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History of the Atom Power Point


History of the Atomic Model
• Around 460 B.C., he was the first to suggest
  atoms existed.
• Believed that atoms were indivisible and
• His research lacked experimental support
                             Democritus model:
                             atom was a tiny, hard,
                             uncuttable sphere
Famous Greek Philosopher
All substances made up of 4 elements:
    Fire - Hot
    Air - light
    Earth - cool, heavy
    Water - wet
    Blend these in different proportions to get all
Did not experiment.

Greeks settled disagreements by

Aristotle was a better debater

His ideas carried through to the
middle ages.
              Middle Ages

Age of Alchemists - tried to change lead to gold.
                          Early Ideas
scientist      •   Joseph Proust
led Dalton     •   Law of Definite Proportions
to the first
               •   Lavoisier
atomic         •   Law of Conservation of Mass
theory.        •   Gay-Lussac
               •   Gases react in small whole number ratios
Law of Definite Proportions
             Chemical compounds
             contain the same elements in
             exactly the same proportions
             by mass regardless of size of
             the sample or source of the
Law of Conservation of Mass

   Mass is neither created or
   destroyed during chemical
   reactions or physical changes
Law of Multiple Proportions
             If 2 or more different
             compounds are composed of
             the same 2 elements, they
             combine in different ratios.
                 JOHN DALTON
• 1803 (2000 years after Democritus)
Dalton's "modern" atomic theory includes:
1. All matter is composed of extremely small particles called atoms.
2. Atoms of the same element are identical in size, mass and other
   properties; atoms of different elements differ in size, mass and
   other properties.
3. Atoms cannot be subdivided, created or destroyed in chemical
4. Atoms of different elements combine in simple whole-number
   ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated or
  Dalton’s Model

ELEMENT A                 ELEMENT B
Atoms of element A        Atoms of element B
are identical             are identical, but
                          different that atoms
                          of element A

                          Atoms of element and
Atoms of element A        B can chemically
and element B can         combine to form a new
physically mix together   compound
                    J.J. Thomson
• 1897
• The first evidence for sub-atomic particles came from experiments
  with the conduction of electricity through gases in sealed glass
  tubes at low pressures, called Cathode Ray Tubes.

•   J.J. Thomson demonstrated that the rays consist of a stream of
    negatively charged subatomic particles which he called electrons.
Cathode Ray Tube Experiments
                   •In Thomson’s experiments,
                   the cathode ray is deflected
                   by a both a magnet and also
                   by an electrically charge metal

                   •Thomson believed the ray
                   was made of negatively
                   charged particles because a
                   positively charged plate
                   attracted the ray and the
                   negative plate repelled it

                   •These negatively charged
                   particles were later named
Thomson’s Experiment

-     Voltage source

      Metal Disks
     Thomson’s Experiment
                 Voltage source
     -                                  +

   Passing an electric current makes a beam
    appear to move from the negative to the
    positive end
   Thomson’s Experiment
                Voltage Source


 By adding an electric field he found that the
  moving pieces were negative.
  Thomson’s (Lord Kelvin’s)Plum
        Pudding Model
• The particles making up the rays acted as if they
  were negatively charged.
• Since elements are neutral, there must be positive
  particles balancing out the negative particles
• Thomson theorized that an atom consisted of a
  “pudding” of evenly distributed positive and
  negative particles.
“Plum Pudding Model”
• 1909
• Millikan and co-workers studied the motion of charged
  oil droplets in an electric field

• Through their work, they determined values for the
  charge and mass of an electron: the electron carries one
  unit of negative charge and its mass is 1/1840 the mass
  of a hydrogen atom

• Millikan’s values are very similar to those we use today
  Rutherford’s Experiment
Believed the plum pudding model of the atom
 was correct.

  Used radioactivity to experiment with
  this idea in mind..
  Alpha particles - positively charged
  pieces given off by uranium.
  Shot them at gold foil which can be
  made a few atoms thick.
Lead    Uranium

                  Gold Foil
            He Expected
The alpha particles to pass through without
 changing direction very much.
The positive charges were spread out
 evenly. Alone they were not enough to stop
 the alpha particles.
What he expected
What he got
How he explained it
Atom is mostly empty.
Small dense,
 positive center
 piece                   .+
Alpha particles
 are deflected by
 it if they get close
• Based on his experiments, he concluded that the atom was made
  of mostly empty space

• The mass and positive charge are concentrated in a small area call
  the nucleus.

• Nucleus: the tiny central core of an atom and is composed of
  protons and neutrons

• Rutherford’s model was also known as the nuclear model.

• Analogy: If an atom is the size of a football field, the nucleus is
  the size of a marble
           James Chadwick
• In 1920, Ernest Rutherford postulated that there
  were neutral, massive particles in the nucleus of
• In 1932, James Chadwick discovered the neutron
  He spent more than a decade searching.
                 Niels Bohr
• Won the Nobel Prize in 1922 for his work on
  atomic structure suggesting that atoms emit
  electromagnetic radiation as the electrons move
  from one fixed energy level to another
• Electrons in energy levels called orbits.
Bohr’s Model

     Wave Mechanical Model
• The three men most responsible for the model are
  Louis de Broglie, Erwin Schrodinger, and Werner
• Louis de Broglie was the first man to openly
  support the idea that electrons exhibit wave
     Wave Mechanical Model
• Erwin Schrodinger developed a wave
  equation for the electrons in an atom. This
  equation accurately predicted the behavior of
  every electron in every atom.
     Wave Mechanical Model
• Werner Heisenberg, through his famous
  uncertainty principle, showed that it is impossible
  to know both the exact location and the exact
  speed of an electron
Wave Mechanical or Electron Cloud

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