Nature’s CloCk



When
sedimentary rock
The Law of Superposition doesn’t tell scientists how old a rock layer is, it only tells them that
is deposited in layers
it isisolder than another.
one layer
deposited horizontally.
Another way to say this is that the Law of Superposition tells scientists the order the layers
Scientists
use this
were deposited.
“Principle of Original
Horizontality”
toofhelp
Any method
dating that only orders events only measures:
them determine the age
of the layers.
The older layers of rock
are found below the
younger layers of rock.
Relative Age



If the Law of Superposition only tells scientists
relativeToage,
how radioactive
can we dating
determine
theto
understand
you
first
need
Any method that determines the numerical age of an object measures:
numerical
ageradioactive
of a rock
layer,
fossilofor
understand
decay
and theaconcept
half-an
artifact?
life.
To determine the numerical age of a geologic
object
fossil,
stone
scientists
To(rock,
understand
radioactive
decaytool,
and the…)
concept
of half- use a
method
Radioactive
Dating.
lifecalled
you need
to understand
the structure
an atom.
So radioactive
dating measures
absoluteof
age.
In radioactive dating, scientists find traces of a
radioactive material in the object and determine
the number of times that material has decayed to
determine the age of the object.
Absolute Age


An atom consists of a
nucleus containing
positively charged
protons and neutral
neutrons.
The negative
electrons surround
the nucleus in an
electron cloud.
Proton
Electron
Neutron
1 protons?
2
proton?
What element has 76
H
Hydrogen
He
C -- Carbon
- Helium
What element has 92 protons?
You Decide!!!!!!!





The number of protons determines the type of
atom.
On the periodic table, the number of protons
is shown as the atomic number.
Since the protons are all positively charged
they want to repel (push away from) each
other.
So what keeps the nucleus of an atom
together?
Enter the neutrons.




Neutrons have a very important job in the nucleus
of an atom.
They supply a strong force that glues the protons
together in the nucleus.
Neutrons are unstable when they are isolated from
protons.
When a neutron is by itself or just very far away
from a proton they break apart (decay) into a
proton and an electron.
If I have two atoms of the same element, they must
Carbon has 6 protons in its nucleus.
have the same number of protons.
 But these same two atoms of the same element can
A stable isotope of carbon also has 6 neutrons in its
have different numbers
of neutrons.
nucleus.
 Atoms of the same element that have different
numbers
of neutrons
are called
isotopes.
The symbol
for this isotope
of carbon
is either:
 Different isotopes of an atom are shown as:

or
or





When a nucleus is unstable it can emit (spit
out) particles or energy.
This is called radiation.
When the nucleus emits particles it changes
the type of atom.
This is called radioactive decay.
There are three types of radiation we will
look at here:
Alpha Decay
Beta Decay
Gamma Radiation
An Alpha Decay changes the atom because during the decay the atom loses 2
protons.
 When the number of protons
in the nucleus of an
is very
compared
to(the
the
number
ofdecay),
Inatom
the example
shownlarge
below, the
parent nucleus
nucleus
before the
Uranium, has 92 protons before it decays.
neutrons, the protons repel each other with more
After
it decays
and emits
alpha particle,
the daughter
nucleus
(what’s
left after
force
than
thean
neutrons
can
provide
to glue
the
the decay) now has 90 protons because it lost 2.
protons together.
This
meanscauses
the uranium
nucleus with
92 neutrons
protons has changed
into
thorium with
 This
protons
and
to
be
expelled
90 protons!
(pushed out) of the nucleus.
 An Alpha Particle (α) is what is expelled from
the nucleus.
 An Alpha particle consists of 2 protons and two
neutrons – it is the nucleus of a helium atom!!!
A beta decay changes the atom because it adds a proton to its nucleus.
 When the number of neutrons in the nucleus
In the beta decay shown below, the parent nucleus is potassium which
of an atom
has 19becomes
protons in itsmuch
nucleus larger
before thethan
decay. the
number of protons the neutrons begin to
After the decay, a beta particle is expelled adding one proton to the
decayofinto
a proton
and
an
electron.
nucleus
the daughter
nucleus
giving
it 20
protons. This makes the
calcium!!!
 The proton isdaughter
addednucleus
to the
nucleus of the
atom.
 This changes the type of atom.
 The electron is expelled (pushed out) from
the nucleus.
 This electron is the beta(β) particle.




After an alpha or beta decay, the daughter
nucleus has a large amount of stored energy
in it.
We say it is excited.
When the nucleus “shakes out” this energy it
releases a high energy gamma (γ).
The release of the gamma ray does not
change the type of atom, although the atom
changed due to the alpha or beta decay.
Below
is a radioactive
link to an animation
half-life.
While you
 When
element of
decays
it decays
gradually
over
time, notnotice:
all at once.
work
with this
animation
 The time it takes half (50%) of the atoms in a
decay
is called
the
half-life
of the
1)substance
What effecttodoes
changing
the
half-life
have
on
substance.
how quickly the dots disappear?
 For example, if you now have 200 atoms of a
2)How
much time
does it then
take for
halfhalf-life
the dotsof
tothe
radioactive
substance,
in one
disappear?you will have 100 atoms.
substance
 That is half of 200.
 The half-life for different substances can be less
Click on
pictureto
to go
to the
than
a the
second
millions
of years.
animation!
 But the half-life of the same substance (isotope)
will remain the same.







A very important radioactive isotope for radioactive
Number
of HalfNumber of
Number of C-14 Fraction of C-14 Percent of C-14
dating
is carbon-14.
Lives
Years
Atoms Left
Atoms Left
Atoms Left
C-14 undergoes a beta decay and decays into
nitrogen-14.
5730
256
1
100%
C-140 has a half-life
of 5730-years.
11,430
128
1/2
50%
This1 means that
on average
½ (50%)
of the C-14
will
2
17,190
64
25%
decay
every 5730-years
into
N-14. 1/4
22,920 started with
32
1/8
Let’s3 assume we
256 C-14
atoms,12.5%
this
4
28,650 5730-years
16
1/16
6.25%
means
that after
we should
have 128
atoms
(1/2 or 34,380
50%).
5
8
1/32
3.125%
After
5730-years4 we should1/64have 64 1.5625%
atoms
6 another40,110
(1/47or 25%).45,840
2
1/128
0.78125%
This8 will continue
until there
are no
more C-14
51,570
1
1/256
0.390625%
atoms
left.
9
57,300
0
0
0%




While an animal or plant is living it takes in
two different isotopes of carbon.
Carbon 12 which is stable and Carbon-14
which is unstable.
After the animal or plant dies, it no longer
takes in C-12 or C-14.
At this point the C-14 in the animal or plant
will decay and not be replaced but the
amount of C-12 will remain the same.



Scientists can use the amount of C-12 in the
animal or plant to determine the amount of C-14
in the animal or plant at its death.
Once the original amount of C-14 is known
scientists can compare this to what is left and
figure out the number of half-lives that have
occurred since the animal or plant died.
Since the half-life of C-14 is 5730 years,
multiplying the number of half-lives that have
occurred by 5730 years gives a very good
estimate of absolute age of the plant or animal.
A clay pot was found buried near an ancient temple . Based on an
analysis of the amount of C-12 in the clay pot, it has been
determined that the pot originally had 0.034 grams of C-14. The
same analysis has determined that the clay pot now contains
0.0010625 grams of C-14. How old is the clay pot?
The best way to
solve this problem
is to use a table to
figure out the
number of halflives:
Mass 0f C14
Number of
Half-Lives
0.034-g
0
0.017-g
1
0.0085-g
2
Each half-life takes 5730years.
0.00425-g
3
This means that the pot is
0.0002125-g
4
5 X 5730 = 28,650 years old.
0.0010625-g
5
5 half-lives have occurred since
the materials that contained
carbon in the clay pot “died”.
Using this technique, almost any sample of organic
material can be ABSOLUTELY dated. There are
a number of limitations, however:
1) First, the size of the archaeological sample is
important. Larger samples are better, because
purification and distillation remove some
matter.
2) Second, great care must be taken in collecting
and packing samples to avoid contamination by
more recent carbon. Also, the sample should be
carefully examined to determine that a carbon
sample location was not contaminated by
carbon from a later or an earlier period.
Using this technique, almost any sample of organic
material can be ABSOLUTELY dated. There are a
number of limitations, however:
3) Third radiocarbon dating has significant upper
and lower limits. It is not very accurate for fairly
recent deposits. In recent deposits so little decay
has occurred that the error factor may be larger
than the date obtained. The practical upper limit
is about 50,000 years, because so little C-14
remains after almost 9 half-lives that it may be
hard to detect and obtain an accurate reading,
regardless of the size of the sample.
Using this technique, almost any sample of organic
material can be ABSOLUTELY dated. There are a
number of limitations, however:
4) Fourth, the ratio of C-14 to C-12 in the
atmosphere is not constant, although it was
originally thought that it was. To compensate for
this variation, dates obtained from radiocarbon
laboratories are now corrected using standard
calibration tables developed in the past 15-20
years.