February 25, 2014
Do Now
What
do you think
“radioactive” means?
http://www.brainpop.com/science/energy/radioactivity/
March 10, 2015
Agenda
Objective
 SWBAT use half-life of radioactive isotopes
to amount of matter.
Agenda
 Do Now
 Notes
 Practice
Homework
 Practice
Questions
Radioactivity
Radioactive Isotopes
 Radioactive isotopes are unstable elements.
They decay, and change into different elements
over time.
Radioactivity
Half-Life
 The half-life of an element is the
time it takes
for half of the material you started with to
decay (change to something else).
 Each element has its own half-life.
Radioactivity
Half-Life (Example)
 An unstable element has a half-life of 2000
years.
 What does this mean?

Every 2000 years, half the sample of element decays
into something else
2000
years
later
1 halflife
2000
years
later
2 halflives
Radioactivity
Radioactive Isotopes
Not all elements are radioactive. Listed below are
some examples of radioactive isotopes:
 Uranium-238
Half-life = 4.5 Billion Years
 Potassium-40
Half-life = 1.25 Billion Years
 Carbon-14
Half-life = 5, 730 Years
Radioactivity
Radioactive Decay
Each element decays into a new element
 Uranium-238
Decays into Lead-206
 Potassium-40
Decays into Calcium-40
 Carbon-14
Decays into Nitrogen-14
Radioactivity
The orange grid below represents 16 grams of Carbon-14 (C-14).
 With each click to a new slide, 1half-life goes by.
 How many years is each click? 5730 Years
 What does the C-14 decay into? Nitrogen-14 (N-14)
Half
Life
% C-14
% N-14
Amount
of C-14
0
100%
0%
16g
1
50%
50%
8g
2
25%
75%
4g
3
12.5%
87.5%
2g
4
6.25%
93.75%
1g
Radioactivity
The orange grid below represents a quantity of Carbon-14 (C-14).
 With each click to a new slide, 1half-life goes by.
 How many years is each click? 5730 Years
 What does the C-14 decay into? Nitrogen-14 (N-14)
Half
Life
% C-14
% N-14
Amount
of C-14
0
100%
0%
16g
1
50%
50%
8g
2
25%
75%
4g
3
12.5%
87.5%
2g
4
6.25%
93.75%
1g
Radioactivity
The orange grid below represents a quantity of Carbon-14 (C-14).
 With each click to a new slide, 1half-life goes by.
 How many years is each click? 5730 Years
 What does the C-14 decay into? Nitrogen-14 (N-14)
Half
Life
% C-14
% N-14
Amount
of C-14
0
100%
0%
16g
1
50%
50%
8g
2
25%
75%
4g
3
12.5%
87.5%
2g
4
6.25%
93.75%
1g
Radioactivity
The orange grid below represents a quantity of Carbon-14 (C-14).
 With each click to a new slide, 1half-life goes by.
 How many years is each click? 5730 Years
 What does the C-14 decay into? Nitrogen-14 (N-14)
Half
Life
% C-14
% N-14
Amount
of C-14
0
100%
0%
16g
1
50%
50%
8g
2
25%
75%
4g
3
12.5%
87.5%
2g
4
6.25%
93.75%
1g
Radioactivity
The orange grid below represents a quantity of Carbon-14 (C-14).
 With each click to a new slide, 1half-life goes by.
 How many years is each click? 5730 Years
 What does the C-14 decay into? Nitrogen-14 (N-14)
Half
Life
% C-14
% N-14
Amount
of C-14
0
100%
0%
16g
1
50%
50%
8g
2
25%
75%
4g
3
12.5%
87.5%
2g
4
6.25%
93.75%
1g
Radioactivity
At this point, how much time has passed by?
 What is the half-life of C-14? 5730 Years
 How many half-lives have gone by? 4 half-lives
 Half-life x # of half-lives = amount of time: 5730 x 4 = 22,920 years
Half
Life
% C-14
% N-14
Amount
of C-14
0
100%
0%
16g
1
50%
50%
8g
2
25%
75%
4g
3
12.5%
87.5%
2g
4
6.25%
93.75%
1g
Radioactivity
Determining Age of a Sample
Scientists use radioactive isotopes found in fossils to
determine the age of the sample

Carbon-14 is found in all organic material (from living
organisms)

Once an organism dies, the organism is no longer
exchanging carbon with the atmosphere and the C-14 in the
organism at the time of death begins to decay.

By determining the ratio of the remaining C-14 to a stable
isotope of carbon (C-14) in the fossil, scientists can figure
out how old it is
Radioactivity
Determining Age of a Sample
Archaeologists found a set of bones in South Africa.
They sent a sample to the lab in order to determine
the ratio of C-14 to C-12 in the bones.
 The lab determined that there the bones had gone
through 2 half-lives
Radioactivity
Determining Age of a Sample
How old are the bones?
 What is the half-life of C-14? 5730 Years
 How many half-lives have gone by? 2 half-lives
 Half-life x # of half-lives = amount of time
5730 x 2 = 11,460 years
Radioactivity
Think, Pair, Share
A
sample of a radioactive isotope has a half-life
of 3000 years. After 12,000 years, how many
half-lives has the sample gone through?
Radioactivity
Guided Questions
2.
Isotope A has a half-life of 5 years. 120 grams of
Isotope A decays in 20 years. How much Isotopes A
is left after 20 years?
What is the half-life of Isotope A?
How many half-lives happened in 20 years?
How much Isotope A is left?
Radioactivity
Guided Questions
3.
The half-life of isotope Y is 3 days. How many days
would it take for 6 grams of Y to decay to 0.75 grams?
How many half-lives does Y need to go through to get to
0.75 grams?
What is the half-life of Y?
How many days would it take to get to 0.75 grams?