Isotopes and Half Lifethe light bulb is a reminder you need to do

something on your notes page

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

Slide 2

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.

Half-Life of a Radioisotope

Slide 3

Parent nucleus

Daughter nucleus

½ life

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?

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

Answer = 2 million years

Determining the Half-Life of Pennies Activity

Slide 4

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 = 100Daughter element = heads down (tails up) = 04. 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.

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.

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 2 mg 1 mg

initial1

half-life 2 3

Slide 5

Radiometric datingThe 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

Using Radioisotopes to Determine the Age of a Rock

Slide 7

Using Radioisotopes to Determine the Age of a Fossil

Slide 8

What radioactive element would you use to determine the age

of anything that was once alive (organic)?

Common Isotope Pairs 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.

Slide 9

This table can be

found in your

data booklet

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.

Slide 10

Use the graph - What is the half life of C-14?A: 5730 years

Carbon’s Half-life Carbon dating only works for

organisms less than 50 000 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

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.

Slide 12

What percentage of Strontium-90 will be left after

2 half lives? A = 25%

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

The Potassium-40 ClockWhat percentage of potassium-40

Is left after 3.9 billion years?

If you had 100 g of

Potassium-40, how many g

of Argon-40 would be

produced after 5.2 billion years?

Slide 14

A = 12.5 %

A: 93.75 g

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

Potassium-40 Clock

Slide 15

Potassium-40 Clock

Slide 16

Notice the percent of each

Potassium isotope in a

banana.

Would you use this isotope

to determine the age

of this banana?

Uranium-235 decay

Slide 17

Use the graph to determine

the half life of U-235

A = 710 million years