1. Half Life & Radioactive Decay
Aim: How is half-life used to determine absolute age?

2. Do Now How old do you think this tree is? How can you tell?
How might tree rings be used to help date past events?

3. Radioactive Decay
Because scientist can’t ask a rock its age, they have had to find a different way of determining the absolute ages of rocks. The solution lies in the smallest unit of matter, the atom. Atoms make up everything on Earth, including you and rocks. The atoms of many chemical elements exist in various forms. Some of these forms are unstable and break down over time into another form. This breakdown– called – radioactivity– is a very useful clock because a particular unstable form of an element always breaks down at the same rate into the same other form.

4. Radioactive Decay Con’t…
Radioactive elements occur naturally in igneous rocks. Scientists use the rate at which these elements decay to calculate the rock’s age.
The rate of decay of each radioactive element is constant– it never changes.
This rate of decay is the element’s half life.

5. I. Radioactive Dating
Using elements that naturally break down to accurately tell how old a rock or fossil is. A. Isotope- when you have several forms of chemical elements that differ in the number of neutrons in their atomic nuclei.

6. II. Half Life
Half-life: is the time it takes for half of the radioactive atoms to decay into a decay product.

7. Half-Life Continued
Decay Product- Daughter product, the element produced by the decay of a radioactive isotope.
By studying the amount of decay product left over from the radioactive elements in a rock, we can determine the absolute age.
Example- If a fossil with C14 is found containing 50% C14 and 50% N14, how old is the rock?

8. Elements Used in Radioactive Dating
Carbon 14-All plants and animals contain carbon-14. As they grow carbon atoms are added to their tissues. After an organism dies, the carbon-14 begins to decay. It changes into the stable nitrogen-14.
Because of it’s short half life (5,700 years) it is only useful in dating fossils up to the age of 50,000 years old.
Carbon Dating

9. Half Life Curve

10. 1/ ,800
1/ ,500
1/ ,200