1. Rate of Radioactive Decay

2. 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. (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.

3. 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)

4. 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
See pages

5. Common Isotope Pairs
There are many radioisotopes that can be used for dating.
Parent isotope = the original, radioactive material.
Daughter isotope = the stable product of the radioactive decay.

6. Radiocarbon Dating & the Potassium-40 Clock
Radiocarbon dating measures the ratio of C-12 and C-14.
When an organism dies, carbon-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.

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