Atoms, Atoms Everywhere!

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					Atoms, Atoms Everywhere!
The History of Atomic Models
   Timeline of the Models*

    ?
 460 BC           1800            1897            1911           Today
Democritus   John Dalton           JJ          Rutherford         Many
 “Atoms”     “Billiard Ball”   Thompson        “Planetary”      scientists
                                 “Plum            1912          “Modern”
                                Pudding”       Niels Bohr
                                              “Bohr Model”


   *There are many more models. These are the ones we’ll cover in class.
Democritus (c. 460 BC)
      Democritus asked: If you keep breaking
       matter in half, how many breaks will you
       have to make before you can’t break it
       apart any further?
      Democritus called the smallest possible bits
       of matter atoms. (indivisible in Greek)
      He had no experiments to support his
       theory.


    Democritus was supposedly known as ‘the laughing philosopher’
    because of his wry amusement at human foibles.
The Ancients – B.C.
   Believed Aristotle's
    theory that everything
    was made up of the
    fundamental
    “elements”
       Earth
       Wind (air)
       Fire
       Water
Aristotle’s Folly

   Unfortunately Aristotle (the more
    popular Greek philosopher) dismissed
    the atomic idea of Democritus as
    worthless. (What?!)
   For more than 2000 years nobody did
    anything to continue Democritus’ work.
   No surprise, we call these the
    “Dark Ages” of atomic theory.
John Dalton (c. 1800 AD)
   English chemist, John Dalton, performed
    experiments with various chemicals and
    showed matter seemed to consist of
    “indivisible” particles (atoms).
   Though he didn’t know about atoms’
    structure, he did know about the Law of
    Conservation of Matter and based his theory
    on this.
    Dalton was an avid weather watcher and discoverer of color blindness
    among other things.
Dalton’s Ideas
“Billiard Ball” or “Marbles”
   Dalton's model says atoms are tiny,
    indivisible, indestructible particles
   Believed each atom had a certain
    mass, size, and chemical behavior
    determined by what kind of elements
    they make up.
   See next slide for the details….
    John Dalton Theory --1800
    (a.k.a “the marble guy”)
   Atoms are the smallest
    particles of nature-indivisible
    and indestructible
   All atoms of the same
    element are identical
   Atoms of different kinds can
    combine to form compounds
   Chemical reactions are atoms
    recombining to form new
    substances
J.J. Thomson (c. 1897)
   In 1897, English physicist J.J.
    Thompson discovered the electron
    and proposed a model for the structure
    of the atom.
   Using a CATHODE RAY TUBE,
    Thomson discovered electrons have
    a negative charge and thought that the
    rest of matter must have a positive
    charge to offset the negative electron.
His Experiment
      JJ Thompson’s Model
      “Plum Pudding”
   Because the beam of light
    traveled from to the positive end
    of the tube he concluded that the
    light had a negative charge
   Because the beam could push a
    paddle wheel he concluded that
    the particle had mass.
   Thompson's model says atoms
    are positively charged spheres
    with negatively charged electrons
    randomly located throughout.
Side Trip…Alpha Particles!

   Around this time scientists also
    discovered alpha rays (particles),
    which had a positive charge.
   Some physicists thought these
    alpha particles were made up
    of the positive parts of JJ
    Thompson’s atom.
       Ernest Rutherford (1911)
   Rutherford as a student worked
    under J.J. Thompson
    supervision at the famous
    Cavendish Laboratories.
   In 1911 Ernest Rutherford
    bombarded atoms with alpha
    rays to investigate the inside of
    the atom.
   The results were, to say the
    least, unexpected!
       Gold Foil Experiment
   Rutherford used
    Radium as the source
    of the alpha particles
    and shot them at a thin
    gold foil like aluminum
    foil but made of gold
   A fluorescent screen
    sat behind the gold foil
    on which he could
    observe
    the alpha particles’
    impact.
   When the particles bounced
    back or were deflected
    Rutherford reasoned that it
    hit something massive and
    positive. This mass became
    know as the nucleus.
   When the alpha particles
    went straight through it hit
    nothing.
   This happened most often
    so the atom is mostly empty
    space.
    Rutherford’s “Planetary Model

    Rutherford’s model said the
    negative electrons orbited a
    positive center (NUCLEUS)like
    our planets orbit the sun.
   The nucleus contained most of the
    mass of the atom
   And the distance between the
    positive center (nucleus) and the
    electrons was huge-like a marble
    in the center of a football field.
   The atom was mostly empty
    space!!!!
One little problem…

   The theory of electricity
    and magnetism predicted
    that opposite charges attract
    each other and the electrons
    should gradually lose energy
    and spiral inward toward the nucleus.
    (BOOM! No more atom.)
Niels Bohr (1912)
   In 1912 a Danish physicist, Niels Bohr
    came up with a theory that said the
    electrons do not spiral into the nucleus
    and came up with some rules for what
    does happen.
   This was a pretty radical approach,
    because for the first time rules had to fit
    the observation regardless of how they
    conflicted with the theories of the time!
    (Aristotle would have been furious).
Previous experiments-White light
gives off all wavelengths of energy- all
colors.
Previous experiments-helium gas only gives
off certain colors thus the line emission
spectra for helium is produced.
      The explanation-

   An electron absorbs
    energy it jumps
    farther away form the
    nucleus
   As the electron falls
    back closer to the
    nucleus it gives off
    the energy as
    colored light.
     How Light Relates to Electron Location

   Bohr observed
    that only certain
    colors were
    given off
   Therefore the
    electron could
    only orbit at
    certain distances
    from the nucleus
Bohr’s Rules
   RULE 1: Electrons can orbit only at
    certain allowed distances from the
    nucleus (energy-levels).
   RULE 2: An atom absorbs energy when
    an electron gets boosted from a low-
    energy orbit to a high-energy orbit.
   Rule 3: Atoms radiate energy when an
    electron jumps down from a higher-
    energy orbit to a lower-energy orbit.
Bohr’s Model of the atom. Light can
excite electrons around atoms and
this gives rise to “quantum levels”.
Are We Done Yet?


   Almost… Cliff Notes version is that
    Niels Bohr came really close, and
    when you add the works of Arnold
    Sommerfeld, Wolfgang Pauli, Louis
    de Broglie, Erwin Schrödinger,
    Max Born, and Werner Heisenberg,
    we arrive at today’s model…
Today’s Model!-Electron Cloud
   Today's model
    says electrons
    are not confined
    to fixed orbits.
   They occupy
    volumes of
    space outside
    the nucleus.

				
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posted:11/16/2012
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