Transcript 12.3 Dating with Radioactivity

Aim: How can the decay of an
organism tell us its age?
Do Now: Think about this… If
you were standing 10 feet
from the wall, and I asked you
to walk half the distance to
the wall, how close would you
be? If I asked you to repeat
this process, would you ever
reach the wall?
Basic Atomic Structure
 Electrons – Orbit the
nucleus (negative electrical
charges).
Atomic number is the
number of protons in the
atom’s nucleus.
 Mass number is the
number of protons plus
neutrons in an atom’s
nucleus.
DO NOT
COPY
I. Isotopes
- the unstable form of an element.
- The atoms have the same number of
protons and different number of
neutrons.
EXAMPLE
DO NOT
COPY
Stable Element, Carbon-12:
-6 Protons
-6 Neutrons
Unstable Element (Isotope), Carbon-14:
-6 Protons
-8 Neutrons
II. Radioactivity refers to energy and
particles emitted from unstable isotopes.
(Isotope)
This is known as radioactive decay
III. Absolute Age : (Radioactive dating)
- Using precise numbers to tell when an
organism lived or an event occurred.
- This can be done by using radioactive
isotopes.
cover of your ESRT.
IV. Radioactive Decay
• Continues at a regular rate (not
affected by outside factors).
• The first isotope is called the parent
nuclei. (unstable)
• When it decays, it turns into the
daughter nuclei. (stable isotope)
A. Half – Life:
- time required for half of the atoms in
a sample of radioactive element to
decay.
**The parent
nuclei (Red)
changes to the
daughter nuclei
(Blue)
V. Dating with Carbon-14
- Used to date most recent organic
material. (organisms and plants)
- Shortest half-life.
- Found in people
Examples: Use your ESRT
How many years will it take for two halflives to pass of C14? 11,400 yrs
• What % of C14 will remain at this point?
25% C14 will remain
• Which isotope has the longest half life?
What is it? Rubidium-87 (87Rb)
Example 1: Use your ESRT
If you can see in a rock sample
there is 1/4 C14 and 3/4 N14, the
sample must have gone through
how many half-lives?
Two half-lives.
How old is this sample?
11,400 yrs old.
EXAMPLE
A student found a fossil and by looking at
the radioactive decay of C14 she found
there to be 1/16th of C14 remaining in the
fossil. How many half lives has this
sample gone through?
The sample has gone through 4 halflives
Half-Life
0
% Parent
Remaining
1/1
Number of
years
0
1
1/2
5,700
2
1/4
11,400
3
1/8
17,100
4
1/16
22,800
• Closure:
Obtain the worksheet form the back, we are
going to work on it together.