nuclear chemistry
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NUCLEAR CHEMISTRY. Section 22-1: The Nucleus. Objectives. Explain what nucleons are. Explain what a nuclide is, and describe the different ways it can be written. Define nuclear binding energy. Explain the relationship between nucleon number and stability of nuclei. The Nucleus. - PowerPoint PPT PresentationTRANSCRIPT
NUCLEAR CHEMISTRY
Section 22-1: The Nucleus
Objectives
1. Explain what nucleons are.
2. Explain what a nuclide is, and describe the different ways it can be written.
3. Define nuclear binding energy.
4. Explain the relationship between nucleon number and stability of nuclei.
The Nucleus
The nucleus is composed of nucleons Protons Neutrons
A nucleus is characterized by two numbers mass number (A; total # of nucleons) atomic number (Z; number of protons)
ZAE
1327Al
total number of nucleons is 27
total number of protons is 13
the number of neutrons is 14
in nuclear chemistry, an atom is referred to as a nuclide.
Subatomic Particlesone atomic mass unit (u) is defined as 1/12th the
mass of a carbon-12 atom
Particle mass in kg mass in u
electron 9.11 x 10-31 kg 5.485 x 10-4 u
proton 1.673 x 10-27 kg 1.0073 u
neutron 1.675 x 10-27 kg 1.0087 u
Mass of 2
4He
The actual mass is 4.00260 amu.
Why the difference in mass? mass defect
Einstein’s Equation
• Energy and mass can be interconverted
• E = mc2
• E- energy, m-mass, c-speed of light
• When protons & neutrons are packed together to form a nucleus, some of the mass is converted to energy and released.
• Mass is lost
Nuclear Binding Energy• Nuclear Binding Energy – the energy released
when a nucleus is formed from protons and neutrons.
• Can also be thought of as the amount of energy required to break apart the nucleus. of an existing atom.
• The higher the binding energy, the more tightly the nucleus is held together.
Binding Energy Curve
• graph peaks at A=56 • the higher the BE, the
more stable the nucleus
• mass number of 56 is maximum possible stability
How Many Neutrons?
Stable nuclides have certain characteristics The number of neutrons in a nucleus can vary as
we have seen Range limited by the degree of instability created
by: having too many neutrons too few neutrons
Stable nuclei do not decay spontaneously Unstable nuclei have a certain probability to decay
Nuclear Stability Facts
• 265 stable nuclides• For light elements (Z 20), Z:N ratio is ~1• Example: helium-4 (2 neutrons, 2 protons)• Z:N ratio increases toward 1.5 for heavy elements• Example: lead-206 (124 neutrons, 82 protons)• For Z > 83 (bismuth), all isotopes are radioactive• This is due to repulsive forces of the protons
Nuclear Stability Facts
Stable nuclei tend to have an even number of nucleons.
Out of 265 stable nuclides, 159 have even numbers of both protons and neutrons.
Only 4 nuclides have odd numbers of both.
The most stable nuclides are those having: 2, 8, 20, 28, 50, 82, 126
Protons, neutrons or total nucleons
Examples:Sn (Z=50) has 10 isotopes; In (Z=49) &
Sb (Z=51) have only 2 isotopesPb-208 has a double magic number
(126n, 82p) & is very stable
“Magic numbers” of protons or neutrons which are unusually stable
Nuclear Reactions
Nuclear Reaction – a reaction that affects the nucleus of an atom.
Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons.
They also give off a large amount of energy and increase their stability in the process.
Nuclear Reactions In a nuclear reaction, the total of the
atomic numbers and the total of the mass numbers must be equal on both sides of the equation.
Example:
49Be + 2
4 612C + 0
1n
49Be + 2
4 612C + 0
1n
Note that when the atomic number changes, the identity of the element changes.
Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons.
Sample Problem
Identify the product that balances the following nuclear reaction:
84212Po ? + 2
4He
Sample Problem
Identify the product that balances the following nuclear reaction:
84212Po 82
208Pb + 24He
Classwork
Problems 2-3, page 704
Homework
Page 724
Problems: 31, 33 and 40
Section 22-2
Radioactive Decay
RadioactivityObjectives
1. Define the terms radioactive decay and nuclear radiation.
2. Describe the different types of radioactive decay.
3. Define the term half-life, and how it relates to stability.
Radioactivity
The spontaneous decay of an unstable nucleus into a more stable nucleus.
Energy is released.
Certain isotopes are just not stable and will spontaneously decay.
Types of Radioactive Decay
All elements with 84 or more protons (Polonium) are unstable, and will undergo radioactive decay.
Naturally occurring radioactive isotopes decay in four primary ways: Alpha particle emission Beta particle emission Gamma radiation emission Positron emission
Alpha Emission Alpha particle (– is two protons and two neutrons bound together and emitted from the nucleus.
They are helium nuclei with a charge of +2.
It has no electrons.
Represented by the symbol: 24He or 2
4Restricted to heavy elements: Ex. uranium
Alpha Emission
Process which is effective to lose a lot of mass form the element.
Example:
84210Po 82
206Pb + 24
The atomic number decreases by two (a new element) and the mass number decreases by four.
Alpha Emission
92
235U 90231Th + 2
4or 24He
Quick way for a large atom to lose a lot of nucleons
Beta Emission Beta particle – is essentially an electron that’s emitted from the nucleus: -1
0
In the nucleus, a neutron is converted (decayed) into a proton and an electron. 0
1n 11p + -1
0
The electron is emitted as a beta particle.
Represented by the symbol: -10e or -1
0
A good way to decrease the number of neutrons.
Beta Emission
By decreasing the number of neutrons you improve the neutron/proton ratio.
Example:
53131I 54
131Xe + -10
The mass number stays the same in going from I-131 to Xe-131 but the atomic number increases by 1. Loss of neutron!
Beta Emission
• 1940K 20
40Ca + -10
• Identity of atom changes
Gamma Emission Gamma rays – are high energy electromagnetic waves emitted from the nucleus.
There is no mass change with gamma emission, only radiation.
Usually occurs immediately following other types of decay.
Not shown in a balanced nuclear reaction.
Gamma Emission
• An example is cobalt-60 (Co-60) which gives off a large amount of gamma radiation.• Co-60 used in the radiation treatment of cancer.
Positron Emission Positron particle – is essentially an electron that has a positive charge.
A proton can be converted into a neutron by emitting a positron 1
1p 01n + +1
0
A useful way to decrease the number of protons.
Represented by the symbol: +10e or +1
0
Example:
1938K 18
38Ar + +10
Positron Emission
Notice that the atomic number decreases by one but the mass number stays the same.
Can be viewed as the opposite of beta emission.
Half-Life
Half-life - the amount of time it takes for one-half of a radioactive sample to decay is called the half-life of the isotope
It is given the symbol: t1/2
No two radioactive isotopes decay at the same rate
Half-LifeUseful application of half-life is radioactive dating. Used to determine the age of things.
Carbon-14 (t1/2 – 5715 years) dating can be used to determine the age of something that was once alive.
Examples include animal and plant species.
Cannot be used to determine the age of rocks.
Half-Life
Radium-226 has a half-life of 1599 years.
Half of a given amount of radium-226 decays in 1599 years.
In another 1599 years, half of the remaining radium-226 will decay.
This will continue until there is a negligible amount of radium-226 remaining.
Half-Life
Decay of radium-226
Half-Life
• Each radioactive element has its own half-life.
• More stable elements decay slowly and have longer half-lives.
• Less stable elements decay quicker and have shorter half-lives
Radioactive Decay Rates
Half-Life
The time required for half of a sample to decay
Half-Life
Number ofHalf-Lives
Fraction ofInitial Amount
Remaining
AmountRemaining (mg)
0 1 20.00 (initial)1 1/2 10.002 1/4 5.003 1/8 2.504 1/16 1.255 1/32 0.625
Half-LifeProblem:
Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope.
First determine the number of half-lives that have elapsed!
Half-LifeAnswer:
First determine the number of half-lives that have elapsed!
57.2 days / 14.3 days = 4 half lives
4.0 mg 2.0 1.0 0.5 0.25 mg 1 2 3 4
Classwork
Page 709Problems 1, 3, 4 and 6
Review HW
Page 724Problems: 31, 33 and 40
Homework
Page 724
Problems 36 - 39
Decay Series
• Some nuclides (particularly those Z>83) cannot attain a stable, nonradioactive nucleus by a single emission.
• The product of such an emission is itself radioactive and will undergo a further decay process.
• Heavy nuclei may undergo a whole decay series of nuclear disintegrations before reaching a nonradioactive product.
Trying To Reach Nuclear Stability
Decay Series – a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached.
The heaviest nuclide of each series is called the parent nuclide.
The nuclides produced are called daughter nuclides.
The Four Known Decay Series
ParentRadioisotope
# of DecaySteps
Final Productof Series
Uranium-238 14 Lead-206
Thorium-232 10 Lead-208
Uranium-235 11 Lead-207
Plutonium-241 13 Bismuth-209
Artificial Transmutations Transmutation – a change in the identity of a
nucleus (element) as a result of a change in the number of its protons.
Artificial radioactive nuclides are radioactive nuclides not found naturally on Earth.
Artificial Transmutation – bombardment of stable nuclei with charged and uncharged particles (protons, alpha particles and neutrons)
Artificial Transmutations Great quantities of energy are required
to bombard nuclei with these particles.
These reactions are usually done by accelerating the particles in a high magnetic or electrical field.
The instrument used is a particle accelerator.
Artificial Transmutations
Used to produce all the elements in the periodic table with more than 92 protons in their nucleus.
Also used to produce technetium (43 protons), astatine (85), francium (87) and promethium (61).
Examples of elements created by artificial transmutation
Section 22-4
Nuclear Fissionand
Nuclear Fusion
Objectives
1. Define the terms nuclear fission, chain reaction and nuclear fusion.
2. Explain how a fission reaction is used to generate power.
3. Discuss fusion reactions.
Nuclear Fission
Nuclear Fission – a very heavy nucleus splits into more stable nuclei of intermediate mass.
This process releases an enormous amount of energy.
Can occur spontaneously or when nuclei are bombarded with particles.
Nuclear FissionExample: Uranium-235
When uranium-235 is bombarded with neutrons, the uranium nuclei becomes unstable.
The nucleus splits into medium-mass parts with the emission of more neutrons.
The mass of the products is less than the mass of the reactants. The missing mass is converted to energy.
92235U + 0
1n 56142Ba + 36
91Kr + 3 01n
The product are barium, krypton and
3 more neutrons.
The reaction is balanced.
Example: Uranium-235
Nuclear FissionWhen fission of an atom bombarded by neutrons produces more neutrons, a chain reaction can occur.
Chain Reaction – a reaction in which material that starts the reaction is also one of the products and can start another reaction.
In the last slide neutrons are generated from the initial reaction and can start a second reaction.
The chain reaction continues until all of the uranium-235 atoms have split or until the neutrons fail to strike uranium-235 nuclei.
Critical Mass - the minimum amount of nuclide that provides the number of neutrons needed to sustain a chain reaction.
Atomic Bombs – uncontrolled chain reactions.
Nuclear Reactors – controlled chain reactions.
Nuclear Fission
Only two fissionable isotopes are used during nuclear reactions.
Uranium-235 and plutonium-239
Uranium-238 (the more abundant isotope) only produces 1 neutron. This will not sustain a chain reaction.
92238U + 0
1n 56145Ba + 36
93Kr + 1 01n
Atomic Bombs
In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is not going to explode.
When time comes for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass.
The resulting chain reaction is uncontrolled, releasing a tremendous amount of energy.
Nuclear Power Plants
The secret to controlling a chain reaction is to control the neutrons.
If the neutrons can be controlled, then the energy produced can be controlled.
That is what takes place with nuclear power plants.
Nuclear Power Plants
In many respects, a nuclear power plant is similar to a conventional power plant (coal, oil, natural gas).
In these power plants the fuel is burned and the heat produced is used to boil water, which in turn is used to make steam.
The steam is then used to turn a turbine that’s attached to a generator that produces electricity.
Nuclear Power Plants
In a nuclear power plant the heat is produced through a fission reaction.
The fissionable isotope (uranium-235) is contained in fuel rods in the reactor core.
Control rods made of boron or cadmium (not fissionable) are also in the reactor core.
Nuclear Power PlantsThe control rods act like neutron sponges to control the rate of radioactive decay of the uranium-235.
Operators can stop a chain reaction by pushing all the control rods into the reactor core, where they absorb all the neutrons.
Operators can pull out the control rods slowly to produce the desired amount of heat.
Nuclear Power PlantsWater is circulated through the reactor core, and the heat generated by the fission reaction is absorbed
The water flows to a steam generator where steam is produced.
The steam then is used to turn the turbine that generates electricity.
The water is recycled in a closed system.
Nuclear Reactor
Nuclear FusionNuclear Fusion – light-mass nuclei combine to form a heavier, more stable nucleus.
Nuclear fusion is essentially the opposite of nuclear fission.
Nuclear fusion releases more energy than nuclear fission.
Nuclear Fusion
Nuclear fusion reactions are what power the sun for light and heat.
4 11H 2
4He + 2 10
Uncontrolled fusion reactions of hydrogen are the source of energy for the hydrogen bomb.
A fission reaction is used to provide the heat and pressure necessary for the fusion reaction.
A hydrogen bomb is about 1,000 times more powerful than an atomic bomb.
Nuclear Fusion
Nuclear Fusion
Nuclear fusion: the goal of scientists has been to control a fusion reaction.
Fusion reactions can provide an unlimited supply of energy that has no nuclear waste.
The product from a fusion reaction is helium, already found in our atmosphere.
Nuclear Fusion
Three major obstacles need to be solved:
1)Temperature - The estimated temperature required to start the reaction is 40,000,000K.
2)Time – nuclei must be held together long enough.
3)Containment – At the required temperature all known material would vaporize.
Classwork
1) Distinguish between nuclear fission and fusion
2) Define chain reaction
3) Explain how fusion can be a good source of energy
Review Homework
Page 724
Problems: 36 - 39
End of Chapter
Radiation Energetics
Alpha Particles relatively heavy and doubly charged lose energy quickly in matter
Beta Particles much smaller and singly charged interact more slowly with matter
Gamma Rays & X-rays high energy more lengthy interaction with matter
Hazards of Radiation Types
§ Alpha Emissions easily shielded considered hazardous if alpha emitting material is
ingested or inhaled
§ Beta Emissions shielded by thin layers of material considered hazardous if a beta emitter is ingested
or inhaled
§ Gamma Emissions need dense material for shielding considered hazardous when external to the body
The Radon Story
Radon-222§ Originates from U-238 which occurs naturally in most
types of granite
§ Radon-222 has a half-life of 3.825 days
§ It decays via alpha emissions
§ This isotope is a particular problem because it is a gas which can leave the surrounding rock and enter buildings with the atmospheric air
