Chapter 21
Nuclear Chemistry
21.1 The Nucleus
Nuclear Notation
• During a nuclear reaction, an atom can gain or
lose protons & neutrons so proper notation
must be used to keep track.
• Since altering the number of protons changes
the identity of an atom, these reactions actually
convert atoms into new atoms, not just new
substances.
• Isotopes must be indicated, as some are
radioactive and can decay further.
• A nucleus is often referred to as a nuclide
when it is shown in nuclear or isotope notation.
Nuclear Notation
• The mass number is shown as a superscript,
and the atomic number is shown as a subscript
below the mass number.
• Remember, mass number = protons + neutrons
Nuclear Reactions
• Nuclear reactions produce a very
large amount of energy, as mass
particles are converted into energy.
• Einstein was the first scientist to discover the
relationship between mass and potential energy:
Where E = energy
E = mc2
m = mass
c = speed of light, 3.0 x 108 m/s
Mass Defect
• Mass defect is caused by a conversion of
mass into energy when the nucleus is
formed.
• Mass defect is defined as the difference
between the mass of an atom & the sum of
the masses of its subatomic particles.
• Since energy is released when the nucleus is
formed, thus mass lost, that energy is called
the nuclear binding energy.
Mass Defect
• Nuclear reactions are the sole method of
the loss of mass in a reaction.
Nuclear
Binding
Energy
E = mc2
c = speed of light
(3.00 x 108 m/s)
Calculating Mass Defect
• The measured mass of a Li-7 atom is 7.01600
amu.
The mass amu = 3.021828 amu
3 protons x 1.007276
of a proton
The mass
4 neutrons x 1.008665
amu = 4.03466 amu
of a neutron
The mass
3 electrons x 0.0005486
amu = 0.0016458 amu
of an electron
7.05813 amu
7.05813 amu - 7.01600 amu = 0.04213 amu
Calculated Mass
Measured Mass
Mass Defect
Calculating Nuclear Binding Energy
• Convert the mass defect into energy using
Einstein’s equation, E = mc2
First convert amu to kg to coincide with the
units for energy: kg·m2/s2 (= J)
0.04213 amu x 1.6605 x10-27 kg = 6.99569 x 10-29 kg
1 amu
E = (6.99569 x 10-29 kg) (3.0 x 108 m/s)2
= 6.29612 x 10-12 J
Nuclear Binding Energy
Binding Energy per Nucleon
• Binding energy per nucleon is the binding
energy divided by the number of nucleons,
or protons & neutrons.
• The higher this value is, the more tightly
the nucleons are held together.
• A higher binding energy per nucleon
indicates a more stable atom.
Nuclear Stability
• For small atoms, the most stable neutronHe-4
N
proton ratio is 1:1
P
N
P
2 protons
2 neutrons
• As atomic number increases, this ratio
Pb-206
change to 1.5:1
124 neutrons
82 protons
• Protons repel each other in the long range
through electrostatic repulsion.
• In the short range, protons attract each
other through nuclear force.
Nuclear Stability
• As the nucleus gets larger, the repulsive
forces increase and more neutrons are
needed to stabilize the nucleus.
• After atomic number 83, the repulsive
force is so great that there are no more
stable nuclei.
• Even numbers of nucleons provide the
most stable nuclei, this indicates that
nucleons are most stable when paired, like
electrons.
Nuclear Shell Model
• Nucleons exist in different energy levels,
like shells, in the nucleus.
• Completed nuclear energy levels are
represented by the following numbers of
nucleons:
2,8,20,28,50,82 & 126
• These numbers are known as magic
numbers.
Nuclear Reactions
• Any reaction that affects the nucleus of an
atom is called a nuclear reaction.
• The total number of nucleons must be the
same on both sides of the equation.
• When a nuclear reaction changes the
identity of an atom by altering its atomic
number, that is called transmutation.
• See sample & practice problems on pg. 684
Chapter 21
Nuclear Chemistry
21.2 Radioactive Decay
Discovering Radioactivity
• Henri Becquerel did experiments
with a phosphorescent uranium
compound. He exposed it to
sunlight then used photograph paper to capture
the emitting energy.
• The uranium would be placed onto the paper,
then developed to show the area where the
image was placed.
• He accidentally discovered that these
compounds spontaneously give off energy,
without prior exposure to sunlight!
Phosphorescence
• Some substances will appear to “glow”
after being exposed to sunlight.
• These substances absorb the energy from
sunlight and the electrons move to higher
energy states.
• As the electrons return to their ground
states, the emit the energy as visible light.
• Some of these substances may also be
radioactive…
Phosphorescence vs. Radioactivity
• Phosphorescent
substances will only emit
energy after exposure to
an energy source. This
involves only a change
in electrons.
• Radioactive substances
can emit energy without
prior exposure to an
energy source. This
involves a change in the
nucleus!
Radioactivity
• A spontaneous emission of
radiation by an unstable
nucleus is called radioactivity.
• Marie & Pierre Curie further
investigated Becquerel’s observations and
determined that for the first time, reactions
involving a change in the nucleus were being
correctly observed.
• All scientists were awarded the Nobel Prize for
Physics in 1903 for this discovery.
Radioactive Decay
• Decay is the emitting of radiation by
radioisotopes. These nuclei are not stable.
• There are too many protons in the nucleus
which causes excessive repulsion.
• There are 5 different kinds of radiation:
alpha ()
beta ()
positron emission
electron capture
gamma ()
Alpha Decay
• Alpha particles consist of helium nuclei, 2 protons & 2
neutrons.
• They are considered “large” particles in comparison to
other forms of radiation.
• These particles collide often, do not pass through
many substances & can be blocked by thin material.
• Whenever a radioisotope decays, or loses, 2 protons
& 2 neutrons it is called alpha decay.
Chemical Equations depicting
Alpha Decay
• Check to see that the law of conservation
of matter is upheld. The mass numbers &
atomic numbers should add up to be equal
on both sides of the equation
Beta Decay
• Beta particles
are smaller &
lighter so they
move faster &
pass through more substances than alpha
particles.
• They are high energy electrons, with a -1
charge.
• They can be stopped by stacked sheets of
metal, heavy clothing or blocks of wood.
Chemical Equations depicting Beta
Decay
• The emitted electron
comes from the
conversion of a
neutron into a
proton & an
electron.
• Since only 1 electron is emitted, the mass
number will stay the same but the atomic
number will increase by one. This is called
transmutation.
Positron
Emission
• Atoms with too many protons, an unstable
nucleus, can convert a proton into a neutron
and emit a positron.
• A positron is a positively charged particle that
has the equivalent mass of an electron.
Positron Emission
Equations
• Atomic number
decreases by one,
mass number stays
the same.
A
Neutrino….
Electron
Capture
• If too many protons are present, another decay
may occur where an inner shell electron is
captured by the nucleus and combined with a
proton to form a neutron; called electron
capture.
Electron Capture
Equations
• Similar to positron
emission, the
atomic number
decreases by one
and mass number
stays the same.
Gamma Decay
• High energy electromagnetic radiation without
mass or charge.
• Since it is able to pass through most
substances, gamma radiation can cause
damage to living cells.
• It is often observed in combination with other
types of decay.
Predicting Decay
• Alpha decay predominates in isotopes with Z > 83.
For isotopes with Z < 83, decay will occur if it is
outside the belt of stability.
• Isotopes that are neutron-rich will undergo beta
decay.
• Isotopes that are neutron-poor will decay by electron
capture or positron decay.
• Lighter isotopes (Z < 50) are more likely to decay by
positron emission.
• Heavier elements are more likely to decay by
electron capture.
Summary
• If you analyze a nuclear reaction & observe the
products, you can determine the type of reaction
that took place:
1- If both the mass number & atomic number
decrease, alpha decay occurred.
2- If only the atomic number increases, beta
decay has occurred.
3- If neither mass number or atomic number
change, gamma decay has occurred.
4- If atomic number decreases and mass number
stays the same, it is positron emission if a
positron is a product, and electron capture is an
electron is a reactant.
• Often, these kinds of decay occur in combinations
and chain reactions.
Decay Series
• A decay series of a chain of decay
reactions.
• These proceed until stable nuclei are
formed.
• The heaviest nuclide formed is the parent
nuclide.
• The other nuclides produced are called
daughter nuclides.
Artificial Transmutations
• Bombardment of stable nuclei with particles to
artificially create larger atoms.
Fermi International Accelerator Laboratory
(underground particle accelerators)
Half-Life
• The rates of nuclear decay are constant and
spontaneous. They cannot be altered.
• Half-Life is half of the amount of time it takes
for a given amount of a radioactive material to
undergo decay and result in stable nuclei.
• By studying the state of decay, you can predict
the age of a fossil or other artifact if you know
the half-life.
• carbon-14, uranium-238, rubidium-87 &
potassium-40 are commonly used for dating
objects.
Half Life
• After 1 half-life, 50% of the sample remains.
• After 2 half-lives, 25% of the sample remains.
Percent of Radioactive
Nuclei Remaining
Half-Life Trend
120
100
80
60
40
20
0
0
1
2
3
4
Number of Half-Lives That Have Passed
Chapter 21
Nuclear Chemistry
21.3 Nuclear Radiation
What is Radioactivity?
• The decay of an atom that results in the emission of
particles from the nucleus = “nuclear reaction”
3 Types:
1. alpha, α, emission
2. beta, β, emission
3. gamma, γ, emission
Smoke Detectors
Paper, Plastic or
Steel Sheeting
Increasing
Strength
Decay naturally occurs in
elements above 83
Water Pipe Leakage, Radiation
Therapy, Sterilizing Medical
Equipment or Packaged Food,
Medical Tracer,
Natural Sources of Radiation
• Radiation from outer space
• cosmic rays from the Sun
• Radioactivity from naturally occurring radioisotopes in
rocks at the surface
• traces of radioisotopes of uranium in granite rocks
• the radioactive gas Radon is formed in the process,
and can build up to harmful levels in cellars
• Radioactivity from naturally occurring radioisotopes
deep in the Earth's core, the energy released keeps the
core very hot and heats the magma in the Earth's
mantle
Smoke Detectors
• Alpha particles emitted from
source ionize the air and
provide the charge
necessary to conduct
current through
the air.
• Charges stick to the heavy
smoke particles and the
current drops, causing the
alarm to buzz.
Detecting Radiation
• Radiation cannot be seen,
heard, smelled, touched or
tasted.
• Geiger counters are used to
detect radiation by
detecting charged particles
that cause a clicking sound
to register on the meter.
• A scintillation counter
measures radiation by
detecting light.
Photographic paper will
also detect close proximity
radiation.
Carbon-14 Dating
• One out of every million carbon atoms is
carbon-14, a radioactive isotope. Since all
living things contain carbon, the amount of
C-14 remains relatively stable over a
lifetime. After death, no more carbon is
taken in, so the amount of C-14 gradually
decreases as the C-14 decays.
• The half-life of carbon is relatively short,
5,730 years.
Other Dating Methods
• Since carbon has a short
half-life, other methods of
dating are used to determine
the age of objects older than 60,000 years.
• Potassium-40
• Uranium-238
• Rubidium-87
t1/2 = 1.25 billion years
t1/2 = 4.5 billion years
t1/2 = 48 billion years
Radioisotopes as Medical Tools
Radioisotopes are used
as tracers to track the
movement of chemicals
in the body.
They are also used in
scanning devices to
visualize organs &
glands.
Environmental Tracers
• Pesticides can be altered to contain a
radioisotope. When they are sprayed onto a
field, they can be traced by detecting the
radioisotope.
• Runoff into water sources, or movement by
animals can be tracked to see the effect of the
initial spraying.
Food Irradiation Applications
• Low dose (up to 1 kGy)
Inhibition of: sprouting potatoes, onions, garlic, ginger, yam
Insect and parasite disinfestation:
cereals, fresh fruit, dried foods
Delay ripening: fresh fruit, vegetables
• Medium dose (1-10 kGy)
Extend shelf life: fish, strawberries, mushrooms
Halt spoilage, kill pathogens: seafood, poultry, meat
• High dose (10-50 Gy)
Industrial sterilization: meat, poultry, seafood, prepared foods
Decontamination: Spices, etc.
Chapter 22
Nuclear Chemistry
22.4 Nuclear Fission & Fusion
Ba-141
U-235
N
N
+
Nuclear
Fission
Kr-92
N
• Fission reactions can release more energy
than a nuclear decay reaction.
• When additional neutrons are introduced
to an atom, it disrupts the ratio of neutrons
to protons which can result in an unstable
nucleus.
• Fission is the splitting of an unstable
nucleus into 2 separate nuclei.
• This was first observed & explained in
1938 by German & Austrian physicists.
Fissionable Isotopes
• U-235 is fissionable, but U-238 is not.
• Pu-239 is fissionable
• Th-232 is fissionable only with a fast
moving neutron.
• Slow-moving neutrons are used for
controlled fission reactions in power
plants.
Animations of Nuclear Fission
Nuclear Fission
When a heavy atomic
nucleus is bombarded
with a neutron…
Two middle weight
atoms result
Neutrons and Energy
also result
= Chain Reaction!
Nuclear power
stations and
bombs
Nuclear Fission
• A chain reaction is when the products of a
reaction can further react, making the products
into reactants.
• These reactions can result in chain reactions if
more fissionable materials are exposed to the
neutrons produced.
• They can be carefully controlled, so energy is
released slowly and utilized to do work.
• If they occur too quickly, an explosion results like
in an atomic bomb.
Animation of a Nuclear Fission
Chain Reaction
Examples of Fission Reactions
235U
+ 1 neutron → 3 neutrons + 93Kr + 140Ba
+ energy
235U
+ 1 neutron → 3 neutrons + 87Br + 146La
+ energy
Fission Reactors
• Nuclear power plants generate energy by using
controlled fission reactions of uranium.
• A nuclear reactor extracts energy from
radioactive fuel.
• Fissionable U-235 is present at approximately
3% in the fuel rods, much less than necessary
for a nuclear explosion. The rest of the uranium
is nonfissionable U-238
Fission Reactors
• More than 100 nuclear power plants are
operating in the United States.
• Nuclear power accounts for 20% of all electricity
used in the United States.
• France uses the most nuclear power
compared to other countries, 70% of
its electricity is attributed to nuclear
reactors.
Other Fission Sources
• Plutonium can also be used in nuclear
reactors, however, during the fission
reaction, more fissionable material is
produced than used.
• Those types are called breeder reactors.
• They are very
dangerous due to
the ability of the
produced Pu-239
to be used in
fission bombs.
Health Concerns
• Nuclear reactors produce very little in the
way of pollutants. Water vapor is the
byproduct of the reaction.
• The radioactive waste from used fuel rods are
difficult and expensive to dispose of.
• The risk of explosions or fires causes great
resistance to building these power plants as
radioactive materials would be released into
the environment.
• Ex: Three Mile Island, PA
Chernobyl, former Soviet Union
Nuclear Fusion
• The opposite of fission, fusion is when 2
nuclei combine to form a larger nucleus.
• A large amount of energy is generated during
nuclear fusion.
• Small nuclei combine to form heavier, more
stable nuclei.
• No radioactive products are produced.
• Fusion reactions are easier to control, so the
risk of explosions & accidents are decreased.
Nuclear Fusion
When two light
atoms join to form
heavy atoms.
Energy is also
released.
Sun, stars and
the hydrogen
bomb
Nuclear Fusion
• Nuclear fusion reactions do not occur as readily
as fission reactions. They require a tremendous
amount of energy to initiate the reaction.
• In the sun & stars, the excess pressure combined
with extremely high temperatures triggers the
fusion reaction.
• On Earth, a temperature of 200
million kelvins would be required.
• There are many technical difficulties associated
with these types of reactions.
• Such as:
a heat-resistant reaction chamber
outcome must exceed input to be
practical
Animations of Nuclear Fusion
D = deuterium
T = tritium
He = helium
n = neutron
Examples of Fusion Reactions
• D + T→ 4He + n
• D + D→ T + p
• D + D → 3He + n
Where:
D = deuterium
T = tritium
p = proton
n = neutron