Chapter 8' Electrons in Atoms:

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Chapter 8. Electrons in Atoms Quantum Theory
and Atomic Structure
In this chapter we discover a colossal revolution
which took place in how scientists think about
the make up of the universe. A change in our
understanding of this magnitude, which took place
mainly in the early 1900s, will not likely be
repeated. During this period between 1890 and
1930 many brilliant minds such as Thomson, Curie,
Planck, Rutherford, Einstein, Bohr, DeBroglie,
Schroedinger and Heisenberg developed a model of
the atom which required a new physicsquantum
physicswhich explains the world of the very
small.
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Atomic Structure
Electrons
J.J. Thompsons Cathode Ray Tube Early Mass
Spectrometer (1897)
- Thompson established a ratio of the charge to
mass for cathode rays by measuring the deflection
of the charge in a magnetic field (basis of mass
spectrometry).
e magnitude of electron charge in C m mass of
electron in g
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• Thompsons Plum Pudding Model

mass of electron (1909)
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Rutherfords Gold Foil Experiment (Ernst
Rutherford and Hans Geiger, 1909)

• the majority of the a-particles passed straight
  through the foil
• some experienced slight deflections
• about 1 in every 8000 suffered serious
  deflections and even fewer bounced back in the
  direction they came

- as if you fired a 15 shell at a piece of
tissue paper and it came back and hit you
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• immediately a problem was realized with
  Rutherfords model of the atom

• if the electrons and the nucleus are to remain
  apart the kinetic energy of the electrons must
  match the potential energy of attraction between
  the electron and nucleus

• classical physics had established that a charged
  particle moving in a curved path at constant
  velocity (ie. accelerating) must emit radiation
  and therefore lose energy

• why does the electron not spiral into the
  nucleus?

• if the Rutherford model was true, subatomic
  particles violate classical physics

• what soon followed were results of experiments
  which required rethinking of the classical
  picture of energy and matter

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The Nature of Light

• what we normally think of as light is the
  visible part of the electromagnetic spectrum, the
  light which we can see

• other types of electromagnetic radiation which
  you may be familiar with are x-ray, ultraviolet,
  infrared, microwave and radiowaves.

• the classical model of electromagnetic radiation
  is that it consists of energy propagated by means
  of electric and magnetic fields that alternately
  increase and decrease in intensity as they move
  through space, the wave model of light

• the wave model of light explains the everyday
  observations of light, but it doesnt explain
  observations on the atomic scale.

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The Wave Nature of Light

• the wavelength, l, is the distance between any
  two adjacent points on a wave and has units of
  length

• the frequency, n, is the number of cycles the
  wave undergoes per second and has units of s-1 or
  Hz

• the speed that a wave travels is the product of
  its frequency and its wavelength
• the wavelength and frequency are interdependent
  because their product, the speed, is a constant
  for all electromagnetic radiation and is commonly
  known as the speed of light,

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eg. A dental hygienist uses x-rays, l 1.00 D,
to take a series of dental radiographs while the
patient listens to a radio station, l 307.5 cm
and looks out the window at the beautiful blue
sky, l 473 nm. What is the frequency of the
electromagnetic radiation from each source?
Solution. You first must be able to convert from
all the above units to m,
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• another characteristic of waves is the amplitude
  which is the height of the crest (or depth of the
  trough) of each wave.

• the amplitude is related to the intensity of the
  radiation or the strength of the electric and
  magnetic fields, a property we perceive as
  brightness

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The Electromagnetic Spectrum
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The Distinction Between Energy and Matter
Refraction
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Diffraction
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Particles
detector
detector
P12 P1 P2
1
2
Gun
wall
one hole at a time
both holes open
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Waves
detector
detector
not I1 I2
1
2
light source or water waves
wall
one hole at a time
both holes open
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• it is apparent from the properties of refraction
  and diffraction that particles and waves are
  different

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The Particle Nature of Light

• blackbody radiation and the quantization of
  energy

1000 K
1500 K brighter and more orange
2000 K even brighter and whiter, ie. all
visible light is being emitted

• blackbody radiation
• all objects emit radiation characteristic of the
  bodys temperature

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• classical physics could not explain the emission
  of light by heated solids. It predicted that the
  intensity of the radiation emitted would increase
  indefinitely at higher energy (lower wavelength).
  The ultraviolet catastrophe.

classical physics
T2 gt T1

• in 1900 Max Planck developed a formula that fit
  the data perfectly. However, he had to make a
  radical assumption which led to a new view of
  energy and a new physics.

T1
Intensity

• classical physics places no limitations on the
  amount of energy a system may possess, whereas
  quantum theory limits this energy to a discrete
  set of specific values. An object emits (or
  absorbs) radiation only in certain quantities of
  energy

ve integer, quantum
frequency
Plancks constant, 6.626 x 10-34 J s
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• when the energy of a system increases or
  decreases, it does so by a tiny jump or quantum

• because n is a positive integer, the atom can
  change its energy only by integer multiples of hv
  so the smallest energy change is,

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How did Plancks idea of quantized energy help
circumvent the Ultraviolet Caltastrophe?

• from Boltzmann, the chance of finding a molecule
  with a particular speed is related to its energy

• Planck said the energies of the substances
  oscillating to emit blackbody radiation were
  distributed according to Boltzmanns distribution
  law

frequency
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