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Historical Development of Atomic Theory and the Structure

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					 HISTORICAL DEVELOPMENT
OF ATOMIC THEORY AND THE
 STRUCTURE OF THE ATOM




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

   Greek philosopher
   First to propose existence
    of smallest particle
   “Atomos” used to describe
    particle
                               Aristotle
                              (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 matter.
       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.
       all flammable 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
                                   (1743-1794)

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




     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
                        (1754-1826)

   Proposed the Law of Constant
    Composition in 1799
   the composition of a substance is
    always the same, regardless of
    how the substance was made or
    where the substance is found
     there are always 2 atoms of
      hydrogen and 1 atom of oxygen
      in a molecule of water.
                      John Dalton
                        (1776-1844)

   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 1808.
     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
                       (1776-1856)

   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
                        (1834-1907)

   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 atom…
                             J. J. Thomson
                                (1856-1940)
   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 neutrality.
New Model of Atom
                    Robert Millikan
                        (1868-1953)

   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
                        (1845-1923)

   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
                         (1852-1908)

   Observed thast fluorescent materials like potassium
    uranyl sulfate, K2UO2(SO4)2 emitted mysteries rays
    like those described by Roentgen
     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 material.
   Demonstrated that the radiation emitted by uranium
    shared certain characteristics with X rays but, unlike
    X rays, could be deflected by a magnetic field and
    therefore must consist of charged particles.
                    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
                          (1887-1915)

   Discovered that the energy of x-rays emitted by the
    elements increased in a linear fashion with each
    successive element in the periodic table
     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
                          (1871-1937)

   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
                           (1858-1947)

   Made observations of the radiation of heated materials
   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
                       (1913-1963)
   Proposed improvement to Rutherford atomic model
     forthis reason, the planetary model of the atom is
      sometimes called the Rutherford-Bohr model.
   Bohr added the idea of fixed orbits, or energy
    levels for the electron traveling around the nucleus
     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 Model
 The attraction of the electron to the nucleus
  would cause it to spiral into the nucleus
 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 nucleus
       but such a collapse does not occur. Rutherford's model was
        unable to explain why there was no collapse
                    Francis Aston
                      (1877-1945)

   Invented the mass spectrograph in 1920
   First person to observe isotopes.
     observed that 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
                                     (1891-1974)

   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 protons).
           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 neutron
   Scientists realized neutrons could be used to probe atomic nuclei
       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 atom
       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
    particles
   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
    determined
       Rather, there are regions inside the atom were electrons are likely to be found.
    Electron clouds – Regions inside an atom where electrons are likely to be
    found.

				
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