Chapter 17
Radioactivity, Nuclear Energy,
and Solar Energy
The Core of Matter
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Chemistry Applied
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Radioactivity
1. Most elements occur as a mixture of
isotopes.
The nucleus of an atom contains protons and neutrons.
Each proton has a charge of +1.
A neutrons has about the same mass as a proton but is
electrically neutral.
Neutrons do not affect the chemical properties of atoms.
The sum of the protons and neutrons in an atom is
called the mass number.
Nuclei of a given element having different mass
numbers are called isotopes.
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Radioactivity
Most elements consist of a homogeneous mixture of
several isotopes.
The mass number is often displayed as a leading
superscript to the elemental symbol generating the
isotopic symbol of the isotope.
Carbon consists of three isotopes,
About 99% of carbon atoms are
protons and 6 neutrons.
About 1% of carbon atoms are
protons and 7 neutrons.
12C, 13C,
12C,
13C,
Only a trace of carbon atoms are
protons and 8 neutrons.
and
14C.
which contain 6
which contain 6
14C,
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which contain 6
Radioactivity
Natural carbon is a mixture of the three isotopes,
13C, and 14C.
12C,
The atomic weight of an element is the average mass
of the natural mixture of its isotopes.
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Radioactivity
2. Radioactive nuclei emit alpha, beta,
or gamma particles.
Most atomic nuclei are stable.
Those that are not spontaneously decompose by
emitting a small energetic particle.
In decomposing, a radioactive nuclei may change the
number of protons in its nucleus and thus change
into a different element.
The nuclei of very heavy elements are prone to this
type of decomposition.
The new nuclei produced by the decomposition may
itself be radioactive.
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Radioactivity
The most common particles emitted by a radioactive
nucleus are alpha, beta, and gamma particles.
Alpha emission
An  particle has a charge of +2 and a mass
number of 4. It is identical to the nucleus of a
4 He atom.
2
The nucleus remaining after alpha emission will
have an atomic number 2 units smaller than the
original nucleus and a mass number 4 units
smaller.
232
90Th

228
88Ra
+
4
2He
Notice that both the total mass numbers and total
nuclear charges balance individually.
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Radioactivity
Beta emission
 Emission occurs when a neutron in the nucleus
splits into a proton and an electron. The
electron is ejected from the nucleus as a high
energy  particle.
Neutron  proton + electron
In  emission, the atomic number of the nucleus
increases by one unit and the mass number of the
nucleus remains the same.
214
82Pb

214
83Bi
+
0
-1e
Radioactivity
Radioactivity
Gamma emission
Gamma particles are actually high energy
photons, and are usually called gamma rays.
Neither the atomic number nor the mass
number of an isotope change as a result of
emitting a gamma ray.
Gamma rays are often emitted in conjunction
with the emission of an alpha or beta particle.
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3. Radioactivity can be dangerous to
living organisms.
 and  particles are ejected from the nucleus
with a very large amount of kinetic energy.
This energy can be transferred during
collisions with molecules in living organisms
and can break bonds or ionize electrons
from atoms or molecules.
The three types of ionizing radiation vary in
their abilities to penetrate matter.
Alpha particles cannot penetrate matter.
They are absorbed immediately at the
surface.
Beta particles can travel about one meter
in air and several millimeters in water or
human tissue.
Radioactivity
Gamma rays penetrate matter efficiently.
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Although alpha and beta particles do not penetrate far
into matter, when ingested or inhaled they can
cause serious tissue damage, especially if it involves
DNA disruption.
Although gamma rays pass completely through tissue,
they lose energy in doing so which is transferred to
the tissue which can cause serious damage.
All three types of radiation are called ionizing radiation
because they produce free radical ions:
H2O + radiation  e- + H2O·+
H2O·+  H+ + ·OH
The free radical hydroxide ion formed can damage
proteins and DNA in the cell.
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Radioactivity
Exposure to excessive amounts of ionizing radiation
can lead to radiation sickness and even death.
This effect is the basis of food irradiation which kills
microorganisms and thereby prevents food spoilage.
Radiation sickness in humans results in nausea and a
drop in the white blood cell count. Genetic damage
may also occur which can lead to cancer or
mutations which can be passed on to subsequent
offspring.
Because radiation affects rapidly dividing cells most
strongly, it is often used to treat cancer. In this case
a radiation dose is give which is lethal to the rapidly
growing cells but sub-lethal to the non-cancerous
cells.
Radioactivity
4. A radioactive isotope decays over
time.
The rate of decay is usually expressed as the half-life, or
t1/2, of the isotope. This is the time it takes for
exactly one half of the nuclei present to decay. This
time is always the same for the isotope, irregardless
of how many nuclei are actually present.
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Radioactivity
The half-life of the isotope
years.
238
92U
is about 4.5 billion
Most isotopes have half-lives much shorter than that of
U-238.
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Nuclear Energy
Fusion: Two very light nuclei combine to form a
heavier one. Again, the mass of the product nuclei
is less than the mass of the light nuclei and energy
is released.
Commercial, controlled fusion is not currently
feasible but may become so in the future.
Nuclear Energy
8. Fission reactors use chain reactions
of uranium-235 to generate energy.
When struck by a neutron, U-235 splits with the
release of several additional neutrons. One possible
reaction is:
1
0n
+
235
92U

142
56Ba
+
91
36Kr
+3
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1
0n
Nuclear Energy
Only a few kilograms of weapons-grade plutonium are
required to construct an atomic bomb so the security
of plutonium stockpiles is a serious concern.
The world stockpiles of plutonium now exceeds 1000
ton and is continues to grow.
Plutonium is an alpha emitter, and as such does not
pose a serious health threat unless it enters your
body.
When exposed to air, however, plutonium forms the
oxide, PuO2, a powdery solid, which if inhaled in even
a very small amount can cause lung cancer.
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Nuclear Energy
11. Uranium mining produces radioactive
contamination of the environment.
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Nuclear Energy
11. Uranium mining produces radioactive
contamination of the environment.
Uranium ore contains a mixture of several radioactive
elements, decay products of the uranium present.
Mining of uranium often causes environmental
contamination by these substances.
The waste produced from refining uranium ores is a
liquid-solid mixture called tailings.
Tailings are held in settling ponds until the solid particles
settle out by gravity. Pollution of local groundwater
can result when these ponds leak or overflow due to
heavy rains.
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Nuclear Energy
When tailings are used as a landfill material, subsequent
building on the site may result in elevated radon
levels in buildings constructed.
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Nuclear Energy
12. The problem of high-level nuclear
waste disposal is unsolved.
There is currently no consensus as to how best to
store radioactive power plant wastes for the long
term.
Currently, spent fuel rods are stored under water at
the power plants where they were created.
Once the shorter half-life isotopes have sufficiently
decayed, the rods can be transferred to dry
storage.
If the plutonium is removed from the rods, the
remaining radioactive waste is re-solidified and
stored.
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Nuclear Energy
Two methods have been proposed to dispose of
excess plutonium.
Cover it and other highly radioactive wastes into a
glass and bury it far underground in metal
canisters.
Convert it to PuO2 and mix it with uranium oxide
to form a mixed oxide fuel for power plants.
Most nuclear waste disposal plans suppose that the
radioactive wastes will be immobilized in a glass or
ceramic form and buried underground.
The burial vaults would be 500-1000 meters deep in
sites having high geological stability and low
permeability.
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Nuclear Energy
Copyright W. H. Freeman and Company ·
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Nuclear Energy
13. The future of fission-based nuclear
energy is uncertain due to the potential
dangers of accidental releases of
radioactivity.
It is not possible for nuclear power plants to blow up like
an atom bomb. The chain reaction for a bomb
requires 95% enrichment of U-235, compared to the
3% maximum enrichment used in power plants.
More likely, and having already occurred, is the
possibility that nuclear material could contaminate
the areas surrounding a reactor in the event of a nonnuclear explosion or thermal melt-down.
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Nuclear Energy
The Chernobyl reactor in the Ukraine underwent an
explosion in 1985 which spread radioactivity over a
wide area, extending all the way to Scandanavia.
A less serious accident at Three Mile Island in
Pennsylvania resulted in the release of a small
amount of radioactivity in 1979.
No new nuclear power plants have been ordered in the
US since the Three Mile Island accident and several
operating nuclear plants in the US and Canada have
subsequently been shut down.
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Nuclear Energy
14. Fusion reactors could produce
enormous amounts of power, but a
number of problems impede their
immediate use.
Fusion results when two light nuclei fuse into a heavier
one. This is the energy source that drives the Sun
and other stars.
The fusion reactions being considered for commercial
energy production involve deuterium, 2H, and tritium,
3H. Deuterium is readily available from water
(0.015% of the hydrogen therein). Tritium would
have to be synthesized by the fission of lithium.
Copyright W. H. Freeman and Company ·
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Nuclear Energy
2
1H
+
2
1H

3
2He
+
1
0n
2
1H
+
3
1H

4
2He
+
1
0n
A fusion reaction produces about 106 times more power
per atom than a typical chemical reaction. This
energy, in the form of heat, would be used to produce
high pressure steam in order to generate electricity.
No feasible fusion power reactors have been constructed
and none are expected in the near future.
If perfected, fusion power should have less serious
environmental consequences than those associated
with fission reactors.
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Tying Concepts Together: The sources of
fission, fusion and solar energy
Both splitting large nuclei and fusing small nuclei
produce energy. The point of maximum nuclear
stability is at mass number 60.
Sustained nuclear fusion reactions, such as in the sun,
generate extremely hot temperatures. The sun’s
surface has a temperature of 6000 oC and emits
light, mainly in the IR and visible regions of the
spectrum.
Trapping of sunlight may solve all or part of our energy
requirements in the future.
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Solar Energy
Solar energy can be
discussed in terms of
etta joules, or EJ, which
is equivalent to 1018 J.
Earth receives 3 million EJ
annually in the form of
sunlight. Capturing
0.01% of this would
satisfy the worlds
annual energy needs.
Solar energy represents a
renewable energy
source.
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Solar Energy
15. Many forms of indirect solar energy
are already used to generate power.
Hydroelectric Power:
Water, evaporated from the Earth’s surface by the
sun, falls as rain, and eventually returns to the sea
via streams and rivers.
During this return, dams can artificially raise the
water level in a river, and the resultant increased
potential energy can be used to drive turbines and
generate hydroelectric power.
The current annual amount of hydroelectric power
generated is about 24 EJ. About 4 times this
amount could be generated using all suitable sites.
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Solar Energy
Hydroelectric power is not pollution free. Flooding
of vegetation by the water retained by the dam can
produce enough greenhouse gases (CH4) to offset
the CO2 savings by not burning fossil fuels.
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Solar Energy
Wind power:
About 33 EJ of energy is potentially available from
wind power. Only 0.05% of this amount is currently
being tapped.
Wind power is feasible in locations that experience
almost constant windy conditions.
Windmills have been used for centuries on farms in
North America to pump water and in Europe,
especially in Holland.
Wind power could be expanded to provide up to
20% of the worlds electricity.
The most elaborate wind farms are in Denmark and
California.
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Solar Energy
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Solar Energy
Biomass Energy:
All biomass is a result of plant photosynthesis, either
directly or indirectly.
Combustion of biomass currently produces 55 EJ of
energy. This process liberates the same amount of
CO2 into the atmosphere as was originally removed
by the plants and thus does not increase the
concentration of greenhouse gases in the
atmosphere.
Domestic and small scale combustion of biomass
does generate considerable pollution in the form of
fine particulate matter, and is quite inefficient.
Technology in use by large-scale installations for
burning biomass avoids this pollution.
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Solar Energy
Biomass can be treated with bacteria and converted
into alcohol fuels. The energy used to produce crops
explicitly for this purpose (corn, sugarcane) can
offset nearly all the CO2 saved by using the biomass
for energy production.
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Solar Energy
Tidal and wave power:
Tidal basins can be dammed and a gate opened and
closed to trap water at high tide. At low tide the
water can be released and the potential energy used
to generate electricity.
Tidal power plants are in operation in France, Nova
Scotia, and Russia.
This energy source is renewable, but have high
capital costs and can only operate twice daily.
Sedimentation often occurs behind the dams which
results in the destruction of tidal mudflats.
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Solar Energy
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Solar Energy
Wave power, or the up and down motion of waves,
can be used to generate electricity.
Currently there are thousands of ocean buoys
powered by this mechanism.
About 20 EJ of energy is potentially recoverable
from waves and tides.
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Solar Energy
16. Solar energy can be used directly in
two ways.
Thermal Conversion:
Heat energy (infrared) from the sun is absorbed and
used to heat water and living space. These needs
account for half of our total energy consumption.
Photo Conversion:
Absorption of UV, visible, and IR photons excites
electrons an absorbing material to higher energy
levels. The excited electrons subsequently cause a
physical or chemical change, rather than generating
heat.
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Solar Energy
17. Thermal conversion can be used to
cook and obtain hot water.
Solar water heaters are used extensively in Australia,
Israel, the southern US, Japan, and other hot areas
receiving lots of sunshine.
Heat is absorbed by a flat, black plate collector, or by
thin black plastic tubes through which water flows.
The hot water is then stored in tanks until it is
used.
Heat exchangers can be added to solar water heating
systems which transfer the heat to circulating air
which can be used to heat buildings.
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Solar Energy
18. Thermal conversion can be used to
obtain electricity.
Sunlight can be focused by mirrors onto a small receiver
where the resulting temperature becomes high enough
to boil water.
Steam generated by the sunlight can be used to drive a
turbine and generate electricity.
To achieve a reasonable efficiency (2nd law of
thermodynamics) the steam must be at a very high
temperature.
This type of system is very efficient if water already near
its boiling point is available from some other source.
This is an example of cogeneration of energy.
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Solar Energy
19. Solar cells produce electricity
directly from sunlight.
Electricity can be directly
generated from sunlight
using the photovoltaic
effect.
In certain materials,
photons of light create
separated positive and
negative charges. These
materials are called
semiconductors and the
positive charge is called
a “hole”.
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Solar Energy
The most common semiconductor used in solar cells is
silicon. A maximum of 28% of the sunlight’s energy
can be captured by a crystalline silicon solar cell.
Amorphous silicon cells (about 14% efficient) are more
commonly used because they are much less expensive
to produce.
Usually many solar cells are combined to form a solar
array in order to increase the available current from
the system.
Solar cells produce DC current which is not easily
transmitted long distances. DC current can be
transformed into AC current, but at the cost of some
power.
DC current from solar cells is ideal for producing H2 gas
by the hydrolysis of water.
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Solar Energy
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Solar Energy
Photovoltaic power is attractive in hot sunny locations
where power demands peak at the same time the
available solar energy peaks.
Currently, solar cells (plus storage) are cheaper than
extending power lines a kilometer or more from
existing power lines, and are competitive with diesel
generators.
Developing countries, many of which have abundant
sunshine, represent the greatest potential market
for this technology.
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Solar Energy
20. Solar energy has both advantages
and disadvantages.
Advantages:
Free and abundant
Low environmental impact
Low operating costs
Large centralized suppliers, and distribution
networks are not needed
High public acceptance as a “natural” form of
energy.
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Solar Energy
Disadvantages:
Intermittent, thus requiring storage or backup
systems
Diffuse large solar collectors are necessary.
High capital costs to set up
Currently receives no tax or regulatory credit in
recognition of its low air pollution and greenhouse
gas emissions
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Summarizing the Main Ideas
A radioactive nucleus spontaneously decomposes by
emitting a small, fast moving particle.
An alpha particle is the same as a helium nucleus.
A beta particle is a high energy electron.
A gamma ray is a very energetic photon of light.
If an alpha particle is emitted, the atomic number of the
nucleus decreases by two and the mass number
decreases by four.
If a beta particle is emitted, the atomic number of the
nucleus increases by one, and the mass number is
unchanged.
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Summarizing the Main Ideas
The mass number of an isotope is indicated by a leading
superscript to the symbol of the element.
Radioactivity is dangerous to living matter because the
emitted particles carry a lot of energy which can break
bonds or ionized biological materials.
Not all nuclei in a sample of a radioactive element decay
at once. They decay at random with one half of the
nuclei decaying in a time called the half-life of the
isotope.
Radon gas is a radioactive element produced by the
decay of U-238. Radon gas can escape from the soil
and enter buildings through cracks in the foundation.
The decay products of Radon are themselves radioactive
and adhere to dust particles which can become lodged
in the lungs, resulting in lung cancer.
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Summarizing the Main Ideas
Nuclear energy is a power source based on the splitting
of heavy nuclei (fission) or the fusion of small nuclei
(fusion).
In either case, large amounts of energy are release
which can be used to generate electrical power.
Some of the products of fission are themselves
radioactive, and the tailings produced during mining
and purification of uranium are also radioactive, both
of which can contaminate the environment.
Fissionable plutonium can be extracted from spent fuel
rods by reprocessing.
Breeder reactors maximize the production of plutonium
Solar energy comes directly or indirectly from the sun.
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Summarizing the Main Ideas
Solar energy is renewable and generally non-polluting.
Indirect forms of solar energy include wind power,
biomass energy and wave power.
Direct absorption of solar energy can occur by either
thermal or photo-conversion mechanisms.
Thermal conversion is generally used to produce hot
water, but can be used to generate electricity from hot
steam.
Solar cells use the photoelectric effect, where a
semiconductor absorbs photons, to produce electricity.
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