Radioactive Decay and Half-Life

The rate of radioactive decay can be compared using a measurement called a __________________________.

Half-Life –

The shorter the half-life, the _________________ the decay.

“Common Isotope Pairs Chart”. This identifies the ____________ isotope (the one that decays) and the

_____________ isotope (one of the decay products). The chart also shows the half-life of the parent and the

dating range that the isotope can be used for in radiometric dating.

Below is an example of what happens to atoms of a radioactive element over time.

Amount of Parent Isotope: Amount of Daughter Isotope:

Amount of Parent Isotope: Amount of Daughter Isotope:

Amount of Parent Isotope: Amount of Daughter Isotope:

Complete the table according to the half-life paper demo. Round off decimal numbers.

# of half-lives Fraction of parent remaining (%

remaining)

Fraction of daughter created (% created)

0

1

2

3

4

If we were to graph the number or percentage of remaining parent nuclei versus time, we would always end

up with a certain pattern. We call these graphs _____________ _____________.

The difference between different isotopes

is the _____________ of

their half-lives.

A decay curve can be used to estimate the

amount of parent isotope remaining or the

amount of daughter isotope produced at

any time after the radioactive sample first

formed.

Example: Uranium-235 Decay 1. What is the half-life of uranium-235?

2. What percentage of uranium remains after

1420 million years?

3. What percentage of uranium (parent) remains

after three half-lives?

4. How many years does it take for 60 percent of the

original amount of uranium-235 to decay?

Practice – Understanding Half-life Graphs

Sample A

1. What is the half-life of sample A?

2. At which time is there 80% of parent isotope remaining?

3. What is the percentage of parent remaining after 3 half lives?

4. How much time will it take for 2 half-lives to pass?

Sample B

1. What is the half-life of Sample B?

2. What percentage of parent will remain after 2 half-lives?

3. What percentage of daughter will be produced after 1 half-life? 2 half-lives?

4. How many half-lives have passed if 6000 years have gone by?

5. What percentage of parent remains after 1000 years?

6. What percentage of daughter has been created after 3500 years?

Uses of Radioactive Decay – Radioactive Dating

Understanding the half lives radioactive elements has enables scientists to determine the ___________ of rocks and the remains of living things such as plants and animals.

1. Which items can be Carbon dated using the isotope Carbon-14?

2. Why can’t all the items shown be dated using Carbon-14?

3. Which other isotopes could be used to find the age of these items.

4. What happens to the amount of radioactive isotope if the item that you’re trying to “date” is too old?

Use the list in your data booklet and state which isotopes you think would be useful for dating material that is:

a) 3000 years old

b) 4 billion years old

Uses of Radioactive Decay – Radioactive Dating Word Problems

Use the Common Isotopes Pairs Chart in your data booklet to help you with some of the following.

1. Which parent isotope has the slowest rate of radioactive decay?

3. If an original 100 g. sample of carbon – 14 decayed for 11,460 years…

a) How many half lives have passed?

b) What mass of nitrogen – 14 would have been produced?

4. Iodine – 131 has a half-life of eight days. If a sample contained 512 g of iodine-131, what mass of iodine

would remain after 32 days?

5. A sample of rock contains 800 g of a radioisotope. How much will remain after three half-lives?

6. A fossil is found to contain 6.25% Carbon-14 and 93.75% Nitrogen-14.

a) How many half lives have passed?

b) How old is the fossil?

7. An original sample containing 100 grams of Uranium-235 undergoes radioactive decay. How much Lead-

207 would there be after 2130 million years?