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20.1 Nuclei and Nuclear Reactions
• Radioactive decay – emission of particles and/or electromagnetic radiation by unstable nuclei
• Radioactivity - Spontaneous emission of particles or electromagnetic radiation
• Nuclear transmutation, results from the bombardment of nuclei by neutrons, protons, or other nuclei.
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Radioactive decay and nuclear transmutation are nuclear reactions, which differ significantly from ordinary chemical reactions.
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• The symbols for subatomic particles are as follows:
• In balancing any nuclear equation, we must balance the total of all atomic numbers and the total of all mass numbers for the products and reactants.
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017833 XAs A
Z
reactant mass numbers = product mass numbers
reactant atomic numbers = product atomic numbers
0 78 A
78 A
1)( 33 Z
34 ZTherefore, X is Se or Se78
34
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20.2 Nuclear Stability
• Nuclear stability determined by a balance between– Coulombic repulsions– Short range nuclear attractions (very strong)– If replusions > attractions, the nucleus is
unstable– If attractions > replusions, the nucleus is
stable
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• Patterns of nuclear stability– Nuclei containing a magic number of protons
and/or neutrons are stable.• The numbers 2, 8, 20, 50, 82, and 126 are
called magic numbers.– There are many more stable nuclei with even
numbers of both protons and neutrons than with odd numbers of these particles.
– All isotopes of the elements with atomic numbers higher than 83 are radioactive.
– All isotopes of technetium (Tc, Z = 43) and promethium (Pm, Z = 61) are radioactive.
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particle emission
electron capture
positron emission
Above the belt
Below the belt
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• Nuclear Binding Energy– Quantitative measure of nuclear stability– The energy required to break up a nucleus
into its component protons and neutrons.– Represents the conversion of mass to energy
that occurs during an exothermic nuclear reaction.
– The difference between the mass of an atom and the sum of the masses of its protons, neutrons and electrons is called the mass defect.
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– According to Einstein’s mass-energy equivalence relationship (E = mc2, where E is energy, m is mass, and c is the velocity of light), the energy released is
– where E and m are defined as follows:
2cm E )(
reactants ofenergy - products ofenergy E
reactants of mass - products of mass m
energy binding nuclear E
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nucleons of number
energy binding nuclear nucleon perenergy binding nuclear
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Calculate a) the nuclear binding energy in
kilojoules/mol and b) the nuclear binding energy
in joules per nucleon of . The exact atomic
mass of bismuth is 208.9804.
Bi20883
1.008665 nmass 10
1.007825 Hmass 11
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amu 83.649475 1.007825 83
amu 126.083125 1.008665 125 amu 209.732600
amu 209.732600- amu 208.9804 m
amu 75220. m
amu 10 6.022
kg 1.00 amu 75220
26. m
a) Nuclear binding energy
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2210 s/m kg10 1.12 E
2827 m/s103.00kg 101.2491 E
J 10 1.12 10E
/mol106.022J 10 1.12 2310 E
J/mol10 6.74 13E
kJ/mol10 6.74 13E
kg 101.2491 27m
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b) Nuclear binding energy per nucleon
J/nucleon105.38nucleons 208
J10 1.12 13
10
E
kJ/mol10 6.74 energy binding Nuclear 13
kJ/mol10 6.74- 13E
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20.3 Natural Radioactivity
• The disintegration of a radioactive nucleus often is the beginning of a radioactive decay series, which is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.
• The beginning radioactive isotope is called
the parent and the product isotope is called the daughter.
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• Kinetics of radioactive decay– First-order kinetics (N – number of radioactive
nuclei at time t , k is the rate constant and is the half-life)
– Used as the basis for dating (14C and 238 U are used depending on material)
time atdecay of rate kNt
0
t kt N
Nln
0.693
k t
21 /
21 /
t
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A piece of linen cloth found at an ancientburial site is found to have a 14C activity of 4.8 disintegrations per minute. Determinethe age of the cloth. Assume that thecarbon-14 activity of an equal mass of living flax (the plant from which linen ismade) is 14.8 disintegrations per minute. The half-life of carbon-14 is 5715 years.
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0
t kt N
Nln
flax living inacitivity C
artifact inactivity C14
14
kt ln
yr101.12 dps 14.8
ps d 4.8ln 14 t
0.693
years5715k
yr101.21 -14 k
yr10 1.0 4t
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20.4 Nuclear Transmutation
• Nuclear transmutation differs from radioactive decay in that transmutation is brought about by the collision of two particles.
• Particle accelerators made it possible to synthesize the so-called transuranium elements, elements with atomic numbers greater than 92.
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Write an equation for the process represented by
AgpPd 10947
10646 ),(
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AgpPd 10947
10646 ),(
emitted particlebombarding particle
p Ag He Pd 11
10947
42
10646
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A particle accelerator uses electric and magnetic fields to increase the kinetic energy of charged species so that a reaction will occur.
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20.5 Nuclear Fission
• Nuclear fission is the process in which a heavy nucleus (mass number > 200) divides to form smaller nuclei of intermediate mass and one or more neutrons.
• Because the heavy nucleus is less stable than its products, this process releases a large amount of energy.
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• nuclear chain reaction, which is a self-sustaining sequence of nuclear fission reactions.
• critical mass, the minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction.
• Applications of nuclear fission– Atomic bomb– Generation of electricity
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• Types of reactors (Using as fuel) – Light water reactor - uses H2O as
moderator used to reduce kinetic energy of neutrons
– Heavy water reactor – uses D2O as moderator• More efficient than light water reactor
– Breeder reactor – produces more fissionable fuel than it uses• Doubling time – time to produce enough
fuel to refuel the original reactor• Can utilize fertile isotopes plutonium-239
and thorium-232
U23592
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20.6 Nuclear Fusion
• Nuclear fusion - the combining of small nuclei into larger one– Exempt from waste disposal issues of fission
• Solar fusion
• Thermonuclear reactions – take place at very high temperatures
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• Promising reactions
• Technical difficulty – confine nuclei at required temperatures– Magnetic confinement– High-power lasers
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• Used in hydrogen (thermonuclear) bombs– High temperatures attained– Contain solid LiD
– Cleaner than fission bombs
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20.7 Use of Isotopes• Chemical analysis
– Use of tracers • Sulfur-35 in the determination of the
structure of thiosulfate
• Photosynthetic pathway using oxygen-18 and 14-carbon
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• Isotopes in medicine– Use of tracers for diagnosis
• Sodium-24 – blood flow• Iodine-131 –thyroid conditions• Iodine -123 – brain imaging
– Major advantage – easy to detect
normal Alzheimer victim
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20.8 Biological Effects of Radiation
• Quantitative measures of radation– curie (Ci): fundamental unit of radioactivity
• Equivalent to 3.70 x 1010 nuclear disintegrations per second
– rad (radiation absorbed dose)• Considers activity• Considers energy• Considers type of radiation emitted• 1 rad = 1 x 105 J/g of tissue irradiated
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– RBE (relative biological effectiveness)• Considers biological effect of radiation
–Part of body irradiated–Type of radiation
– rem (roentgen equivalent for man)
• Chemical basis for radiation damage– Ionizing radiation produces radicals– Radicals (free radicals) – molecular
fragments with unpaired electrons
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– e and the hydroxyl radical can form other radicals
– In tissues radicals can attack and destroy membranes, enzymes, DNA, etc.
• Radiation damage– Somatic (affect the organism within its
lifetime)– Genetic (inheritable changes and gene
mutations)
Key Points
• Nuclei and nuclear reactions– Radioactive decay– Nuclear transmutations– Particles involved in nuclear reactions– Balancing nuclear reactions
• Nuclear stability– Type of interactions involved– Pattern of stability
• Magic numbers• Odd/even numbers of nucleons
– Nuclear binding energy• Mass defect• Einstein’s mass-energy equivalence
relationship• Calculation nuclear binding energy
–Per mole of nucleons–Per nucleon
• Natural radioactivity– Radioactive decay series
– Kinetics of radioactive decay– Dating based on radioactive decay
• Carbon-14 dating• Uranium-238 dating• Potassium-40 dating
• Nuclear Transmutation– Transuranium element– Particle accelerators
• Nuclear fission– Nuclear fission reactions
• Nuclear chain reactions• Critical mass
– Generation of electric power• Light water reactors• Heavy water reactors• Breeder reactors
– Nuclear fusion• Solar nuclear reactions• Thermonuclear reactions• Potential for generation of electric power• Thermonuclear bombs