The fossil below is that of a Coelophysis
and was found in upstate N.Y.
Approximately how old is the rock?
Scientists made the geologic time scale
using fossils and call it a relative time
scale, they can give a range.
Radioactive Decay: Scientists have
devised a way of using the decay rate
of Radioactive Isotopes (Atoms).
Atoms are supposed to have the same
number of protons and neutrons
No Big deal. Eventually it breaks
down into its normal stable self
Carbon 14 is unstable (parent), while
Nitrogen 14(daughter) is stable.
Carbon 14 is unstable:
It has an uneven number of protons
and electrons
Once the original element changes it can
never go back. Whatever is lost by the
“parent” is gained by the “daughter”
This breakdown is predictable, and is measured
in half life which is the time required for ½ (50%)
the radioactive element to change to the decay
product.
If no time has passed that means
you have 100% of your original
• element and 0% new element.
If ONE half life has passed that
means you have 50% original and
?
After every HALF LIFE, you cut
the original product in HALF.
So after one half life you have
50/50
How much of the original is left
after TWO half lives?
Keep cutting the original in half.
Number of Half Original Product Decay Product
lives
0
100%
0%
1
50%
50%
2
25%
75%
3
12.5%
87.5%
4
6.25%
93.75%
Unstable
Stable
You can tell how old something is
by counting how many parent and
daughter isotopes you have.
For example equal amounts of C 14
and N 14 means it has gone through 1
half life. 5,700 years.
How many years is 2 half lives?
How much C14 is left?
Half
Lives
0
1
2
3
Original remaining
# of years
If 100 grams of pure C 14 starts to
decay, how much N 14 will be
created in 11,400 years? How many
½ lives is that?
A 200 gram rock sample contains 25
grams of K 40, what is the age of
this rock?
A rock sample contains 75%
Argon and 25% Potassium.
How many half lives have
passed, and how long did it
take?
After three half-lives, how much of the
original sample of uranium-238 remains?
If an original sample of radioactive uranium-238
had a mass of 400 grams, what is the total
amount of the uranium-238 sample that would
be left after 9 billion years?
If the half-life of the radioactive material is
1,000 years, what is the age of the sample
represented by the diagram?
Draw what you think the box will
look like after 3 half lives