Absolute Dating by
the Use of
Radioactive Isotopes
What is Absolute Dating?

Age of fossil or rock is given in years
instead of relative terms like before and
after, early and late.

Does not mean it isn’t without error

Radiometric dating is the most
common type of absolute dating.
Atoms and Isotopes: Let’s
review the basics
Chemical symbol
for element
Mass #
(protons +
neutrons)
A
Z
X

The number of protons in an atom
determines which element it is.

If you change the # of protons, the
element changes & the mass
changes.

If you change the # of neutrons, the
element stays the same, but the
mass changes

ISOTOPES - atoms of the
same element that have
different numbers of neutrons
Atomic # (protons)
http://www.saskschools.ca/curr_content/c
hem30/images/e_deuterium.jpg
Why Are Some Isotopes Radioactive?



Isotopes that have the right amount of neutrons are
called stable. They always stay the same.
Some isotopes have a few too many neutrons or not
enough - This makes them unstable and radioactive.
The nuclei of these radioactive atoms change or
decay by giving off radiation in the form of particles or
electromagnetic waves until the atom reaches a
stable state.
Radioactive Decay







During radioactive decay, the number of protons in
the atom changes, and one element transforms into
another.
Parent isotopes decay into daughter isotopes.
Radioactive Decay is like popping popcorn.
Each radioactive parent always
decays to a specific daughter.
There is no way to predict
which atoms will decay first.
Radioactive atoms decay at a
specific rate.
Once they decay, they can
not change back.
How Long Does Radioactive Decay Take?

Half-Life - the time it takes for half of the
radioactive or parent isotopes in a sample to
decay to daughter isotopes.



Each parent has a 50% chance of decaying during
1 half-life.
Measured in seconds, minutes, years, etc.
Each isotope has its own unique half-life.

From thousandths of a second to billions of years
Starting the Stopwatch

If you measure the ratio of parent to daughter isotopes, you can
determine how many half-lives have passed.
Find the half-life of the parent isotope

# of half-lives  length of half-life = age of sample

Example: 3 half-lives; 1 half-life = 200 years

Atoms Don’t Age the Way We Do
1
2
3
4
Start with 16 baby
aliens
Have 70 year half-lives
4 half-lives = 280 years
Each atom has a 50%
chance of decaying
during a half-life.
http://www.colorado.edu/physics/2000/isotopes/ima
ges/age280_baby.jpg
How to Choose Which Isotope to Use



K-40: feldspar & mica
Uranium: zircon
Estimate the age of your
sample and choose an
isotope with an appropriate
range.
First find out what minerals
are in your sample. The
minerals in your rock need to
have the element you want to
use for dating.
Carbon-14 can only be used
to date samples that were
once living (organic)

Ex: Wood, bone, cloth,
paper
Let’s Practice Absolute Dating