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Nuclear Chemistry
Chapter 10
• What happens during nuclear
decay?
• What are the three types of
nuclear radiation?
• How does nuclear radiation affect
atoms?
• What devices can detect nuclear
radiation?
Key Concepts
• 1896 – French physicist Antoine Henri Becquerel
• Experiments with uranium salts
• Thought that the salts – which glow after exposure to
sunlight – would produce X-rays while they glowed
• Discovered the salts didn’t need to be exposed to
sunlight to emit rays; uranium salts emitted rays that
had never been discovered before
Nuclear Decay
• Radioactivity – the process in which an unstable atomic
nucleus emits charged particles and energy.
• Becquerel’s experiment marked the discovery of
radioactivity.
• Any atom containing an unstable nucleus is called a
radioactive isotope or radioisotope.
Radioactivity
• Isotopes can be stable or unstable (radioisotopes).
• Radioisotopes spontaneously change into other isotopes over
time.
• When the composition of a radioisotope changes, it is said to
undergo nuclear decay.
• During nuclear decay, atoms of one element can change into
atoms of a different element altogether.
• Example: uranium-238 decays into thorium-234
Radioisotopes
• Scientists can detect a radioactive substance by
measuring the nuclear radiation it gives off.
• Nuclear radiation is charged particles and energy that are
emitted from the nuclei of radioisotopes.
• Common types of nuclear radiation include:
• Alpha particles
• Beta particles
• Gamma rays
Types of Nuclear Radiation
• Alpha particle – a positively charged particle made up of two
protons and two neutrons (same as a He nucleus).
• It has a 2+ charge.
• When uranium-238 decays, it emits alpha particles.
• Common symbol for alpha particles 42𝐻𝑒
• Subscript = atomic number (# of protons)
• Superscript = mass number (#protons + #neutrons)
• Other common symbol for alpha particles α (Greek letter)
Alpha Decay
• Alpha decay refers to nuclear decay that releases alpha
particles.
• Alpha decay is an example of a nuclear reaction.
• Like chemical reactions, nuclear reactions can be
expressed as equations.
238
92𝑈
→
234
90𝑇ℎ
Alpha Decay
+ 42𝐻𝑒
• Alpha particles are the least penetrating type of nuclear
radiation.
• Most travel no more than a few centimeters in the air.
• Can be stopped by a sheet of paper or by clothing.
Alpha Decay
• Beta particle – an electron emitted by an unstable
nucleus.
• During beta decay, a neutron decomposes into a proton
and an electron.
• Common symbols for alpha particles −10𝑒 or β
• Because of its single negative charge, a beta particle is
assigned an atomic number of -1 and a mass number of 0.
234
90𝑇ℎ
→
Beta Decay
234
91𝑃𝑎
+
0
−1𝑒
• The proton stays trapped in the nucleus, the electron is
released.
• Due to their smaller mass and faster speed, beta particles
are more penetrating than alpha particles.
• Can pass through paper, stopped by thin sheet of metal.
Beta Decay
• Gamma ray – a penetrating ray of energy emitted by an
unstable nucleus.
• Symbol for a gamma ray is γ.
• Gamma radiation has no charge and no mass.
• Like x-rays and visible light, gamma rays are energy
waves that travel through space at the speed of light.
Gamma Decay
• Atomic number and mass number of the atom remain the
same but the energy of the nucleus decreases.
• Gamma decay often accompanies alpha or beta decay.
• Gamma rays are much more penetrating than either α or β
particles. It can take several centimeters of lead or several
meters of concrete to stop gamma radiation.
Gamma Decay
• You are exposed to nuclear radiation everyday.
• Background radiation – nuclear radiation that occurs
naturally in the environment.
• Radioisotopes in air, water, rocks, plants, and animals all
contribute to background radiation.
• Cosmic rays are also a source (cosmic rays are streams of
charged particles from outer space).
• Background radiation levels are generally low enough to be
safe.
Effects of Nuclear Radiation
• Radon is a naturally occurring radioactive element that is formed in the
decay chain of uranium-238.
• Radon gas is produced underground as the uranium in rocks and soil
decays.
• As the radon seeps up through the ground, it can get into buildings by
passing through cracks or holes in their foundation.
• Most areas – amount of uranium in rocks and soil is very small.
• Higher concentrations of uranium and its minerals are commonly found
in light colored igneous rocks, granite, dark shale, phosphate-containing
sedimentary rocks, and metamorphic rock derived from these rocks.
Soils derived from these rocks also have high uranium concentrations.
• EPA - RADON
Radon
• When nuclear radiation exceeds background levels, it can
damage the cells and tissues of your body.
• Nuclear radiation can ionize atoms.
• When cells are exposed to nuclear radiation, the bonds
holding together the proteins and DNA molecules may
break – cells may no longer function properly.
• α and β particles and γ rays are all forms of ionizing
radiation.
Effects of Nuclear Radiation
• Devices used to detect nuclear radiation:
• Geiger counters
• Film badges
• Geiger counter –
measures ionizing
radiation with a gasfilled tube .
Detecting Nuclear Radiation
• Film badge - often worn by people who work with or near
radioactive materials, monitors exposure to nuclear
radiation.
Detecting Nuclear Radiation
Film Badge
Radioactive Isotopes
and Half Life
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Radioactive Isotopes
Radioactive elements are unstable. They decay, and change into
different elements over time.
Not all elements are radioactive. Those that are listed below are
the most useful for geologic dating of fossils are:
U-238
K-40
C-14
Half-life = 4.5 Billion Years
Half-life = 1.25 Billion Years
Half-life = 5, 730 Years
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Radioactive Decay and Half Life
Here are some facts to remember:
1.
The half-life of an element is the
time
it
takes for half of the material you started with to
decay.
2.
Each radioisotope has it’s own half-life
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Radioactive Decay and Half Life
3. Each isotope decays into a new
isotope
C14 decays into N14
4. The half-life of each element is constant. It’s like a clock
keeping perfect time.
Now let’s see how we can use half-life to determine
the age of a rock, fossil or other artifact.
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Radioactive Decay and Half Life
3. Each isotope decays into a new
isotope
C14 decays into N14
4. The half-life of each element is constant. It’s like a clock
keeping perfect time.
Now let’s see how we can use half-life to determine
the age of a rock, fossil or other artifact.
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The grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
14
0
100%
0%
no ratio
1
50%
50%
1:1
After 1 half-life (5730 years), 50% of
the C14 has decayed into N14. The ratio
of C14 to N14 is 1:1. There are equal
amounts of the 2 elements.
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The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red .
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
14
0
100%
0%
no ratio
1
50%
50%
1:1
2
25%
75%
1:3
Now 2 half-lives have gone by for a total
of 11,460 years. Half of the C14 that was
present at the end of half-life #1 has now
decayed to N14. Notice the C:N ratio. It
will be useful later.
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The blue grid below represents a quantity of C14. Each time you click,
one half-life goes by and you see red.
Ratio of
Half
% C14
%N14
C to N
C14 – blue
N14 - red
lives
14
14
0
100%
0%
no ratio
1
50%
50%
1:1
2
25%
75%
1:3
3
12.5%
87.5%
1:7
After 3 half-lives (17,190 years) only
12.5% of the original C14 remains. For
each half-life period half of the material
present decays. And again, notice the
ratio, 1:7
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What is the half life
represented in this graph?
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Time
Half-lives
Parent
Remaining
Daughter
0
1/1
0
1
1/2
½
2
1/4
3/4
3
1/8
7/8
4
1/16
15/16
5
1/32
31/32
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PRACTICE PROBLEMS
Sodium-24 has a half-life of 20 hours. How much sodium-24 will
remain in an 18.0 g sample after 60 hours?
After 40 days a 2.0 g sample of phosphorus-32 contains only
0.5 g of the isotope. What is the half-life of phosphorus-32?
Carbon-14 has a half-life of 5730 years. How “old”
would a real fossil be if it has gone through 3 carbon14 half-lives?
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