The Atom and the Quanta

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The Atom and the Quanta

• Models of Atoms
• Niels Bohr constructed a model of an atom where
  the electrons circle the nucleus
• Newton constructed the particle model of light.
  He believed that light was composed of many tiny
  particles
• Huygens belief that light was a wave phenomenon
  was proven by Maxwell and Hertzs electromagnetic
  wave model

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Light Quanta

• Einstein believed that light was made up of
  quanta energy, or little particles called photons
• A photons energy is proportional to its
  frequency
• Energy and frequency of a photon is related by
  Plancks constant h in the equation E h f
• h 6.626 x 10-34 Js

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The Photoelectric Effect

• Photoelectric effect is supported by the particle
  theory of light. When light falls upon a
  surface, electrons can be dislodged
• If the frequency of the photons is too low, then
  emission of electrons wont happen, no matter
  what intensity of light is present

4
Matter Has Wave-like Properties

• De Broglie came up with idea that all matter has
  wave properties
• h
• wave length (l) momentum
• Electrons have a detectable wavelength that is
  smaller than light, can be diffracted and
  undergoes wave interferences under the same
  conditions as light

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Electron Waves

• Niels Bohrs planetary model helped explain the
  atomic spectra of the elements
• Spectral lines are cause by electron transitions
  between energy levels
• Different energy levels result in different
  orbits for the electrons
• Each element has an unique electron wave length
  and thus only certain electron energy levels were
  possible
• Electron wave lengths increase for orbitals of
  increasing radii

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Relative Size of Atoms

• 1. The radius of the atoms of each element
  is unique to
  that element
• 2. As nuclear charge increases, additional
  electrons are added to the outer orbits.
  As more outer orbits are added, the inner
  orbits shrink to keep the size of the
  elements relatively similar
• 3. Ionization energy - the amount of
  energy needed to knock an electron out of an
  atom completely

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Quantum Physics

• Quantum mechanics - the study of motion in the
  micro world
• Newtonian laws for large object do not apply for
  objects in the micro world
• Subatomic measurements there are many fundamental
  uncertainties. ex(the position and momentum of
  an electron)

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Radioactivity and Nuclear Energy

• Nucleons
• Protons-- determine atom identity, have charge
• Neutrons--add mass and stability, neutral charge
• Both are thought to be made of smaller particles
  quarks
• a. Proton 2 up 1 down quark 1 charge
• b. Neutron 2 down 1 up quark 0 charge
• Nucleons held together by strong nuclear force
• a. Not strong at distance, only in contact
• b. Neutrons necessary to provide enough strong
  force to overcome proton repulsion
• c. More protons present, more neutrons needed to
  balance
• d. Above 83 protons, no possibility to balance,
  atoms unstable

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Binding Energy

• . . . is the energy required to decompose the
  nucleus into its components.
• has a binding energy
• per nucleon of 8.79 MeV
• Iron-56 is the most stable nucleus.

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Radioactivity

• Neutrons are unstable by themselves, break up to
  a proton electron, stable only when near
  protons
• Breakdown of neutrons or atoms is radioactivity
• Forms of radioactive decay
• Alpha--He nucleus, 2 protons 2 neutrons
• Beta-- Electron from decay of a neutron
• Gamma-- High energy photon wave released when
  a or b is ejected
• Penetration
• Alpha--stopped by paper or clothes, but very
  damaging if gets internal
• Beta--stopped by Al foil, can damage skin if in
  contact
• Gamma--goes through many materials heavy metals
  like Pb can stop it because it has many
  electrons to resonate with g

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Types of Radioactive Decay
4

• alpha production (?) helium nucleus
• beta production (?)

He
2
0
e
?
1
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Types of Radioactive Decay

• gamma ray production (?)
• positron production
• electron capture (inner-orbital electron is
  captured by the nucleus)

0
e
1
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Radioisotopes

• Stable atoms are not radioactive
• Radioisotope has more or less neutrons than would
  be stable for an element
• Example is tritium , H-3, hydrogen with 2
  neutrons--1 neutron breaks making an extra
  proton, turning it to He-3

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Decay Series

• A radioactive nucleus reaches a stable state by a
  series of steps.

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Half-life

• Time for one-half of a radioisotope to decay and
  change into another atom
• Transmutation--the result of radioactivity is
  changing of one element into another
• This can be done artificially, which is how
  elements beyond U were formed

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Rate of Decay

• rate kN
• The rate of decay is proportional to the number
  of nuclides. This represents a first-order
  process.

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Half-Life

• . . . the time required for the number of
  nuclides to reach half the original value (No/2).

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Half Life Calculations and Remaining Amounts
X-- amount left after time t Xo -- amount
present at time 0 ln X -kt Xo
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Nuclear Transformation

• The change of one element into
• another.

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Radioactive Dating

• Carbon dating depends upon a continuous supply of
  C-14 from the atmosphere
• As long as organism is alive, it has constant
  amount of C-14. After it dies, the C-14 decays,
  and its disappearance can act as a clock to tell
  how long ago it died. Not reliable for organisms
  in water or under water shield
• Uranium dating depends on when the rock
  solidified, and how much end material was present
  then.
• Not measurable accurately--rocks 200 years old
  were claimed to be 200 million. Other rock
  dating methods have given widely different
  values, even less than 10,000 years

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Energy and Mass

• When a system gains or loses energy it also gains
  or loses a quantity of mass.
• ?E ?mc2
• ?m mass defect
• ?E change in energy
• If ?E ? (exothermic), mass is lost from the
  system.

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Mass-Energy Equivalence

• Energy in nuclear reactions comes from conversion
  of matter to energy
• Einstein recognized this in his equation E
  mc2
• Masses of nucleons are greater outside a nucleus
  than in it--some of mass becomes binding energy
• Binding energy is released in both fission and
  fusion reactions

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Nuclear Fission and Fusion

• Fusion Combining two light nuclei to form a
  heavier, more stable nucleus.
• Fission Splitting a heavy nucleus into two
  nuclei with smaller mass numbers.

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Nuclear Energy

• Nuclear Fission
• Splitting of heavy atomic nucleus--electromagneti
  c force overwhelms nuclear forces
• Requires activation energy--usually the force of
  a colliding neutron
• If split releases additional neutrons, the
  fission can repeat indefinitely in a chain
  reaction
• If enough fissionable material is present,
  (critical mass), chain reaction can result in
  tremendous release of heat energy in a nuclear
  explosion
• Chain reactions do not occur in nature due to
  small quantities of fissionable atoms which are
  surrounded by neutron absorbers

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Fission Processes
A self-sustaining fission process is called a
chain reaction.
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Nuclear Reactors

• Captures heat from fission reaction by
  controlling its rate, heat used to make steam for
  electricity generating
• Control accomplished with neutron absorbing
  cadmium or boron steel rods
• Advantages--much power from very little fuel, no
  pollution of air or water
• Disadvantages--possible meltdown if uncontrolled,
  nuclear wastes of long half life--radiation
  hazard, nuclear proliferation by unstable
  governments
• Fissionable elements-- U-235, Pu-239--both easily
  fissionable
• Breeder reactor--creates more nuclear fuel by
  allowing U-238 to be bombarded by neutrons,
  creating Pu-239

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Key Parts of a Fission Reactor

• Reactor Core 3 moderator and
  control rods.
• Coolant
• Containment Shell

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Breeder Reactors

• Fissionable fuel is produced while the reactor
  runs ( is split, giving neutrons for the
  creation of )

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Nuclear Fusion

• 1. Energy released when two small nuclei collide
  and fuse
• 2. Often called thermonuclear reaction due to
  very high activation energy needed
• 3. Likely source of energy for stars, which have
  gravity to hold particles in and high
  temperatures 15 million oC
• 4. Controlling fusion is goal of nuclear
  physicists today--requires 350 million oC to be
  self sustaining

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Fusion Reactors

• Present fusion reactions are not self sustaining
  due to instabilities in the plasma needed for
  reaction
• Containing the hot plasma also a problem--
  magnetic fields are used to hold it
• Many other schemes to attain fusion have been
  postulated
• a. Laser fired--high energy lasers zap fuel
  pellets
• b. Muon-shielded H fusion--muons are heavy
  electrons with very short half life
• Fusion is promising as energy source if it can be
  harnessed
• a. No pollution or nuclear waste
• b. Extremely abundant source of fuel--H from water

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Biological Effects of Radiation

• . . . depend on
• 1. Energy of the radiation
• 2. Penetration ability of the radiation
• 3. Ionizing ability of the radiation
• 4. Chemical properties of the radiation source

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Radiation and Man

• Radioactive tracers--some elements can be made
  artificially radioactive and then injected into
  living organisms to trace metabolic pathways
• Exposure to radioactivity--most is from natural
  sources
• Stars, earth minerals--56
• X-rays from medical/dental--42
• Less than 3 from man-made sources

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Homework for Quantum and Nuclear Physics

• Chapt 27p913ff 5, 7, 13, 16, 37, 39
• Chapt 28p949ff 1, 13, 17
• Chapt 29p986ff 17, 19, 25, 26, 35, 36
• Chapt 30p1026ff 1, 3, 10
• YES, THIS IS Serway/Faughn!!!
• Alsoget and do the Atomic Phyz MC XCR test