The History of the Atom

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The History of the Atom

• Early Greeks to Present

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Democritus (460 B.C.-370 B.C.) Greek
Philosopher

• Among the first to suggest the existence of
  atoms.
• Believed that atoms were indivisible and
  indestructible.
• No experimental support.
• Ideas not based on scientific method.

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Aristotle (384 B.C.-322 B.C.)Greek Philosopher

• Believed all matter was made up of atoms.
• Gave matter the name hyle.
• No experimental data.

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John Dalton (1766-1844)English Chemist and
School Teacher

• Used experimental methods.
• Transformed Democrituss ideas on atoms into a
  scientific theory.
• Responsible for Daltons Atomic Theory.

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Daltons Atomic Theory

• All elements are composed of tiny indivisible
  particles called atoms.
• Atoms of the same element are identical. The
  atoms of one element are different from those of
  any other element.

• Atoms of different elements can physically
  combine in simple whole-number ratios to form
  compounds.
• Chemical reactions occur when atoms are
  separated, joined, or rearranged. Atoms of one
  element are never changed into atoms of another
  element.

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J.J. Thomson (1856-1940) Excuse me... how can
you discover a particle so small that nobody has
ever seen one?

• 1897-English Physicist
• Discovered the electron.
• Used cathode ray tube experiment to explain his
  theory.

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Cathode-Ray Tube
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Cathode Ray-Tube(Crookes Tube)
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Cathode-Ray Tube Experiment

• Thomson performed experiments that involved
  passing electric current thru gases at low
  pressure.
• He sealed the gases in glass tubes fitted at both
  ends with metal disks called electrodes.
• Electrodes were connected to a source of
  electricity.

• Two ends of the tube
• 1. cathode- negatively charged electrode.
• 2. anode- positively charged electrode.
• Cathode Ray- glowing beam that travels from the
  cathode to the anode.

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Cathode Ray Tube Experiment Continued

• The beam traveled across the tube from the
  cathode to the anode.
• The beam was deflected by negative charges, but
  was attracted to positive charges.
• He concluded that the particles that made up the
  beam must be negatively charged. He called them
  electrons.

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Plum Pudding Model

• J.J. Thomson proposed this model of the atom.
• Electrons were stuck into a lump of positive
  charge, similar to raisins stuck in dough.

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Robert A. Millikan (1868-1953)U.S. Physicist

• 1916- Calculated the mass of electron.
• Values for electron charge and mass reported in
  1916 are very similar to those accepted today.
• Electrons carry exactly one unit of negative
  charge.
• Electrons have a mass of 1/1840 the mass of a
  hydrogen atom.

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Eugen Goldstein (1850-1930)German Physicist

• Observed rays traveling in the direction opposite
  to that of the cathode rays.
• He called these rays canal rays and concluded
  they were positively charged.
• Now called protons.
• Mass of a proton is 1840 times that of an
  electron.

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James Chadwick (1891-1974)English Physicist

• 1932-Confirmed the existence of another particle
  with no charge- neutron.

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Ernest Rutherford (1871-1937)Student of Thomson

• 1911- Tested the theory of atomic structure using
  the gold-foil experiment.
• Suggested a new theory base on his results.

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Rutherfords Gold Foil Experiment
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Summary of Gold-Foil Experiment

• A narrow beam of alpha particles was directed at
  a very thin sheet of gold foil (1 atom
  thickness).
• According to the previous theory, the particles
  should have passed thru.
• Most particles passed through and some of them
  bounced back or were deflected.

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Rutherfords Conclusions

• Atoms are made up of mostly empty space.
• Atoms have a positive center called a nucleus.
• Most of the mass of the atom is located in its
  nucleus.
• Improvement over the Plum-Pudding Model, but
  still incomplete.

Plum Pudding Model
Rutherfords New Atom
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Niels Bohr (1885-1962)Danish Physicist
Bohr debating Quantum Theory with Albert Einstein
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Bohr Model of the Atom

• Planetary Model was correct.
• Electrons were in orbits around the positively
  charged nucleus.
• Based on Quantum Physics.

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Electron Cloud Model

• Electrons are not in perfect orbits around the
  nucleus.
• Imagine, as the electron moves it leaves a trace
  of where it was.
• This collection of traces quickly begins to
  resemble a cloud. 

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Thanks to the following Websites

• http//regentsprep.org/Regents/physics/phys05/cato
  model/cloud.htm
• www.wikipedia.com
• http//www.upscale.utoronto.ca/GeneralInterest/Har
  rison/BohrModel/BohrModel.html
• http//www.chemsoc.org/timeline/pages/1911.html