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Nuclear Chemistry Chapter 18
Nuclear Chemistry Chapter 18
Hein and Arena Eugene Passer Chemistry Department Bronx Community College© John Wiley and Sons, Inc
Version 2.0
12th Edition
2
Chapter Outline18.1 Discovery of Radioactivity
18.2 Natural Radioactivity
18.3 Alpha Particles, Beta Particles and Gamma Rays 18.10 The Atomic Bomb
18.11 Nuclear Fusion18.4 Radioactive Disintegration
Series
18.5 Transmutation of Elements
18.9 Nuclear Power
18.12 Mass-Energy Relationships in Nuclear Reactions
18.13 Transuranium Elements
18.6 Artificial Radioactivity
18.7 Measurement of Radioactivity
18.8 Nuclear Fission
3
18.118.1
Discovery of Discovery of RadioactivityRadioactivity
4
Roentgen• In 1895 Wilhelm Konrad Roentgen
discovered X-rays.• Roentgen observed that a vacuum
discharge tube enclosed in a thin, black cardboard box had caused a nearby piece of paper coated with the salt barium platinocyanide to glow with phosphorescence.
5
Roentgen• From this and other experiments he
concluded that certain rays, which he called X-rays, were emitted from the discharge tube, and penetrated the box and caused the salt to glow.
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Becquerel• Shortly after Roentgen’s discovery,
Antoine Henri Becquerel attempted to show a relationship between X-rays and the phosphorescence of uranium salts.
• Becquerel wrapped a photographic plate in black paper, sprinkled a sample of a uranium salt on it, and exposed it to sunlight.
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Becquerel• When Becquerel attempted to repeat
the experiment the sunlight was intermittent.
• He took the photographic plate wrapped in black paper with the uranium sample on it and placed the whole setup in a drawer.
8
Becquerel• Several days later he developed the
film and was amazed to find an intense image of the uranium salt on the plate.
• He repeated the experiment in total darkness with the same result.
9
Radioactivity is the spontaneous emission of particles and/or rays from the nucleus of an atom.
Becquerel• This proved that the uranium salt emitted
rays that affected the photographic plate and that these rays were not a result of phosphorescence due to exposure to sunlight.
Elements having this property are radioactive.
• Two years later, in 1896, Marie Curie coined the name radioactivity.
10
Rutherford• In 1899 Rutherford began to investigate
the nature of the rays emitted by uranium.• He found two particles in the rays. He
called them alpha and beta particles.
• Rutherford’s nuclear atom description led scientists to attribute the phenomenon of radioactivity to reactions taking place in the nuclei of atoms.
11
Villiard• The gamma ray, a third type of emission
from radioactive material, was discovered by Paul Villiard in 1900.
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DefinitionsDefinitionsDefinitionsDefinitions
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nucleon a proton or a neutron mass number the total number of nucleons in the nucleus.
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isotope atoms of the same element with different massesnuclide any isotope of any atom
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Isotopic NotationIsotopic NotationIsotopic NotationIsotopic Notation
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17
C66 protons
126 protons + 6 neutrons
A nuclide of carbon
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88 protons
168 protons + 8 neutrons
O
A nuclide of oxygen
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88 protons
178 protons + 9 neutrons
O
A nuclide of oxygen
20
88 protons
188 protons + 10 neutrons
O
A nuclide of oxygen
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18.218.2
Natural RadioactivityNatural Radioactivity
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• Radioactive elements continuously undergo radioactive decay or disintegration to form different elements.
• Radioactivity is a property of an atom’s nucleus. It is not affected by temperature, pressure, chemical change or physical state.
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radioactive decay the process by which a radioactive element emits particles or rays and is transformed into another element.
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• Each radioactive nuclide disintegrates at a specific and constant rate, which is expressed in units of half-life.
• The half-life (t1/2) is the time required for one-half of a specific amount of a radioactive nuclide to disintegrate.
1/2t
226 88
1620 yrs1.0 g Rn
1/2t
226 88
1620 yrs0.5 g Rn 226
880.25 g Rn
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The half-life of 131I is 8 days. How much 131I from a 32-g sample remains after five half-lives?
131 53 I131
53 I
Take a perpendicular line from any multiple of 8 days on the x-axis to the line on the graph.
half-lives
number of days
amount remaining
0
32 g
1
8
16 g
2
16
8 g
Trace a horizontal line from this point on the plotted line to the y-axis and read the corresponding grams of 131I.131
53I
3
24
4 g
4
32
2 g
5
40
1 g
18.1
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• Nuclides are said to be either stable (nonradioactive) or unstable (radioactive).
• Elements that have an atomic number greater than 83 are naturally radioactive.
• Some of the naturally occurring nuclides of elements 81, 82 and 83 are radioactive and some are stable.
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• No stable isotopes of element 43 (technetium) or of element 61 (promethium) are known.
• Only a few naturally occurring elements that have atomic numbers less than 81 are radioactive.
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• Radioactivity is believed to be a result of an unstable ratio of neutrons to protons in the nucleus.
• Stable nuclides of elements up to about atomic number 20 generally have about a 1:1 neutron-to-proton ratio.
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• When the neutron to proton ratio is too high or too low, alpha, beta, or other particles are emitted to achieve a more stable nucleus.
• In elements above atomic number 20, the neutron-to-proton ratio in the stable nuclides gradually increases to about 1.5:1 in element number 83 (bismuth).
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18.318.3
Alpha Particles,Alpha Particles,Beta Particles and Beta Particles and
Gamma RaysGamma Rays
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Marie Curie, in a classic experiment, proved that alpha and beta particles are oppositely charged.
radiation passes between the poles of an electromagnet
a radioactive source was placed inside a lead block
Alpha rays are less strongly deflected to the negative pole.
Gamma rays are not deflected by the magnet.Beta rays are strongly deflected to the positive pole.
three types of radiation are detected by a photographic plate
18.1
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Alpha ParticlesAlpha ParticlesAlpha ParticlesAlpha Particles
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It consists of two protons and two neutrons.
It has a mass of 4 amu.
It has a charge of +2.
The symbols of an alpha particle are
An alpha particle is a helium nucleus.
42He
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Loss of an alpha particle from the nucleus results in loss of 4 in the mass number (A) loss of 2 in the atomic number (Z)
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238 234 92 90U Th + α
Formation of thorium from the radioactive decay of uranium can be written as
or238 234 4 92 90 2U Th + He
mass number decreases by 4
atomic number decreases by 2
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To have a balanced nuclear equation• the sum of the mass numbers (superscripts)
on both sides of the equation must be equal.
• the sum of the atomic numbers (subscripts) on both sides of the equation must be equal.
238 234 4 92 90 2U Th + He
sum of mass numbers = 238
sum of atomic numbers = 92
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Beta ParticlesBeta ParticlesBeta ParticlesBeta Particles
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Its charge is -1.
The symbols of the beta particle are
The beta particle is identical in mass and charge to an electron.
0-1e
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A proton and a beta particle are formed by the decomposition of a neutron.
The beta particle leaves the nucleus and the proton remains in the nucleus.
n p + e10
11
0 -1
42
Loss of a beta particle from the nucleus results in
234 234 90 91Th Pa + β
– no change in the mass number
– an increase of 1 in the atomic number
Pa U + e234 91
234 92
0 -1
43
Gamma RaysGamma RaysGamma RaysGamma Rays
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It is emitted by radioactive nuclei.
It has no measurable mass.
It has no electrical charge.
The symbol of a gamma ray is
A gamma ray is a high energy photon.
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Loss of a gamma ray from the nucleus results in
– no change in the mass number
– no change in atomic number
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Write an equation for the loss of an alpha particle from the nuclide 194Pt. 194
78 Pt
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78 protons
Pt78194
78 protons + 116 neutrons
A nuclide of platinum
Atomic number(number of protons in the nucleus)
Mass number(sum of protons and neutrons in the nucleus)
Write an equation for the loss of an alpha particle from the nuclide 194Pt. 194
78 Pt
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Write an equation for the loss of an alpha particle from the nuclide 194Pt. 194
78 Pt
Loss of an alpha particle, 4He, results in a decrease of 4 in the mass number and a decrease of 2 in the atomic number.
42 He
Mass of new nuclide: A-4 194 – 4 = 190or
A = mass number
Atomic number of new nuclide: Z-2 78 – 2 = 76orZ = atomic number
Element number 76 is Os, osmium.
194 190 4 78 76 2Pt Os + He
The equation is
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What nuclide is formed when 194Ra loses a beta particle from its nucleus?
228 88 Ra
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88 protons
Ra88228
88 protons + 140 neutrons
A nuclide of radium
Atomic number(number of protons in the nucleus)
Mass number(sum of protons and neutrons in the nucleus)
What nuclide is formed when 194Ra loses a beta particle from its nucleus?
228 88 Ra
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The loss of a beta particle from a 194Ra nucleus means a gain of 1 in the atomic number with no essential change in mass.
228 88 Ra
228 228 0 88 89 -1Ra Ac + e
Mass of new nuclide: A-0 228 – 0 = 228or
A = mass number
Atomic number of new nuclide: Z-(-1)Z = atomic number
88 + 1 = 89or
What nuclide is formed when 194Ra loses a beta particle from its nucleus?
228 88 Ra
The equation is
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Penetrating PowerPenetrating Powerof Radiationof Radiation
Penetrating PowerPenetrating Powerof Radiationof Radiation
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The ability of radioactive rays to pass through various objects is in proportion to the speed at which they leave the nucleus.
Thin sheet of aluminum – stops and particles.
18.2
Thin sheet of paper – stops particles.
5-cm lead block – will reduce, but not completely stop radiation
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18.418.4
Radioactive Radioactive Disintegration SeriesDisintegration Series
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The naturally occurring radioactive elements with a higher atomic number than lead fall into three orderly disintegration series. Each series proceeds from one element to the next with the loss of either an alpha or a beta particle, finally ending in a nonradioactive nuclide.
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– The uranium series starts with 238U and ends with 206Pb.
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82– The thorium series starts with 232Th and
ends with 208Pb. 90
82– The actinium series starts with 235U and
ends with 207Pb.92
82
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A fourth series begins with the synthetic element plutonium.
– The neptunium series begins with 241Pu and ends with 238Bi.
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83
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18.3
The uranium disintegration series. 238U decays by a series of alpha () and beta () emissions to the stable nuclide 208Pb.
238 92 U
206 82 Pb
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18.518.5
TransmutationTransmutationof Elementsof Elements
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226 222 88 86Ra Rn + α
210 210 0 82 83 -1Pb Bi + e
Transmutation is the conversion of one element into another by either natural or artificial means.
Transmutation occurs spontaneously in natural radioactive disintegrations.
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Alchemists for hundreds of years attempted to transmute mercury and lead into gold by artificial means. They were never successful.
The first artificial transmutation occurred in 1919 when Rutherford succeeded in producing oxygen from nitrogen.
14 4 17 1 7 2 8 1N + He O + H
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Some of these transmutations are:
7 1 43 1 2Li + H 2 He
209 2 210 1 83 1 84 0Li + H Po + n
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18.618.6
Artificial Artificial RadioactivityRadioactivity
65
Irene and Frederick Curie observed that the bombardment of aluminum-27 with alpha particles resulted in the emission of neutrons and positrons.
27 413 2Al + He neutrons and positrons
66
When alpha particle bombardment is halted neutron emission stops, but positron emission continues.
This suggested that neutron emission and positron emission were a result of separate reactions.
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Further investigation on their part showed that when aluminum-27 is bombarded with alpha particles phosphorous-30 is produced.
27 4 30 113 2 15 0Al + He P + n
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Phosphorous-30 is radioactive, has a 2.5 minute half-life, and decays to silicon-30 with the emission of a positron.
30 30 015 14 +1P Si + e
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30 30 015 14 +1P Si + e
The radioactivity of nuclides produced in this manner is known as artificial or induced radioactivity.
The Joliot-Curies received the Nobel Prize in chemistry in 1935 for the discovery of artificial, or induced, radioactivity.
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18.718.7
MeasurementMeasurementof Radioactivityof Radioactivity
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Radiation from a radioactive source can be measured by a variety of instruments.
– Geiger counters
– film badges
– scintillation counters
72
The curie is the unit indicating the rate of decay of a radioactive substance.
One curie (Ci) = 3.7 x 1010 disintegrations per second.
This very high radiation level is the amount of radiation emitted by 1 gram of radium in one second.
Because a curie is so large the millicurie (one thousandth of a curie) and the microcurie (one millionth of a curie) are more commonly used.
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18.818.8
Nuclear FissionNuclear Fission
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235 1 139 94 1 92 0 56 36 0U + n Ba + Kr + 3 n
In nuclear fission, a heavy nuclide splits into two or more intermediate sized fragments when struck in a particular way by a neutron
235 1 144 90 1 92 0 54 38 0U + n Xe + Sr + 2 n
As the atom splits, it releases energy and two or three other neutrons, each of which can cause another nuclear fission.
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Characteristics of Nuclear Fission
1. Upon absorption of a neutron, a heavy nuclide splits into one or more smaller nuclides (fission products).
2. The mass of the nuclides ranges from abut 70-160 amu.
3. Two or more neutrons are produced from the fission of each atom.
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Characteristics of Nuclear Fission
4. Large quantities of energy are produced as a result of the conversion of a small amount of mass into energy.
5. Many nuclides produced are radioactive and continue to decay until they reach a stable nucleus.
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Each time fission occurs three neutrons and two nuclei are produced.
18.5
Fission of 235U235 92 U
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• In a chain reaction the products cause the reaction to continue or magnify.
• For a chain reaction to continue, enough fissionable material must be present so that each atomic fission causes, on average, at least one additional fission.
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• The minimum quantity of an element needed to support a self-sustaining chain reaction is called the critical mass.
• Since energy is released in each atomic fission, chain reactions provide a steady supply of energy.
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18.6
Fission and chainreaction of 235U.
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18.918.9
Nuclear PowerNuclear Power
83
• A nuclear power plant is a thermal power plant in which heat is produced by a nuclear reactor instead of by combustion of fossil fuel. The major components of a nuclear reactor are:b– an arrangement of nuclear fuel, called the
reactor core.
– a control system, which regulates the rate of fission and thereby the rate of heat generation.
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85
• In the United States breeder reactors are used to generate nuclear power.
• Breeder reactors use U3O8 that is enriched with scarce, fissionable U-235.
• In a breeder reactor, excess neutrons convert nonfissionable isotopes, such as U-238 or Th-232, to fissionable isotopes, Pu-239 or U-233.
238 1 239 239 239 92 0 92 93 94U + n U Np Pu
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18.1018.10
The Atomic BombThe Atomic Bomb
87
• The atomic bomb is a fission bomb.
• “Wild” or uncontrolled fission occurs in an atom bomb, whereas in a nuclear reactor the fission is carefully controlled.
• A minimum critical mass of fissionable material is required for a bomb.
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• When a quantity of fissionable material smaller than the critical mass is used, too many neutrons escape and a chain reaction does not occur.
• The fissionable material of an atomic bomb is stored as two or more subcritical masses and are then brought together to achieve a nuclear detonation.
89
• Uranium-235 and plutonium-239 are the nuclides used to construct an atomic bomb.
238 1 239 239 239 92 0 92 93 94U + n U Np Pu
• 99.3% of uranium is nonfissionable uranium-238. Uranium-238 can be transmuted to fissionable plutonium-239.
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18.1118.11
Nuclear FusionNuclear Fusion
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Nuclear fusion is the process of uniting two light elements to form one heavier element.
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The masses of the two nuclei that fuse into a single nucleus are greater than the mass of the nucleus formed by their fusion.
3 2 4 11 1 2 0H + H He + n + energy
tritium deuterium
3 1 41 1 2H + H He + energy
3.0150amu
1.0079amu
4.0026amu
4.0229amu 4.0229 amu – 4.0026 amu = 0.0203 amu
The difference in mass is released as energy.
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• Fusion reactions require temperatures on the order of tens of millions of degrees for initiation.
• Such temperatures are present in the Sun but have been produced only momentarily on earth.
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Fusion power will be far superior to fission power because
– Virtually infinite amounts of energy are possible from fusion power.
– While uranium supplies are limited, deuterium supplies are abundant.
– It is estimated that the deuterium present in a cubic mile of seawater used as fusion fuel can provide more energy than the petroleum reserves of the entire world.
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Fusion power will be far superior to fission power because
– Fusion power is much “cleaner” than fission power.
– Fusion reactions (unlike uranium and plutonium fission reactions) do not produce large amounts of long-lived and dangerously radioactive isotopes.
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18.1218.12
Mass-Energy Relationship inMass-Energy Relationship inNuclear ReactionsNuclear Reactions
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• In fission reactions, about 0.1% of the mass of the reactants is converted into energy.
7 1 4 43 1 2 2Li + H He + He + energy
7.016 g 1.008 g 4.003 g 4.003 g
8.024 g
8.024 g – 8.006 g = 0.018 g
8.006 g
• In fusion reactions as much as 0.5% of the mass of the reactants may be changed into energy.
The energy equivalent of this mass is 1.62 x 1012J.
This is 4,000,000 times greater than the energy released from the combustion of 1 mol of carbon.
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• The mass of a nucleus is less than the sum of the masses of the protons and neutrons that makes up that nucleus.
• The difference between the mass of the protons and the neutrons in a nucleus is known as the mass defect.
• The energy equivalent to this difference in mass is known as the nuclear binding energy.
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• The nuclear binding energy is the amount of energy that would be required to break a nucleus into its individual protons and neutrons.
• The higher the binding energy, the more stable the nucleus.
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• Neutrons and protons attain more stable arrangements through nuclear fission or fusion reactions.– when uranium undergoes fission the
products have less mass (greater binding energy) than the reactants.
– when hydrogen and lithium fuse to form helium, the helium has less mass (greater binding energy) than the hydrogen and lithium.
101
Calculate the mass defect and the nuclear binding energy for an particle (helium nucleus).
1.0 g = 9.0 x 1013J when converted into energy
Data: proton mass = 1.0073 gneutron mass = 1.0087 g/mol mass = 4.0015 g/mol
Solution: First calculate the sum of the individual parts of an particle:
= 2.0146 g/mol2 x 1.0073 g 2 protons:2 neutrons: 2 x 1.0087 g = 2.0174 g/mol
4.0320 g/mol
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Calculate the mass defect and the nuclear binding energy for an particle (helium nucleus).
Mass defect:
the difference between the mass of the particle and its component parts.
mass defect = 0.0305 g/mol
mass of component particles = 4.0320 g/molmass of particle = 4.0015 g/mol
Nuclear binding energy:
convert mass defect to its energy equivalent:
g0.305
mol
13 J9.0 x 10
g
12 J= 2.7 x 10
mol
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18.1318.13
Transuranium Transuranium ElementsElements
104
transuranium elements follow uranium in the periodic table and have atomic numbers greater than 92.
They are synthetic radioactive elements; none of them occur naturally.
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• The first transuranium element, number 93, was discovered in 1939 by Edwin McMillan who named it neptunium for the planet Neptune.
238 238 0 93 94 -1Np Pu + e239 239 0 93 94 -1Np Pu + e
• In 1941, element 94, plutonium, was identified as a beta-decay product of neptunium.
Plutonium is one of the most important fissionable elements known today.
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