Radioactivity and Nuclear Chemistry. Nuclear Chemistry Nuclei that are unstable and spontaneously...
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Chapter 19 Radioactivity and Nuclear Chemistry
Radioactivity and Nuclear Chemistry. Nuclear Chemistry Nuclei that are unstable and spontaneously decompose are said to be radioactive. Nuclear chemistry
Nuclear Chemistry Nuclei that are unstable and spontaneously
decompose are said to be radioactive. Nuclear chemistry is the
study of nuclear reactions and their uses in chemistry. Nuclear
energy is produced from the radioactive nuclei and accounts for 20%
of our electrical generation in the US.
Slide 3
Radioactivity Atomic number = number of protons in the nucleus
Mass number = number of protons and neutrons in the nucleus Protons
and Neutrons collectively called nucleons.
Slide 4
Radioactivity Isotopes = atoms with the same atomic number, but
have different mass numbers. Each element can have one or more
isotopes that occur naturally. Example) Uranium has three isotopes:
U-234, U-235, U- 238 Each isotope has an abundance. U-238 = 99.3%,
U-235 = 0.7%, and U-234 = trace
Slide 5
Radioactivity Most isotopes are stable and only a few are
unstable. Unstable isotopes are called radioisotopes. A nuclear
equation is used to describe a decay process. Parent Nuclei
Daughter Nuclei
Slide 6
Types of Decay There are a few common types of particles found
in nuclear reactions. 1. Alpha = a helium nuclei (He-4). This is a
massive particle, but relatively low energy. 2. Beta = an electron.
The electron comes from the neutron changing it into a proton.
Light mass, but higher energy. 3. Gamma = release of a photon of
energy. Very light mass with very high energy.
Slide 7
Types of Decay 4. Positron = a positively charged electron.
Converts a proton into a neutron. Like the beta particle except
charge is plus one. 5. Neutrons can be generated in some reactions.
These have a moderate mass and energy. 6. Electron capture (EC) =
the capture of an inner shell electron by the nucleus. Converts a
proton into a neutron.
Slide 8
Slide 9
Balancing Nuclear Decays The sum of all the mass numbers must
be equal. The sum of all the atomic numbers must be equal. The
atomic number identifies the nuclide or particle. LEP #1
Slide 10
Fundamental Forces ForceRangeStrengthMediators Strong Nuclear10
-15 m 10 38 gluons Weak10 -18 m 10 25 W and Z bosons
Electromagneticinfinite 10 36 photons Gravitationinfinite 11
???
Slide 11
Patterns of Nuclear Stability Stability of nuclei depends on
many factors and no one factor allows us to predict stability. The
Strong Force holds the nucleons together. So powerful that it can
hold together like charges (protons). The Weak Force describes the
nuclear decay processes.
Slide 12
Neutron / Proton Numbers For Atomic number of 20 or less, a 1:1
ratio is preferred. Odd number elements may have exactly one more
neutron than protons. For Atomic numbers greater than 20, more and
more neutrons are required. Hg-200 has a 1.5:1 ratio. For Atomic
numbers greater than 83, all are radioactive.
Slide 13
Band of Stability Shows stable isotopes. To the left and less
than 83 protons will decay via Beta particle. To the right and less
than 83 protons will either decay via positron or do an EC. Greater
than 83 will decay via an alpha particle.
Slide 14
Very Heavy Isotopes Many very large nuclei will decay via a
series of alpha and beta emissions this is called a decay
series.
Slide 15
Magic Numbers Certain numbers of protons and neutrons have a
special stability. These are referred to as Magic numbers. protons
= 2, 8, 20, 28, 50, or 82 neutrons = same as above, but also 126
When a nucleus has a magic number of both protons and neutrons,
then nucleus is particularly stable.
Slide 16
Even, Odd Evens are favored over odds. Even protons and
neutrons = 157 stable isotopes. Even, Odd = 53. Odd, Even = 50 Odd,
Odd = only 5! obvious one is N-14 LEP #2
Slide 17
Nuclear Transmutations A nuclear reaction can be induced by
colliding two nuclei together. Rutherford was first to do this in
1919. Used to produce the very largest man-made isotopes. LEP #3
LEP #4
Slide 18
Rates of Decay Radioactive decay follows first order kinetics.
Same as Ch. 14
Slide 19
Rates of Decay
Slide 20
Carbon Dating Carbon-14 is produced in the upper atmosphere
from the nuclear reaction of N-14 with a solar neutron. The C-14 is
radioactive and undergoes decay with a half-life of 5730 years.
Assumption is made that C-14 levels have been stable for the past
50,000 years.
Slide 21
Carbon Dating Thus, all living things both plants and animals
have a steady state amount of C-14 until death. After death, the
C-14 slowly decays and can be compared to levels in living things.
Can provide ages for between 100 50,000 years old. LEP #5
Slide 22
Other Dating Techniques Rocks can be aged by comparison of
their U-238 to Pb-206 masses. Rock must contain U-238 at
formation,but be free of any lead. In time, U-238 decays back to
Pb-206 (see decay series earlier). Calculations using these
techniques. LEP #6
Slide 23
Detection Photographic material incorporated into a badge that
a worker wears. Geiger tube contains Argon gas inside of a tube
that has a positively charged wire.
Slide 24
E=mc 2 The energy associated with a nuclear reaction is due to
the loss of mass which is converted to energy. Even a very tiny
loss of mass can produce a huge quantity of energy. Say, for
example, 1 x 10 -6 kg is lost. Then, E = (1 x 10 -6 kg)(3.00 x 10 8
m/s) 2 = 9 x 10 10 J
Slide 25
E = mc 2 The energy produced is many orders of magnitude larger
than ordinary exothermic reactions. Example: The decay of one mole
of U-238 produces 50,000 times more energy than the combustion of
one mole of CH 4. This is why nuclear energy is so attractive! LEP
#7
Slide 26
Nuclear Binding Energies Scientists in the 1930s discovered
that the mass of every nuclei after hydrogen is always LESS than
the sum of the individual masses of the protons and neutrons that
make them up. Example: Mass of He-4 = 4.0015amu Mass of 2p + 2n =
4.03188amu
Slide 27
Nuclear Binding Energies Missing mass is called the mass
defect. For He-4 = 4.03188amu 4.00150amu = 0.03038amu This is then
converted to an energy per nucleon. (0.0000308kg) x (3 x 10 8 m/s)
2 = 2.772 x 10 12 J 2.772 x 10 12 J / 6.02 x 10 23 atoms/mole =
4.60 x 10 -12 J 4.06 x 10 -12 J / 4 nucleons = 1.15 x 10 -12
J/nucleon LEP #8
Slide 28
Nuclear Binding Energies
Slide 29
Nuclear Fission Nuclear Fission the process of splitting larger
nuclei into smaller ones. U-235, U-233, and Pu-239 will undergo
fission when the nucleus is struck by a slow moving neutron. The
heavier nuclei does not split the same way rather a whole variety
of nuclear reactions result.
Slide 30
Nuclear Fission All fission reactions produce two smaller
nuclei and several neutrons. One possible reaction for U-235 is:
Note that the 3 neutrons produced can strike another U-235 nuclei
and split it as well.
Slide 31
Chain Reactions Because each U-235 that splits generates two or
more neutrons, the possibility of a chain reaction occurs. Critical
Mass the minimum amount of U-235 necessary to maintain the chain
reaction. This means that exactly one neutron will continue the
reaction each time. Supercritical Mass exceeds the critical mass.
Results in an uncontrolled chain reaction.
Slide 32
Splitting of U-235
Slide 33
Critical Mass
Slide 34
Nuclear Weapons A nuclear weapon (bomb) can be constructed if
you have two or more sub-critical masses of U-235, which when
combined would produce a critical mass. The critical mass of U-235
is about 1 kilogram. Problem: Naturally occurring Uranium contains
only 0.7% U-235. Solution: Must separate the U-235 from the other
isotopes.
Slide 35
Enrichment Enrichment is the process by which the quantity of
U- 235 present in a sample is increased by removing the other
undesirable isotopes. This is NOT easy to do! U.S. used gaseous
diffusion of UF 6 back in the early 1940s to obtain enough U-235.
Current methods involving using centrifuges.
Slide 36
Weapon Design Problem: sub-critical masses need to be kept
separate until the weapon is deployed. Then, they must be combined
to produce the super-critical mass. Solution: Implosion of
sub-critical masses forces them together. First design was
relatively simple.
Slide 37
Basic Weapon Design
Slide 38
Nuclear Reactors The energy of a nuclear reaction can be
captured in a nuclear reactor. Uranium ore is enriched to about 3%
U-235 and converted to UO 2. The UO 2 pellets are then encased in
either Zr or stainless steel tubes and referred to as fuel rods.
Rods composed of Cd or B, which are good absorbers of neutrons are
also constructed and are referred to as control rods.
Slide 39
Simple Reactor Design
Slide 40
Slide 41
Fast Breeder Reactor A proven, yet unused method to make more
nuclear fuel than it consumes. Uses fast neutrons and heats a
liquid metal like sodium.
Slide 42
Nuclear Wastes Fission products accumulate as the reactor
operates. Fuel rods must be replaced or reprocessed periodically.
Every year about 1/3 of the fuel rods are replaced or repacked.
When replaced, the spent fuel rods are still highly radioactive and
are stored on site in large water pools.
Slide 43
Nuclear Wastes One of the side products is Pu-239 another
fissionable isotope. Pu-239 can be separated from the other wastes
and is easily weaponized. US foreign policy.
Slide 44
Fusion The Sun and other stars use a different type of nuclear
reaction called fusion. Fusion occurs when two or more smaller
nuclei are squeezed together to make a larger isotope. The net
reaction on the Sun is: 4 H He + 2 e + This requires very high
temperatures and pressures of the type found only in stars.
Slide 45
Fusion Fusion on this planet can be achieved in a special
reactor. The lowest energy reaction for fusion is: There is enough
deuterium (H-2) and tritium (H-3) present in the worlds oceans to
supply us with fusion energy forever. Why is this not feasible? LEP
#9
Slide 46
Fusion A novel process for fusion has been proposed by Dr.
Robert Bussard (deceased). Uses a Boron-11 and H-1 collision to
generate three alpha particles. Research efforts can be followed
at: http://focusfusion.org/http://focusfusion.org/
Slide 47
Biological Effects of Radiation We all receive some radiation
whether we want it or not. Background radiation comes from many
sources including: Food K-40 Air Rn-222 Ground U-238 Also are
exposed to man-made sources like X-rays, nuclear medicine, air
travel, and cigarettes. Total background average is about
360mrem.
Slide 48
Ionizing Radiation When molecules absorb radiation it can lose
an electron. For example, when radiation strikes a water molecule:
H 2 O + radiation H 2 O + + 1e - That ion then reacts with a second
water molecule : H 2 O + + H 2 O H 3 O + + OH
Slide 49
Free Radical The OH has an odd number of electrons and is
called a free radical. Any free radical is highly reactive and can
cause biomolecules to form free radicals. Free radicals can also
interfere with electron transfer reactions.
Slide 50
Radiation Doses Two factors are combined: rad = radiation
absorbed dose = 1 x 10 -2 J/kg of body tissue RBE = multiplier that
depends on the particle RBE = 1 for beta and gamma, RBE = 10 for
alpha rem = rad x RBE
Slide 51
Damages By Particle Alpha = least penetrating (skin is enough
protection), but have highest RBE. Can cause great damage
internally in soft tissues like the lungs. Beta = can penetrate the
upper layers of the skin (thick clothing provides protection).
Gamma = can penetrate completely.
Slide 52
Total Effects Damage depends on the activity, source, and
whether it is internal or external exposure. Tissues most affected
are those that reproduce rapidly like the skin, marrow, and
intestinal linings. Tissues least affected are those that undergo
little or no cell division like the brain, muscles, and
nerves.
Slide 53
Radiation Levels A whole body exposure for an adult of ___rem
is called an LD 50 level. A ___rem exposure or more will kill all
white blood cells in the body. Exposure of less than 25rem has no
noticeable effect, but long term health effects are unknown. OSHA
limit for workers is ___rem/year.
Slide 54
Medical Applications Radioisotopes are used in two unique ways.
1. Diagnostic Use low doses with short half-lives are injected into
the body. These help to illuminate a targeted organ or region in
the body. 2. Therapeutic Use high doses with short half-lives
target tumor cells.