Name _________________________ Hour ________
Chemistry Inquiry: Half life and Carbon-14 dating
Half life
Read the tables to help answer the questions to the right.
Radioactive
substance
A
Half life
5 years
Initial amount
of substance
Amount of
substance after
one half life
Amount of
substance after
two half lifes
400 atoms
2. After how much time did the substance have 200
atoms?
200 atoms
100 atoms
Radioactive
substance
B
Half life
15 seconds
Initial amount
of substance
Amount of
substance after
15 seconds
Amount of
substance after
30 seconds
1. What is the half life of substance A?
3. After how much time did the substance have 100
atoms?
800 atoms
4. What is the half life of substance B?
5. How many atoms of substance B was a left after
15 seconds?
400 atoms
200 atoms
6. How many atoms of substance B was left after 30
seconds?
a. 1/8th
b. A quarter
c. 1/2
1. With the information found above, what is a half life?
Half life and Carbon-14 inquiry. Haggerty
Page 1
Fill in the tables using the information you know about half-lifes.
Radioactive
substance
C
Radioactive
substance
D
Half life
1.3 billion years
Half life
___________
100 atoms
Initial amount
of substance
Amount of
substance after
1 million years
Amount of
substance after
2 million years
Initial amount of
substance
Amount of
substance after
___________
Amount of
substance after
two half lifes
50 atoms
___________
___________
10 atoms
5 atoms
Half life practice
1. If we start with 1,000 atoms of a radioactive substance, how many would remain after
a. one half-life?_________
b. two half-lifes? _________
c. three half-lifes? ___________
d. four half lifes? ________
2. If we start with 48 atoms of a radioactive substance, how many would remain after
a. one half-life?_________
b. two half-lifes? _________
c. three half-lifes? ___________
d. four half-lifes? _______
3. If we start with 16 atoms of a radioactive substance, how much will remain after three
half-lifes? _____________
4.
If we start with 120 atoms of a radioactive substance, how many will remain after three
half-lifes? _______________
5.
A radioactive substance of 1200 atoms has a half life of 10 years. After how many years
will the substance have
a.
b.
c.
6.
600 atoms? __________
300 atoms? __________
75 atoms? ___________
A radioactive substance of 80,000 atoms has a half life of 1 million years. The substance
is now 5,000 atoms.
a. How many half lifes occurred?____________
b. How many years did this take? ____________
Half life and Carbon-14 inquiry. Haggerty
Page 2
Carbon-14 Dating
Carbon-14 is a radioactive material that is formed when cosmic radiation hits Nitrogen14 and turns it to Carbon-14. Carbon-14 is unstable and decays back into Nitrogen-14 with a
half life of 5730 years. This means that it takes 5730 years for half of the Carbon-14 atoms to
decay back into Nitrogen-14. All living things have the same proportion of Carbon-12 to
Carbon-14. Since Carbon-14 continues to decay while Carbon-12 does not, it can be measured
to find the age of a dead organism.
Use the information and graph above to answer the following questions.
1. What is the half life of Carbon-14? _____________________
2. If there is only 12.5% of the Carbon-14 is remaining, how old is the material containing
the Carbon-14? ________________________
3. I have a sample of material containing 600 atoms of Carbon-14. It is said to be 10,740
years old. How many atoms of Carbon-14 did the material have to start with? Show
your work!
Half life and Carbon-14 inquiry. Haggerty
Page 3
4. Napoleon Dynamite found a fossil of a liger (bred for its skills in magic). It is known
that at it’s time of death, a liger would have 3,000 atoms of Carbon-14. At the time
Napoleon found the fossil, there are 187.5 atoms of Carbon-14. How old is the fossil?
Show your work!
Other radioactive substances
Answer the following questions:
1. If I started with 50 atoms of Uranium-238, how many atoms would I have after 4,
470,000,000 years? ______________
2. If I started with 1,000 atoms of Radium-226, how many would be left after 4,800 years?
________________
3. After 16 days, I found 100 atoms of Iodine-131 and 100 atoms of Radon-222.
a. How many atoms of Radon-222 were in the material originally? ____________
b. How many atoms of Iodine-131 were in the material originally? ____________
Half life and Carbon-14 inquiry. Haggerty
Page 4