Do Now – 5 Minutes
Topic – How Do We Know How Old the
Earth is?
• Turn to your partner and discuss:
• How can we figure out how old a fossil
is?
• What happens when something
decays?
• What does half-life mean?
Homework
Geologic Time Scale Project
If you need to submit your presentations, do
so immediately.
After tomorrow it is going to be considered
late.
How Old is the Earth?
We can figure out how old the Earth is based
on the age of layers of rock.
How Old is the Earth?
We can also look at fossils and use
radioactive dating to determine the age of
the Earth.
The Atom
All objects that exist are all made of atoms.
The nucleus is made up of protons (+),
neutrons (0), and electrons (-).
The Atom
In a normal, stable atom, the number of
protons is equal to the number of neutrons.
Carbon-12
6 (+), 6 (0)
Nitrogen-14
7 (+), 7 (0)
Decay
However, there are naturally occurring
variations in atoms, called isotopes.
Isotopes have more neutrons than a normal,
stable atom.
To become more stable, the atom will decay –
changing into a different, more stable atom.
Decay
14
14
6
7
Depending on the element, this decay can
happen very quickly or a LONG time.
The half-life of a radioactive material is
the amount of time it takes for half of the
material to decay into another element.
Half-Life
The half-life of Carbon-14 is 5,730 years.
Time = 0
C-14
11,460 years
2 half-lives
5,730 years
1 half-life
N-14
1/2
N-14
3/4
C-14
1/2
C-14
Half-Life
Over time, the amount of your original
substance decreases exponentially.
Mass of Element versus Half-Life Cycles
120
Mass (kg)
100
80
60
40
20
0
1
2
3
4
5
Half-Life Cycles
6
7
Sample Problem
A radioactive material has a half-life of
one year. If you start with 100 grams of
the material, how much material is left
after 6 years?
Time (years)
0
1
2
3
4
5
6
Mass of Element
100
Half-Life Example Problems
If the half-life of a certain isotope is 1620
years, how much of that element will
remain after 1620 years? After 3240
years?
After 1620 years, half of the original
substance will remain.
After 3240 years, one quarter of the
original substance will remain.
Half-Life Example Problems
If you start with a 120-gram sample of
Radium, how much will be left after 44
days? (Missing something?)
(Wait… what’s the half-life of Radium)?
The half-life of Radium is 11 days.
7.5 grams will remain after 44 days.
Half-Life Example Problems
An 80-gram sample of Hydrogen decays,
leaving 2.5 grams of Hydrogen. How long
would this take? (The half-life of
Hydrogen is 12.3 years)
61.5 years
Half-Life Example Problems
You start with 200 grams of Thallium 207.
After 20 minutes, there is only 12.5
grams of Thallium left. What is the halflife of the decay process?
5 minutes
Half-Life
A radioactive element will decay below
detectable levels in about ten to fifteen
half-lives.
The following table lists ALL radioactive
nuclei with a half-life greater than 1000
years.
Half-Life
Isotope
Holmium-166m
Berkelium-247
Radium-226
Molybdenum-93
Holmium-153
Curium-246
Carbon-14
Plutonium-240
Thorium-229
Americium-243
Half-life (yr)
1,200
1,380
1,600
4,000
4,570
4,730
5,730
6,563
7,340
7,370
Rel. Abundance
none
none
trace
none
none
none
trace
none
none
none
Half-Life
Curium-245
Curium-250
Tin-126
Iodine-129
Niobium-94
Plutonium-239
Proactinium-231
Lead-202
Lanthanium-137
Thorium-230
Nickel-59
8,500
9,000
10,000
15,700
20,300
24,110
32,760
52,500
60,000
75,380
76,000
none
none
none
none
none
none
trace
none
none
none
none
Thorium-230
Calcium-41
Neptunium-236
Uranium-233
Rhenium-186m
Technetium-99
Krypton-81
Uranium-234
Chlorine-36
Curium-248
Bismuth-208
Plutonium-242
77,000
103,000
154,000
159,200
200,000
211,000
229,000
245,500
301,000
340,000
368,000
373,300
trace
none
none
none
none
none
none
trace
none
none
none
none
Aluminum-26
Selenium-79
Iron-60
Beryllium-10
Zircon-93
Curium-247
Gadolinium-150
Neptunium-237
Cesium-135
Technetium-96
Dysprosium-154
Bismuth-310m
Mietnerium-53
717,000
1,130,000
1,500,000
1,510,000
1,530,000
1,560,000
1,790,000
2,144,000
2,300,000
2,600,000
3,000,000
3,040,000
3,740,000
none
none
none
none
none
none
none
none
none
none
none
none
none
Technetium-98
Palladium-107
Hafnium-182
Lead-205
Curium-247
Uranium-236
Niobium-92
Plutonium-244
Samarium-146
Uranium-236
Uranium-235
Potassium-40
4,200,000
6,500,000
9,000,000
15,300,000
15,600,000
23,420,000
34,700,000
80,800,000
103,000,000
234,200,000
703,800,000
1,280,000,000
none
none
none
none
none
none
none
none
none
none
rare
rare
Uranium-238
Rubidium-87
Thorium-232
Lutetium-176
Rhenium-187
Lanthanium-138
Samarium-147
Platinum-190
Tellurium-123
Osmium-184
Gadolinium-152
Tantalum-180m
4,468,000,000
4,750,000,000
14,100,000,000
37,800,000,000
43,500,000,000
105,000,000,000
106,000,000,000
650,000,000,000
>1 x 10^13
>5.6 x 10^13
1.08 x 10^14
>1.2 x 10^15
common
common
common
rare
common
rare
common
rare
rare
rare
rare
rare
Xenon-124
Indium-115
Zinc-70
Hafnium-174
Osmium-186
Samarium-149
Neodymium-144
Samarium-148
Cadmium-113
Cerium-142
Tungsten-183
Vanadium-50
Lead-204
>1.6 x 10^14
4.41 x 10^14
>5 x 10^14
2.0 x 10^15
2.0 x 10^15
>2 x 10^15
2.29 x 10^15
7 x 10^15
7.7 x 10^15
>5 x 10^16
>1.1 x 10^17
1.4 x 10^17
1.4 x 10^17
rare
common
rare
rare
common
common
common
common
common
common
common
rare
common
Chromium-50
Tungsten-184
Calcium-48
>1.8 x 10^17
>3 x 10^17
>6.3 x 10^18
common
common
common
Molybdenum-100 1.0 x 10^19
common
Neodynium-150
Zircon-96
Selenium-82
Tellurium-130
Xenon-136
Tellurium-128
Stable Isotopes
common
common
common
common
common
common
**
>1.1 x 10^19
>3.8 x 10^19
1.1 x 10^20
7.9 x 10^20
>2.4 x 10^21
2.2 x 10^24
infinite
And Then?
Using other radioactive dating techniques
(Pb isochron method), geologists have
established that to a great degree of
accuracy, Earth is ~4.6 billion years old.
http://www.sciencechannel.com/tvshows/greatest-discoveries/videos/100greatest-discoveries-radiometricdating.htm
Organic Material
Why is Carbon-14 important in
determining how old organic material is?
About 18% of mass in a human is made
up of Carbon.
But how do we get Carbon-14 into our
bodies?
Creation of Carbon-14
Cosmic Rays
(radiation)
Forms C-14
Collision with
atmosphere (N14)
C-14 combines with
oxygen to form carbon
dioxide (CO2)
Carbon Dating Problem
Imagine we find the fossil of a trilobite
(521 Mya) during an excavation.
Knowing what we know about the half-life
of Carbon-14, why would be difficult to
use carbon dating on the trilobite fossil?
Carbon Dating Problem
Instead of using carbon dating, what
could we do instead to figure out how old
the trilobite is?