1. Rate of Radioactive Decay

2. How old is this tree?
What are some methods you can use to date this tree?
Counting rings
What if you don’t want to cut it down?
What if it’s not a tree?

3. How old is it?
How do scientists find out how old things like these cave paintings of horses are?
One method is to use radioactivity.
The nuclei of radioactive elements decay at a fixed rate. (This is called a half-life)
Measuring the amount of radioactive material remaining compared to stable material lets us calculate how long the material has been decaying.
Using radiocarbon dating, these cave paintings of horses, from France, were determined to have been drawn years ago.
Mesure how radioactivity has changed over time and calculate age of remains
All organisms on earth contain carbon, carbon 14 decays when organism dies without being replaced – ratio of carbon 14 to carbon 12 decreases – can tell us how old it is
Radiocarbon dating – determine how old something is by measuring amount of carbon 14 remaning

4. Half-life Half-life: time required for ½ the original nuclei to decay.
The half-life for a radioactive element is a constant rate of decay.
Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-90 today, there will be 5 g remaining in 29 years.
Half-lives 1 2 3
Years 29 58 87
Strontium-90
10.0g
5.0g
2.5g
1.25g
(29x2)
(29x3)

5. Radioactive Decay Curve
Decay curves show the rate of decay for radioactive elements
Strontium-90
½-lives 1 2 3 4 5
Years 29 58 87
116
145 %
100 50 25
12.5
6.25
3.125
mass
10g 5g
2.5g
1.25g
.625g
.3125g
Radon-222 has short half life – 3.8 days
Uranium-238 has long half life – 4.5 billion years
See pages

6. Common Isotope Pairs
There are many radioisotopes that can be used for dating.
This table can be found on p. 307 and on your data booklet.
Parent isotope – isotope that undergoes radioactive decay
The production of daughter isotope can be a direct reaction or result of a series of decays
Parent isotope = the original, radioactive material.
Daughter isotope = the stable product of the radioactive decay.

7. Radiocarbon Dating & the Potassium-40 Clock
Radiocarbon dating measures the ratio of C-12 and C-14.
When an organism dies, C-14 stops being replenished and slowly decays.
Radiocarbon dating only works for organisms less than years old.
Potassium-40 clock measures the ratio of K-40 and Ar-40.
Rutherford created an application for half life radioisotopes when he used constant rate of decay as a clock to help determine the age of Earth
At every age of the rock, different amounts of parent and daughter
After ONE HALF LIFE, equal amounts of parent & daughter – intersection
More daughter – rock is older than one half life

8. Assignment Classwork: Homework: Practice Problems Q. 1-4 Page 306
Workbook Pages