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Historical Development of Atomic Theory and the Structure of the Atom

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     Judy Vondruska
       PAN Workshop
        July 18, 2010
                    (460-370 BC)

   Greek philosopher
   First to propose
    existence of smallest
   “Atomos” used to
    describe particle
                             (384-322 BC)

   Greek philosopher
   Believed in the four
    elements of air, earth,
    water and fire
   Felt that regardless of the
    number of times you cut a
    form of matter in half, you
    would always have a
    smaller piece of that
       this view held for 2000
        years primarily because
        Aristotle was the tutor of
        Alexander the Great.
Model of Atom
Discoveries/theories without
understanding the nature of the atom…
        Johann Becher (1635-1682) and
           Georg Stahl (1660-1734)
   Developed the Phlogiston theory which
    dominated chemistry between 1670 and 1790.
       allflammable materials contain phlogiston, a substance
        without color, odor, taste, or mass that is liberated in
        burning. Once burned, the "dephlogisticated" substance
        was held to be in its "true" form, the calx.
             problem with the theory was that burning of metals resulted in
              an increase in the mass. This problem was solved by
              assigning negative mass to phlogiston.
                     Antoine Lavoisier

   known as the “Father of Modern Chemistry”
   Proposed the Law of Conversation of Mass
    which represents the beginning of modern

     Sidebar: was associated with a tax-collecting firm and was married to
     the daughter of the one of the firm's executives. This relationship with
     the tax firm led to Lavoisier's beheading at the guillotine in 1794.
                 Joseph Proust

   Proposed the Law of
    Constant Composition in
   the composition of a
    substance is always the
    same, regardless of how the
    substance was made or
    where the substance is found
          are always 2 atoms of
     there
     hydrogen and 1 atom of
     oxygen in a molecule of water.
                  John Dalton

   Developed the concept of the mole and
    proposed a system of symbols to represent
    atoms of different elements
   Proposed the Law of Multiple Proportions.
     when elements combine, they do so in the ratio of
     small whole numbers. For example carbon and
     oxygen react to form CO or CO2, but not CO1.8
             Joseph Gay-Lussac
                      ( 1778-1850)

   Announced Law of Combining Volumes in
     the relative volumes of gases in a chemical
      reaction are present in the ratio of small integers
      (assuming all gases are at the same temperature
      and pressure).
     showed that at the same temperature and
      pressure, two volumes of hydrogen gas reacted
      with one volume of oxygen gas to produce two
      volumes of water (as a gas).
              Amadeo Avogadro

   Proposed what is now known as Avogadro's
    Hypothesis in 1811
     at the same temperature and pressure, equal
      volumes of gases contain the same number of
      molecules or atoms
     when combined with Gay-Lussac's Law of
      Combining Volumes, the only possible formulas
      for hydrogen, oxygen and water are H2, O2 and
      H2O, respectively
     hypothesis not widely accepted for another 50 yrs
              Dimitri Mendeleev

   Proposed the periodic law
    and developed the first
    periodic table in 1869.
     Medeleev's   table was
     arranged according to
     increasing atomic weight and
     left holes for elements that
     were yet to be discovered
The changing view of the
                        J. J. Thomson
   Identified the negatively charged electron in the cathode ray
    tube in 1897
       deduced that the electron was a component of all matter and
        calculated the charge to mass ratio for the electron.
                 e/m = -1.76 x 108 coulombs/g
   Thomson and others also studied the positive rays in the
    cathode ray tube and discovered that the charge to mass ratio
    depended on filling gas in the tube. The largest charge to
    mass ratio (smallest mass) occurred when hydrogen was the
    filling gas. This particle was later identified as the proton.
               e/m = +9.58 x 104 coulombs/g
   Proposed the "plum pudding" model of the atom.
       the volume of the atom is composed primarily of the more
        massive (thus larger) positive portion (the plum pudding). The
        smaller electrons (actually, raisins in the plum pudding ) are
        dispersed throughout the positive mass to maintain charge
New Model of Atom
                 Robert Millikan

   Determined the unit charge of the electron in
    1909 with his oil drop experiment at the
    University of Chicago. Thus allowing for the
    calculation of the mass of the electron and
    the positively charged atoms.
              e = -1.60 x 10-19 coulombs
              Wilhelm Roentgen

   Attention was drawn to a glowing fluorescent
    screen on a nearby table
     determined   that the fluorescence was caused by
      invisible rays originating from the partially
      evacuated glass Hittorf-Crookes tube he was
      using to study cathode rays (i.e., electrons)
     these rays penetrated the opaque black paper
      wrapped around the tube.
   Roentgen had discovered X rays (1895)
           Antoine Henri Becquerel

   Observed thast fluorescent materials like
    potassium uranyl sulfate, K2UO2(SO4)2 emitted
    mysteries rays like those described by
     Found  that uranium emitted radiation without an
      external source of energy such as the sun.
   Becquerel had discovered radioactivity, the
    spontaneous emission of radiation by a
   Demonstrated that the radiation emitted by
    uranium shared certain characteristics with X
    rays but, unlike X rays, could be deflected by a
                 Pierre Curie (1859-1906)
                 Marie Curie (1867-1934)
   Marie coined the term radioactivity
   After chemical extraction of uranium from the
    ore, Marie noted the residual material to be
    more "active" than the pure uranium
     She concluded that the ore contained, in addition
     to uranium, new elements that were also
     radioactive. This led to their discoveries of the
     elements of polonium and radium

    Sidebar: After being killed crossing the street, Pierre's teaching
    position at the Sorbonne was given to Marie. Never before had a
    woman taught there in its 650 year history!
                  Henry Moseley

   Discovered that the energy of x-rays emitted
    by the elements increased in a linear fashion
    with each successive element in the periodic
     proposed  that the relationship was a function of
     the positive charge on the nucleus
       this rearranged the periodic table by using the atomic
        number instead of atomic mass to represent the
        progression of the elements
       this new table left additional holes for elements that
        would soon be discovered
                   Ernst Rutherford

   Proposed the nuclear atom as the result of the
    gold-foil experiment in 1911
     (Activity)

     proposed   that all of the positive charge and all of
      the mass of the atom occupied a small volume at
      the center of the atom and that most of the
      volume of the atom was empty space occupied by
      the electrons
       Developed   planetary orbit model of atom
New Model of Atom
                      Max Planck

   Made observations of the radiation of heated
   Showed (from purely formal / mathematical
    foundations) that light must be emitted and
    absorbed in discrete amounts (quanta) if it was to
    correctly describe observed
     Prior to then light had been considered as a continuous
      electromagnetic wave, thus the discrete nature of light
      was completely unexpected
     the energy of each quantum is equal to the frequency of
      the radiation multiplied by the universal constant: E=f*h,
      where h is 6.63 x 10-34 Js.
                    Niels Bohr
   Proposed improvement to Rutherford atomic
        this reason, the planetary model of the atom is
     for
      sometimes called the Rutherford-Bohr model.
   Bohr added the idea of fixed orbits, or energy
    levels for the electron traveling around the
     hismodel allowed for the idea that electrons can
      become "excited" and move to higher energy
      levels for brief periods of time
         Problem with Rutherford-Bohr
 The attraction of the electron to the
  nucleus would cause it to spiral into the
 According to the electromagnetic theory, if
  a charged particle were accelerated
  around another charged particle then there
  would be a continuous radiation of energy
       the loss of energy would slow down the speed of the
        electron and eventually the electron would fall into the
       but such a collapse does not occur. Rutherford's model
        was unable to explain why there was no collapse
                 Francis Aston

   Invented the mass spectrograph in 1920
   First person to observe isotopes.
     observedthat there were three different kinds of
     hydrogen atoms. While most of the atoms had a
     mass number of 1, he also observed hydrogen
     atoms with mass numbers of 2 and 3.
   Modern atomic masses are based on mass
    spectral analysis
   His work led Rutherford to predict the
    existence of the neutron
        or Na-23                 or Na-24

Isotope: one of two or more atoms having the
          same number of protons but
          different numbers of neutrons
                        James Chadwick

   Was a collaborator of Rutherford's
   Discovered the neutron in 1932
       research was showing that the nucleus of an atom contains more
        than just protons. This was because the atomic mass of the atom
        of most elements was greater than the atomic number (number of
           Electrons contributed nothing to the atomic weight
       chose to bombard boron with alpha particles and analyze the
        interaction of the neutral particles with nitrogen. These particular
        targets were chosen partly because the masses of boron and
        nitrogen were well known.
           conservation of energy was used to determine the mass of the
   Scientists realized neutrons could be used to probe atomic
       lead to bombardment of Uranium with neutrons (Fermi)
       observation of nuclear fission
New Model of Atom
        Erwin Schrödinger (1887 – 1961)
        Werner Heisenberg (1901-1976)
   Schrodinger combined some particle behavior
    with wave behavior as suggested by deBroglie
    and formulated mathematical model for hydrogen
       Derived equation which gives the probability of finding
        the electron at some point in a 3-dimensional space at
        any given instant; gives no information about the path
        the electron follows
           solutions from equation yield information about probability
            maps or shapes on different energy levels
   Develop the wave-particle duality of subatomic
   Electrons exist in different places at different
    points in time, but it is impossible to say where the
    electron will be at a given time
                  New Model of Atom
   Electrons do not travel in definite paths around the nucleus
   The exact path or position of moving electron cannot be predicted or
       Rather, there are regions inside the atom were electrons are likely to be
    Electron clouds – Regions inside an atom where electrons are likely
    to be found.

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