Aim: How do scientists read rocks
for an exact 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.
 Protons – are in the
atom’s nucleus.
 Neutrons - are in the
atom’s nucleus.
DO NOT
COPY
I. Isotopes
- the unstable form of an element
(more neutrons than it should which makes it
Radioactive)
UNSTABLE
STABLE
UNSTABLE
II. Radioactivity refers to energy and particles
(neutrons) emitted from unstable isotopes.
(Isotope)
This process is known as Radioactive Decay.
(makes the element stable)
III. Radioactive Decay
• Not affected by outside factors!
• Isotopes (unstable) decay into new stable elements.
• These Isotopes and their new elements are found in
rocks and can be measured.
- We call the Radioactive
Isotopes, Parents.
- Decayed elements are
called Daughters.
IV. Using Radioactive Decay:
(Radioactive dating, Radiometric Dating, Carbon dating,
Absolute Dating)
- Knowing the rate of decay and the ratio of Parent
Isotopes to Daughter elements, we can figure out
how old the object is.
A. Half – Life
- time required for
50% of the Parent
Isotopes to decay
into daughter
elements.
cover of your ESRT.
Which color dot represents the Parent elements?
Which represents the Daughters?
V. 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)
If you can see in a rock sample there
is 25% (1/4) C14 and 75% (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.
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
What is the half life of
Strontium 90?
25 years
THE CHART THAT MAKES HALF-LIFE EASIER
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:
Take out the worksheet you took from the
back