Early Atomic Models

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Early Atomic Models

• Dalton, Thomson, Bohr, Rutherford

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Thomson Model

• JJ Thomson obviously knew about Daltons atomic
  theory when he discovered the electron and
  characterized the proton that was first observed
  by Goldstein.
• Thomson took this information and developed a
  model of the atom

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Thomson Model

• This model became known as the plum-pudding model
• This model said that the atom was a diffuse
  positive charge and the electrons were randomly
  scattered throughout the atom.

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Thomson Model

• In the picture, the blue region is the diffuse
  positive charge and the red spheres represent the
  electrons randomly placed in the atom.

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Problems with the Plum-Pudding Model

• Did not explain
• the number of protons and electrons and how this
  related to atoms of different elements
• How protons and electrons were arranged in the
  atom
• The loss or gain of electrons to form ions.

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Other known work

• There was other scientific work that was well
  known during this time.
• Much of this work was unexplained.
• One of these experiments had its beginnings from
  Isaac Newton.

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White Light Spectrum

• Isaac Newton was one of the first scientists to
  split white light into its component colors using
  a prism

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CRT Experiment

• One of the first things scientists did was to
  take the CRT and run the light through a prism
  just like Newton did with the white light.

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CRT Tube
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http//www.physchem.co.za/Atomic/Hydrogen20Spectr
um.htmHydrogen Spectrum from CRT
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Spectra of other substances

• All substances have a spectrum. More complex
  substances have more complex spectra. However,
  in the beginning, we didnt understand the most
  simple spectrum.
• It turns out that the spectra observed for
  different substances act like chemical
  fingerprints that actually reveal the identity of
  the substance.

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Spectra of 3 Pure Substanceshttp//spiff.rit.edu/
classes/phys314/lectures/spectra/spectra.html

• Hydrogen Spectrum
• Helium Spectrum
• Carbon Spectrum

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Niels Bohr

• Bohr was aware of these different spectra
• Hydrogen was postulated to be the simplest of all
  substances because it had the lightest mass.
• He concentrated on hydrogens spectrum

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Bohr Atom

• Bohr postulated that the light that was seen in
  the spectrum was due to electron transitions.
• We call this model of the atom the solar system
  model of the atom.

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Bohr Atom

• In order to explain discrete spectra, Bohr found
  that atoms obey three basic rules
• Electrons have only certain energies
  corresponding to particular distances from
  nucleus. As long as the electron is in one of
  those energy orbits, it will not lose or absorb
  any energy. The energy orbits are analogous to
  rungs on a ladder electrons can be only on rungs
  of the ladder and not in between rungs.
• The orbits closer to the nucleus have lower
  energy.
• Atoms want to be in the lowest possible energy
  state called the ground state (all electrons as
  close to the nucleus as possible).

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Hydrogen Atom
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Bohr Atom

• Applet illustrating the Bohr atom

http//lectureonline.cl.msu.edu/mmp/kap29/Bohr/ap
p.htm
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Two different modelsWhich one is right?

• Plum Pudding Model
• Thomson Model
• Diffuse positive charge
• Electrons scattered throughout diffuse positive
  charge
• Revealed little about the structure of the atom,
  or how atoms of different elements are different.

• Solar System Model
• Rutherford-Bohr Model
• All positive charge in one central location in
  the atom.
• Atomic spectra are produced from electron
  transitions between orbits.
• Predictions only worked for hydrogen.

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Rutherford Au Foil Expt

• While Thomson and Bohr were debating how atoms
  were put together, Ernest Rutherford was busy
  developing an experiment that would help to
  answer the question and reveal the structure and
  location of the subatomic particles in the atom.
• This experiment was proposed by Rutherford and
  carried out by two of his graduate students
  Geiger and Marsden.
• Became known as Rutherford Gold Foil Expt.
• Draw on board for test illustration

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http//www.sci.tamucc.edu/pals/morvant/genchem/ato
mic/page7.htm
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Two different modelsWhich one is right?

• Thomson Model
• Au foil experiment should either allow all of the
  alpha particles (He2) to pass through untouched
  (inconsistent with the nature of solidsshould be
  tightly packed positively charged spheres) or
  allow no alpha particles to pass through which
  would be consistent w/ a solid.

• Bohr Model
• Unsure of what to expect but since atom should be
  mostly empty space, expected all of alpha
  particles to pass straight through untouched.

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Rutherford Au Foil Expt Applet

• This applet illustrates what they saw.
• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford/
• Did not understand the experiment for about a
  year.
• Bohr model was consistent with the observed
  experiment.

• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford/

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Rutherford Atom

• All positive charge of atom was in nucleus
• Essentially all mass was in the nucleus

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Rutherford-Bohr Model

• The Au Foil Expt proved several things about the
  structure of the atom
• All of the positive charge of the atom resides in
  a very small part of the atomwe call this the
  nucleus (from biology).
• The nucleus occupies a very small part of the
  atoms total volume (only lt0.1 of He2 ions
  deflected by Au nuclei).
• Electrons must be located outside of the nucleus.
• Most of mass of the atom is in the nucleus.

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Complications with the Rutherford Bohr Atom

• Rutherford-Bohr atomic model worked very well for
  hydrogenit made predictions for hydrogen and the
  predictions were correct (the sign of a good
  model is that it can make correct predictions
  about as yet unknown phenomenon)
• BIG PROBLEM every other element (including
  helium!)the Bohr model could not be modified to
  make any predictions about any other element.

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• http//chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHE
  RFD.html

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Other scientific work was becoming available

• The history of the theories of light
• Wave-particle duality of nature

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The history of the theories of light

• LIGHT IS A WAVE
• 1600s Newton supported Democritus and he also
  believed light was particulate in natureneither
  opinion carried experimental evidence
• 1801 Thomas Young experimentally demonstrated
  that light exhibited wave-like characteristics
  through interference patternsthis was
  experimental proof of wave behaviorwhat was
  required to answer the question.

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How did we know light was a wave?

• Interference Patterns the definitive proof of
  wave behavior.
• Page 371 applet for circular wave interference
  patterns
• http//lectureonline.cl.msu.edu/mmp/kap13/cd371.h
  tm

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The history of the theories of light

• LIGHT IS A PARTICLE
• 1600s Newton believed light had particle
  behaviorno experimental evidence.
• 1900 Max Planck proposed the revolutionary idea
  that light had particle-like behavior based upon
  his work concerning blackbody radiation
• Blackbody radiation could only be fully explained
  with this hypothesis

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The history of the theories of light

• Photoelectric Effect this was another
  experiment that was being done in the early 1900s
  that was unexplained
• Draw on board for test illustration

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Photoelectric Effect Applet

• http//lectureonline.cl.msu.edu/mmp/kap28/PhotoEf
  fect/photo.htm

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Photoelectric Effect

• No one understood this experiment. No one had
  any explanation. This went on for about 3 years.
• Then, Einstein read Max Plancks work on
  blackbody radiation and his idea of the
  corpuscular (particle-like) behavior of light and
  used this idea to explain the photoelectric
  effect.
• 1921 Nobel Prize in Physics

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Photoelectric Effect Explained

• If light behaved like a particle, then if the
  particle had enough energy, it would knock an
  electron off of the metal surface, but only if it
  had enough energy.
• Low energy light did not eject an electronthey
  were reflected as expected.
• High energy light ejected electronsunexpected
  event unless we consider light as a particle with
  energy.
• We call these light particles photons and they
  have specific energies called quanta of energy.

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Who was right? Was light a wave or a particle?

• 1923 Luis de Broglie was aware of this renewed
  controversy
• To explore this further, he asked a rather
  unusual question.
• He asked, Since light, which has no mass,
  appears to have particle-like behavior, could
  particles that have mass behave like waves?

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Wave-Particle Duality of Nature

• The wave nature of the electron was
  experimentally confirmed in 1927 by C J Davisson,
  C H Kunsman and L H Germer in the United States
  and by G P Thomson (the son of J J Thomson) in
  Aberdeen, Scotland. De Broglie's theory of
  electron matter waves was later used by
  Schrödinger, Dirac and others to develop wave
  mechanics. (http//www-gap.dcs.st-and.ac.uk/histo
  ry/Mathematicians/Broglie.html)

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The Wave Particle Duality of Nature

• Today, we know that particles behave like waves
  and waves behave like particles.
• De Broglie developed an equation that illustrates
  this point. We will not look at this equation
  this year, but you will get it in freshman
  college chemistry and physics.
• 1929 Nobel Prize for this work he did as a
  graduate student.

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Web Sites

• http//web.phys.ksu.edu/vqm/tutorials/hydrogen/
• http//galileo.phys.virginia.edu/classes/252/Bohr_
  Atom/Bohr_Atom.html
• http//www.colorado.edu/physics/2000/index.pl
• http//www-gap.dcs.st-and.ac.uk/history/Mathemati
  cians/Broglie.html