1. 4/27/2017
Isotopes and Half Life
the light bulb is a reminder you need to do something on your notes page

2. Half-Life of a Radioisotope
The time for the radiation level to fall (decay) to one-half its initial value
Imagine a situation where you are given a chocolate cake BUT
you are told you can only eat ½ of the cake each day.
We would say the cake has a ½ life of 1 day.
Day 1
dish has 100%
cake
Day 2
50% cake and
50% empty dish
Day 3
25% cake and
75% empty dish
Day 4
12.5% cake and
87.5% empty dish
Slide 1

3. Half-Life of a Radioisotope
Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability.
Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation.
The most common types of radiation are called alpha, beta, and gamma radiation, but there are several other varieties of radioactive decay.
Slide 2

4. Half-Life of a Radioisotope
Daughter nucleus
Parent nucleus
If you had 8 g of this parent nucleus and it has a half life of 1 million years,
how long would it take to produce 6 g of the daughter nucleus?
Answer = 2 million years
Day 1
8 g
parent
1 million years later
4 g parent
4 g daughter
2 million years later
2 g parent
6 g daughter
Slide 3

5. Determining the Half-Life of Pennies Activity
The rate at which a radioisotope decays can be simulated by tossing pennies and letting them land randomly. In this lab you will use the tossing of pennies to generate a radioisotope DECAY CURVE.
Procedure:
1. On the back of your graph paper, make a table that looks like the following. You will need more than 5 shakes.
2. Place 100 pennies into the dish and turn so that all the heads are facing up.
3. In your table, complete the first row.
Parent element = heads up = 100
Daughter element = heads down (tails up) = 0
4. Put the lid on the dish and shake vigorously five times. Remove lid and count number of pennies with heads up (parent element) and heads down (daughter element). Record this in the second row of your table.
5. Remove the pennies that are heads down (tails) from the dish. Now you have only parent elements again (heads up)
6. Repeat steps 4 and 5 until you have no pennies left in your dish.
Slide 4

6. Half-Life of a Radioisotope
4/27/2017
Half-Life of a Radioisotope
The time for the radiation level to fall (decay) to one-half its initial value
decay curve
8 mg 4 mg mg mg
initial 1
half-life 2 3
Slide 5

7. Radiometric dating
The oldest known tree on earth is a bristlecone pine tree growing in the California White Mountains = 4780 years old
How do we determine how old this tree is?
Slide 6

8. Using Radioisotopes to Determine the Age of a Rock
Slide 7

9. Using Radioisotopes to Determine the Age of a Fossil
What radioactive element would you use to determine the age
of anything that was once alive (organic)?
Slide 8

10. Common Isotope Pairs This table can be found in your data booklet
There are many radioisotopes that can be used for dating.
The rate of decay remains constant, but some elements require one step to decay while others decay over many steps before reaching a stable daughter isotope.
Carbon-14 decays into nitrogen-14 in one step.
Uranium-235 decays into lead-207 in 15 steps.
Thorium-235 decays into lead-208 in 10 steps.
This table can be
found in your
data booklet
Slide 9

11. Radiocarbon Dating
All living organisms on the planet have Carbon atoms - because they’re organic.
Carbon dating measures the ratio of carbon-12 to carbon-14.
Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio.
Carbon-14 is formed at a nearly constant rate in the upper atmosphere by the bombardment of nitrogen-14 with neutrons from cosmic radiation. The carbon-14 is eventually incorporated into atmospheric carbon dioxide.
When an organism dies, carbon-14 stops being created and slowly decays.
The ratio of C-14:C-12 decreases with time
We use this to carbon date.
Use the graph - What is the half life of C-14?
A: 5730 years
Slide 10

12. Carbon’s Half-life
Carbon dating only works for organisms less than years old.
Why? What carbon’s half life?
The half-life for carbon-14 is 5,730 years.
Using carbon dating, these cave paintings of horses, from France, were drawn 30, 000 years ago.
Slide 11

13. The Rate of Radioactive Decay
Half-life measures the rate of radioactive decay.
Half-life = time required for half of the radioactive sample 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,
How many grams will you have in 29yrs?
___5 g_____________
Decay curves show the rate of decay for radioactive elements.
What percentage of Strontium-90 will be left after
2 half lives?
A = 25%
Slide 12

14. The Potassium-40 Clock
Radioisotopes with very long half-lives can help determine the age of very old things.
The potassium-40/argon-40 clock has a half-life of 1.3 billion years.
Argon-40 produced by the decay of potassium-40 becomes trapped in rock.
Ratio of K-40 : Ar-40 shows age of rock.
Slide 13

15. The Potassium-40 Clock What percentage of potassium-40
Is left after 3.9 billion years?
A = 12.5 %
If you had 100 g of
Potassium-40, how many g
of Argon-40 would be
produced after 5.2 billion years?
A: g
Slide 14

16. Potassium-40 Clock What is it?:
A tool to determine how old the earth is
Looks at molten rock and analyzes how much K-40 is inside compared to Ar-40
When rock is produced from magma, all gasses are driven out; including the noble gas Ar-40
At this point the potassium radioisotope clock is set to ZERO!
After long periods of time, the K-40 (PARENT ISOTOPE) in this rock is converted to Ar-40 (DAUGHTER ISOTOPE) through radioactive decay
Slide 15

17. Potassium-40 Clock Notice the percent of each Potassium isotope in a
banana.
Would you use this isotope
to determine the age
of this banana?
Slide 16

18. Uranium-235 decay Use the graph to determine the half life of U-235
A = 710 million years
Slide 17