1. By: HULK And Bruce Banner
7.2 Half-Lives
By: HULK
And Bruce Banner

2. Radiocarbon Dating
Used to calculate the age of plant and animal remains
While organism alive carbon-14 to carbon-12 ratio stays same
When organism dies, carbon-14 decays without being replenished
Puny scientists like Bruce Banner measure ratio of carbon-14 to carbon-12 to find how old organism is

3. Rate of Radioactive Decay
Radioactive decay is measured using half-lives
Half-life is constant for any radioisotope
Half-life is the time it takes for half a sample of a radioisotope to decay
Ex: Strontium-90 has a half-life of 29 years
If you had 100g of strontium, after 29 years there would be 50g left

4. Decay Curve Graph of decay of radioisotope
After each half-life, amount of radioisotope drops by half

5. Decay Curve For example:
Iodine-131 is used for treating thyroid cancer
It has a half-life of 8 days
Suppose you have 20g of iodine-131
You can find out how much will remain after 16 days:
16 days = 2 half-lives
20𝑔 × 1 2 × 1 2 =5𝑔

6. Common Isotope Pairs Isotope that decays called parent isotope
Stable product(s) of parent isotope's decay called daughter isotope(s)
On next slide, some common isotope pairs that can be used for radioisotope dating

7. Half-Life of Parent (y) Effective Dating Range (y)
Common Isotope Pairs
Isotope
Half-Life of Parent (y)
Effective Dating Range (y)
Parent
Daughter
Carbon-14
Nitrogen-14
5730
Up to
Uranium-235
Lead-207
710 million
> 10 million
Potassium-40
Argon-40
1.3 billion
to 3 billion
Uranium-238
Lead-206
4.5 billion
Thorium-235
Lead-208
14 billion
Rubidium-87
Strontium-87
47 billion

8. Potassium-40 Clock Potassium-40 has a half-life of 1.3 billion years
It’s daughter isotope is argon-40
When rock is created from lava, the argon-40 is forced out leaving only potassium-40
After time the potassium-40 decays and creates argon-40 which gets trapped in the rock
When scientists examine the rock they can use the amount of argon-40 to find the age of the rock
Since these rocks were probably formed at the beginning of the earth’s life, the age of the rock is close to the age of the earth

9. Potassium-40 Clock
As the amount of potassium-40 decreases, the amount of argon-40 increases
Using the graph and the ratio of potassium-40 to argon-40, scientists can find how old the rock is

10. Video

11. Quiz Which two isotopes get compared in radiocarbon dating?
What is used to measure radioactive decay rate?
Does a half-life change for a given radioisotope?
What is a decay curve?
What is the daughter isotope of uranium-235?
What happens to the amount of argon-40 as the amount of potassium-40 decreases?

12. Answers Carbon-12 and carbon-14 Half-life No
A graph of the decay of a radioisotope
Lead-207
The amount of argon-40 increases