Intro to Nuclear Chemistry

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Intro to Nuclear Chemistry

• DECEMBER 12

http//www.chem.orst.edu/graduate/pics/Reactor.jpg

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How does a nuclear reactor work?
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g
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How does a small mass contained in this bomb
cause

• Nuclear Bomb of 1945 known as fat man

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this huge nuclear explosion?
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Is there radon in your basement?
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Notation

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Nucleons

• Protons and Neutrons

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• The nucleons are bound together by a strong force
  called binding force.

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Isotopes

• Atoms of a given element with
• same protons
• but
• different neutrons

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• H H H

http//education.jlab.org/glossary/isotope.html
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Isotopes of Carbon
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Radioactive Isotopes

• Isotopes of certain unstable elements that
  spontaneously emit particles and energy from the
  nucleus.
• Henri Beckerel 1896 accidentally observed
  radioactivity of uranium salts that were fogging
  photographic film.
• His associates were Marie and Pierre Curie.

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Marie Curie born 1867, in Poland as Maria
Sklodowska

• Lived in France
• 1898 discovered the elements polonium and radium.

http//www.radiochemistry.org/nuclearmedicine/pion
eers/images/mariecurie.jpg
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Marie Curie a Pioneer of Radioactivity

• Winner of 1903 Nobel Prize for Physics with Henri
  Becquerel and her husband, Pierre Curie.
• Winner of the sole 1911 Nobel Prize for
  Chemistry.

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RADIOACTIVITY

• Emission of rays and particles from unstable
  nuclei.
• When a nucleus is emitting rays or particles it
  is said that is DECAYING or is disintegrating.

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Stability of nuclei

• Depend on the ratio between the neutrons and
  protons. Too many or too few neutrons lead to an
  unstable nucleus. All elements with more than 83
  protons are unstable.

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Transmutation

• When the nucleus of one element is changed into
  the nucleus of another element. IT CAN ONLY
  HAPPEN IN A NUCLEAR REACTION!!!

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Nuclear Reactions

• The chemical properties of the nucleus are
  independent of the state of chemical combination
  of the atom.
• In writing nuclear equations we are not concerned
  with the chemical form of the atom in which the
  nucleus resides.
• It makes no difference if the atom is as an
  element or a compound.
• Mass and charges MUST BE BALANCED!!!

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Types ofRadioactive Decay
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SeparationAlphaBetaGamma.MOV Separation of
Radiation
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Alpha Decay

• Emission of alpha particles a
• helium nuclei
• two protons and two neutrons
• charge 2e 
• can travel a few inches through air
• can be stopped by a sheet of paper, clothing.

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Alpha Decay
Uranium Thorium
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Alpha Decay
http//education.jlab.org/glossary/alphadecay.gif
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Alpha Decay

• Loss of an ?-particle (a helium nucleus)

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Alpha Decay

• Mass changes by 4
• The remaining fragment has 2 less protons
• Alpha radiation is the less penetrating of all
  the nuclear radiation (it is the most massive
  one!)

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Alpha decay

• When a nucleus emits alpha particles.
• Atomic number decreases by 2.
• Mass number decreases by 4.
• Neutrons decrease by 2.

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Beta Decay

• Loss of a ?-particle (a high energy electron)

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Beta Decay

• Beta particles b electrons ejected from the
  nucleus when neutrons decay
• ( n -gt p b- )
• Beta particles have the same charge and mass as
  "normal" electrons.

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Beta Decay

• Beta particles b electrons ejected from the
  nucleus when neutrons decay
• n -gt p b-
• Beta particles have the same charge and mass as
  "normal" electrons.
• Can be stopped by aluminum foil or a block of
  wood.

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Beta Decay

• When a neutron becomes a proton and emits an
  electron.
• Atomic Number or number of protons increases by
  1
• Number of neutrons decreases by one.
• Mass number remains the same.
•  

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Beta Decay
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Beta Decay
Thorium
Protactinium
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Beta Decay

• Involves the conversion of a neutron in the
  nucleus into a proton and an electron.
• Beta radiation has high energies, can travel up
  to 300 cm in air.
• Can penetrate the skin

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Beta decay

• Write the reaction of decay for C-14

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Positron Emission

• When a proton changes to a neutron emits a
  positron.
• Atomic number (number of protons)decreases by 1
• Number of neutrons increase by 1.
• Mass number remains same

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Gamma Emission

• Loss of a ?-ray (high-energy radiation that
  almost always accompanies the loss of a nuclear
  particle)

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Gamma Decay

• Gamma radiation g electromagnetic energy that
  is released. 
• Gamma rays are electromagnetic waves.
• They have no mass.
• Gamma radiation has no charge.
• Most Penetrating, can be stopped by 1m thick
  concrete or a several cm thick sheet of lead.

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3 Main Types of Radioactive Decay

• Alpha a
• Beta b
• Gamma g

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Examples of Radioactive Decay

• Alpha Decay
• Po ? Pb He
• Beta Decay p? n e
• n ? p e
• C ? N e
• Gamma Decay
• Ni ? Ni g
• (excited nucleus)

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Which is more penetrating? Why?
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December 14

• Nuclear stability
• Half life
• HW review book
• Question 34 to 47

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Nuclear Stability

• Depends on the neutron to proton ratio.

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Band of Stability
Number of Neutrons, (N)
Number of Protons (Z)
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What happens to an unstable nucleus?

• They will undergo decay
• The type of decay depends on the reason for the
  instability

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What type of decay will happen if the nucleus
contains too many neutrons?

• Beta Decay

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Example
14
0
14

• C ? N e
• In N-14 the ratio of neutrons to protons is 11

-1
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7
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• Nuclei with atomic number gt 83 are radioactive

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Radioactive Half-Life (t1/2 )

• The time required for one half of the nuclei in a
  given sample to decay.
• After each half life the mass of sample remaining
  is half.
• Different Isotopes have different half lives. Use
  table N

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Common Radioactive Isotopes
Isotope Half-Life Radiation
Emitted Carbon-14 5,730 years b,
g Radon-222 3.8 days a Uranium-235 7.0 x
108 years a, g Uranium-238 4.46 x 109 years
a
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Radioactive Half-Life

• After one half life there is 1/2 of original
  sample left.
• After two half-lives, there will be
• 1/2 of the 1/2 1/4 the original sample.

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Graph of Amount of Remaining Nuclei vs Time
AAoe-lt
A
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Example

• You have 100 g of radioactive C-14. The half-life
  of C-14 is 5730 years.
• How many grams are left after one half-life?
  Answer50 g
• How many grams are left after two half-lives?

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Problem

• If 80 g of a radioactive sample decays to 10 g in
  30 min what is the elements half life?

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• How many days will take a sample of I-131 to
  undergo three half life periods?

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• What is the total mass of Rn-222 remaining in an
  original mass 160 mg sample of Rn-222 after 19.1
  days?

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Measuring Radioactivity

• One can use a device like this Geiger counter to
  measure the amount of activity present in a
  radioactive sample.
• The ionizing radiation creates ions, which
  conduct a current that is detected by the
  instrument.

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Transmutations

• To change one element into another.
• Only possible in nuclear reactions never in a
  chemical reaction.
• In order to modify the nucleus huge amount of
  energy are involved.
• These reactions are carried in particle
  accelerators or in nuclear reactors

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Nuclear transmutations

• Alpha particles have to move very fast to
  overcame electrostatic repulsions between them
  and the nucleus.
• Particle accelerators or smashers are used. They
  use magnetic fields to accelerate the particles.

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Particle Accelerators(only for charged
particles!)

• These particle accelerators are enormous, having
  circular tracks with radii that are miles long.

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Cyclotron

• Nuclear transformations can be induced by
  accelerating a particle and colliding it with the
  nuclide.

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Neutrons

• Can not be accelerated. They do not need it
  either (no charge!).
• Neutrons are products of natural decay, natural
  radioactive materials or are expelled of an
  artificial transmutation.
• Some neutron capture reactions are carried out in
  nuclear reactors where nuclei can be bombarded
  with neutrons.

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Mass defect

• The mass of the nucleus is always smaller than
  the masses of the individual particles added up.
• The difference is the mass defect.
• That small amount translate to huge amounts of
  energy ?E (?m) c2
• That energy is the Binding energy of the nucleus,
  and is the energy needed to separate the nucleus.

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Energy in Nuclear Reactions

• For example, the mass change for the decay of 1
  mol of uranium-238 is -0.0046 g.
• The change in energy, ?E, is then
• ?E (?m) c2
• ?E (-4.6 ? 10-6 kg)(3.00 ? 108 m/s)2
• ?E -4.1 ? 1011 J This amount is 50,000 times
  greater than the combustion of 1 mol of CH4

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Types of nuclear reactionsfission and fusion

• The larger the binding energies, the more stable
  the nucleus is toward decomposition.
• Heavy nuclei gain stability (and give off energy)
  if they are fragmented into smaller nuclei.
  (FISSION)

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• Even greater amounts of energy are released if
  very light nuclei are combined or fused together.
  (FUSION)

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Nuclear Fission

• Nuclear fission is the type of reaction carried
  out in nuclear reactors.

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Nuclear Fission

• Bombardment of the radioactive nuclide with a
  neutron starts the process.
• Neutrons released in the transmutation strike
  other nuclei, causing their decay and the
  production of more neutrons.

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Nuclear Fission

• This process continues in what we call a nuclear
  chain reaction.

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Controlled vs Uncontrolled nuclear reaction

• Controlled reactions inside a nuclear power
  plant
• Uncontrolled reaction nuclear bomb

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Nuclear Reactors

• In nuclear reactors the heat generated by the
  reaction is used to produce steam that turns a
  turbine connected to a generator.

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Nuclear Reactors

• The reaction is kept in check by the use of
  control rods.
• These block the paths of some neutrons, keeping
  the system from reaching a dangerous
  supercritical mass.

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FUSION

• Combining small nucleii to form a larger one.
• Require millions of K of temperature

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Fusion

• 1H 1H ? 2H 1e energy
• 1H 2H ? 3He energy
• 3He 3He ? 4He 21H energy
• Reaction that occurs in the sun
• Temperature 107 K
• Heavier elements are synthesized in hotter stars
  108 K using Carbon as fuel

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Nuclear Fusion

• Fusion would be a superior method of generating
  power.
• The good news is that the products of the
  reaction are not radioactive.
• The bad news is that in order to achieve fusion,
  the material must be in the plasma state at
  several million kelvins.

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Nuclear Fusion(thermonuclear reactions)

• Tokamak apparati like the one shown at the right
  show promise for carrying out these reactions.
• They use magnetic fields to heat the material.
• 3 million K degrees were reached inside but is
  not enough to begin fusion which requires 40
  million K

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Fission is the release of energy by splitting
heavy nuclei such as Uranium-235 and Plutonium-239
Fusion is the release of energy by combining two
light nuclei such as deuterium and tritium
D-T Fusion
4He 3.52 MeV
D
Neutron 14.1 MeV
T

• The goal of fusion research is to confine fusion
  ions at high enough temperatures and pressures,
  and for a long enough time to fuse
• This graph shows the exponential rate of progress
  over the decades

• How does a nuclear plant work?
• Each fission releases 2 or 3 neutrons
• These neutrons are slowed down with a moderator
  to initiate more fission events
• Control rods absorb neutrons to keep the chain
  reaction in check

Controlled Fission Chain Reaction
Confinement Progress
There are two main confinement approaches
The energy from the reaction drives a steam cycle
to produce electricity

• Magnetic Confinement uses strong magnetic fields
  to confine the plasma
• This is a cross-section of the proposed
  International Thermo-nuclear Experimental Reactor
  (ITER)

Nuclear Power Plant

• Nuclear Power produces no greenhouse gas
  emissions each year U.S. nuclear plants prevent
  atmospheric emissions totaling
• 5.1 million tons of sulfur dioxide
• 2.4 million tons of nitrogen oxide
• 164 million tons of carbon
• Nuclear power in 1999 was the cheapest
• source of electricity costing 1.83 c/kWh
• compared to 2.04 c/kWh from coal

• Inertial Confinement uses powerful lasers or ion
  beams to compress a pellet of fusion fuel to the
  right temperatures and pressures
• This is a schematic of the National Ignition
  Facility (NIF) being built at Lawrence Livermore
  National Lab

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Uses of radioisotopes

• Medicine
• Medical imaging trace amounts of short half
  life isotopes can be ingested and the path of the
  isotope traced by the radiation given off
• cancer treatment radiation kills cancerous
  cells more easily than healthy cells

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• Sterilisation ? rays can be used to kill
  germs and hence sterilise food and plastic
  equipment
• Industry used to trace blockages in pipes, or
  to test the thickness of materials (by putting a
  source on one side of the material and detector
  on the other)

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• Carbon dating
• Once a living organism dies, it is no longer
  taking in any Carbon.
• C14 is radioactive, and decays over time.
• By measuring the activity of C14 in an object and
  comparing it with the amount of C14 which was
  present initially you can estimate when the
  organism died

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Smoke detectors

• a radioactive source ionises the air between
  two electrodes. Thus current flows between them
• If smoke particles enter this space they stick to
  the ions and the current is reduced.
• This reduced current triggers the alarm