANIC dating.
RECENT because C-14’s half-life is so short we only use it for samples that are under 60,000 years old.
ORGANIC because all organic (living) material is Carbon based
We use Uranium for older rocks due to its extremely long half-life. (Uranium-238 is used to date rocks that are approximately the age of the Earth!)

17. L.O: SWBAT carbon-14 is used to determine absolute age of fossils
The table below gives information about the radioactive decay of carbon-14.
What is the amount of the original carbon-14 remaining after 34,200 years?
a. 1/8 g
b. 1/16 g
c. 1/32 g
d. 1/64 g
L.O: SWBAT carbon-14 is used to determine absolute age of fossils

18. L.O: SWBAT carbon-14 is used to determine absolute age of fossils
The table below gives information about the radioactive decay of carbon-14.
After how many years will 1/128 of a gram of the original carbon-14 remain?
a. 22,800 yr
b. 28,500 yr
c. 34,200 yr
d. 39,900 yr
L.O: SWBAT carbon-14 is used to determine absolute age of fossils