October 7, 2014
CHAPTER 4: RADIOACTIVITY
Radioactivity
•  These unstable nuclei emit radiation to
attain more stable atomic
configurations in a process called
radioactive decay
•  During radioactive decay, unstable
atoms lose energy by emitting one of
several types of radiation
What are the forms of radiation?
1. Alpha particle- a helium nucleus (two
protons and two neutrons)
- When an alpha particle is released from a
nucleus, a new atom is formed (new atomic
number)
2. Beta particle- high speed electron.
Created at the instant of decay since
there are no electrons in the nucleus
3. Gamma rays – high frequency, high energy
x-rays
Alpha Decay
Beta Decay
+
Gamma Decay
Radioactivity Facts
Q: What is the most dangerous or damaging
form of radiation?
A: gamma rays
Q: Why is it dangerous?
A: can damage DNA in living things
Q: How do we detect radiation?
A: Geiger counter (a particle entering the tube of
a Geiger counter triggers an electric current and
registers as a pulse that is recorded as a sound and a
count by the detector)
Shielding from Radiation
•  What materials are best for shielding a
person from radiation?
–  Usually a lead vest/high density
materials
•  Examples of materials:
–  Lead, concrete, paper, wood, plastics, sheet
metals
Examples
•  Which type of decay is this? Circle the
radiation particle.
•  ANSWER: ALPHA DECAY
Examples
•  Which type of decay is this? Circle the
radiation particle.
•  ANSWER: BETA DECAY
ALPHA
BETA
GAMMA
SYMBOL
^2↑4↓𝐻𝑒 ^−1↑0↓𝑒 ^0↑0↓γ MASS
Heavy
Light
No Mass
HOW
NUCLEUS
CHANGES
Decrease mass
number by 4
Decrease atomic
number by 2
Increases
atomic by 1
No change to
nucleus
ABILITY TO
G0 THRU
MATERIAL
LOW
MEDIUM
HIGH
PROTECTED
BY…
Skin
Lead
DANGER
Low
Paper,
clothing
Moderate
High
Radiochemical dating
The radioactive decay of
elements can be used to
estimate the date of older
objects
Isotope
Half Life
Carbon-14
5,730 years
Potassium-40
1.25 billion years
Uranium-238
4.5 billion years
Radiochemical dating
-The decay of
the carbon-14
isotope is used
to estimate the
age of formerly
living objects
up to 50,000
years old
How Carbon-14 Is Produced
Cosmic Rays
(radiation)
Forms C-14
Collision with
atmosphere (N-14)
C-14 combines with
oxygen to form carbon
dioxide (CO2)
Carbon Dating
Once an organism dies, C-14 decays into N-14.
14
14
6
7
1 half-life
2 half-lives
3 half-lives
Radioactive Decay and Half Life
Here are some facts to remember:
1. The half-life of an element is the time it
takes for half of the material you
started with to decay.
2. Each element has it’s own half-life
3. Each element decays into a new element
4. The half-life of each element is
constant
16
The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and turns red.
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
0
100%
0%
14
no ratio
As we begin notice that no
time has gone by and that
100% of the material is C14
17
The grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
14
0
100%
0%
no ratio
1
50%
50%
1:1
After 1 half-life (5730 years), 50% of
the C14 has decayed into N14. The ratio
of C14 to N14 is 1:1. There are equal
amounts of the 2 elements.
18
The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red .
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
14
0
100%
0%
no ratio
1
50%
50%
1:1
2
25%
75%
1:3
Now 2 half-lives have gone by for a total
of 11,460 years. Half of the C14 that was
present at the end of half-life #1 has now
decayed to N14. Notice the C:N ratio. It
will be useful later.
19
The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
14
0
100%
0%
no ratio
1
50%
50%
1:1
2
25%
75%
1:3
3
12.5%
87.5%
1:7
After 3 half-lives (17,190 years) only
12.5% of the original C14 remains. For
each half-life period half of the material
present decays. And again, notice the
ratio, 1:7
20
Carbon Dating Sample Problem
An artifact is discovered by archaeologists. Analysis
reveals that 12.5% of the original carbon-14 remains in
the artifact. Using carbon dating, what is the
approximate age of the artifact? The half-life of C-14
is 5,730 years.
•  Step 1: Determine how many half-lives have
passed
100/2 = 50/2 = 25/2 = 12.5
3 half-lives
•  Step 2: Calculate the number of years
3 x 5,730 years = 17,190 years
Carbon Dating
•  After about 50,000 years, there is only a
very small amount of C-14 left so it can’t
be used for dating
u  Scientists
use isotopes
with much longer halflives for older objects:
Isotope
Half Life
Carbon-14
5,730 years
Potassium-40
1.25 billion years
Uranium-238
4.5 billion years
Radiochemical Dating Problem
Which radioactive isotope should be used to date these
objects: U-238 or C-14?
•  Tyrannosaurus rex fossil
U-238 (older than 50,000 years)
•  A meteorite that landed at MHS
U-238 (older than 50,000 years)
•  An Egyptian mummy
C-14 (less than 50,000 years)
•  An ancient wooden tool found in the mountains in Peru
C-14 (less than 50,000 years)
M&M Half-Life Lab
Intro to lab
1.  Know the following
terms…
–  Radioactive dating
–  Half-life
–  Parent-isotope
–  Daughter-isotope
2.  Read Directions.
3.  This will be done in
groups of about 3-4.
Number of Parent Isotope
M&M Half-Lives
Number of Half-Lives