Radiometric Dating
•Uses radioactive
elements to determine
the absolute age (age in
years) of a rock or fossil
• Only some
materials can be
dated using this
method
Volcanic Ash
Organic material
Igneous Rocks
Some sedimentary rocks
Structure of the Nucleus - Review
Which elements are these?
(protons are shown in red
and neutrons in white.)
They are both carbon. Both have 6 protons. i.e. they
both have an atomic number of 6.
•These are two isotopes (varieties) of carbon.
- same chemical properties, but different physical
properties (e.g. how they behaving in nuclear reactions)
- different number of neutrons, therefore different atomic
masses
• Some isotopes are unstable and therefore radioactive.
Two isotopes of carbon:
Carbon-12
Carbon-14
common
and stable
12
6
C
Mass number = 12
rare &
radioactive
14
6
C
Mass number = 14
p+ = 6
p+ = 6
n0 = 6
n0 = 8
Mass number = #p+ + #no 12
C
6
+
Atomic number = #p
Because radioactive isotopes are unstable, they undergo
radioactive decay:
• Energy is released (in the form of radiation)
• The atom transforms into a completely different element
Parent Material
(the original unstable
atom)
Radioactive
Decay
Daughter Material
(the stable product left
after decay happens)
“When I was young I used to feel so
alive, so dangerous! In fact, would you
believe I started life as Uranium-238?
Then one day I accidentally ejected an
alpha particle … now look at me, a
spent old atom of lead-206. Seem that
all my life since than has been nothing
but decay, decay, decay…”
Half life: the time it takes for half of
the parent radioactive element to
decay to a daughter product.
• Each isotope has a particular
Half-life. This allows us to date
the sample containing radioactive
isotopes.
12.5%
50%
25%
100%
% of Radioactive Material
Radioactive Decay Curve
Time
• Different radioactive isotopes have
different half lives
Good for dating really old
volcanic layers and meteorites
Carbon -14 is good for dating
actual remains. To a limit of
about 60 000 years
Example 1:
A baby mammoth found
frozen in a glacier is
found to contain one
quarter of its original
carbon-14. Determine
its age.
Elements for Radioactive Dating: Page 8 of Data Booklet
Time elapsed since death
(years)
Amount Remaining
5.73 x 103 (5730)
1/2
11460
1/4
The baby mammoth is 1.15 x 104 years old.
Example 2:
A sample of orthoclase is dated
using potassium–40. A mass
spectrometer analysis shows
that 20% of the potassium-40 is
present in the sample. Use this
data to determine the age of
the orthoclase sample.
Elements for Radioactive Dating: Page 8 of Data Booklet
1. Determine if potassium-40 is the parent nuclei
2. Use decay curve to determine # of half lives elapsed
3. (2.3 half-lives)(1.26 x 109 a/half-life) = 2.9 x 109 a
The orthoclase is 2.9 x 109 years old.
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