Atomic Theory

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Atomic Theory
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Lets Take a Trip Through Time!

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Democritus460 370 B.C.

• There are various basic elements from which all
  matter is made
• Everything is composed of small atoms moving in a
  void
• Some atoms are round, pointy, oily, have hooks,
  etc. to account for their properties
• Ideas rejected by leading philosophers because
  void no existence

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First Concept of an Atom

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John Dalton1766-1844

• Each element is composed of extremely small
  particles called atoms
• All the atoms of a given element are identical,
  but they differ from those of any other element
• Atoms are neither created nor destroyed in any
  chemical reaction
• A given compound always has the same relative
  numbers and kinds of atoms

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Daltons Atomic Theory (1808)
1. Elements are composed of extremely small
particles called atoms. 2. All atoms of a
given element are identical, having the same
size, mass and chemical properties. The atoms of
one element are different from the atoms of all
other elements. 3. Compounds are composed of
atoms of more than one element. In any compound,
the ratio of the numbers of atoms of any two of
the elements present is either an integer or a
simple fraction (Law of Definite Proportion). 4.
A chemical reaction involves only the separation,
combination, or rearrangement of atoms it does
not result in their creation or destruction.
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Daltons Model

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Law of Multiple Proportions
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Law of Conservation of Mass
8 X2Y
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Modern Atomic Theory

• Atoms cannot be subdivided, created, or
  destroyed.
• Atoms can be destroyed via nuclear reactions but
  not by chemical reactions.
• Atoms of a given element are identical in size,
  mass, and other properties
• There are different kinds of atoms (differing by
  their masses) within an element that are known as
  "isotopes.

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J.J. Thomson(Discovery of Electron)1856-1940

• Discovered electron 1897 Cathode Ray Experiment
• Plum Pudding model 1904
• Electrons in a soup of positive charges
• Discovered isotopes 1913

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Discovery of Electron

• J. J. Thomson(1897)
• British physicist at Cambridge University
• Cathode Ray Experiment Hypotheses
• Cathode rays are charged particles (which he
  called "corpuscles").
• These corpuscles are constituents of the atom.
• Observation
• -the rays bended with a magnet
• -the electrometer measured a large amount of
  negative charge

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Cathode Ray Tube
J.J. Thomson, measured mass/charge of e- (1906
Nobel Prize in Physics)
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The Electron

• J.J. Thomson 1897 - CRT experiment
• Negative Charge
• e-
• Actual Mass 9.11 x 10-28g
• Relative mass information - discovered later
  1/1840 the mass of proton

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Plum Pudding Model

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Millikans Experiment
Measured mass of e- (1923 Nobel Prize in Physics)
e-
charge -1.60 x 10-19 C Thomsons charge/mass of
e- -1.76 x 108 C/g
e- mass 9.10 x 10-28 g
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Ernest Rutherford1871-1937

• Nucleus Theory 1910
• alpha particle gold foil experiment
• An atoms mass is mostly in the nucleus
• The nucleus has a positive charge (Moseley)
• Electrons in fixed orbit

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Rutherford Gold Foil Experiment

• Bombard the gold foil with alpha
  particles,positively charge particles w/ 4x of
  the mass of H
• Only expectslight deflection
• EvidenceWide-angledeflection
• Click for Interactive Demonstration

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Experiment Conclusion

• atoms positive charge is concentrated in the
  nucleus
• Volume of a nucleus is very small compared with
  the total volume of an atom
• The nucleus is heavier than electron
• Suggested that the atom might resemble a tiny
  solar system, with a massive, positively charged
  center circled by only a few electrons
• proton (p) has opposite()charge of electron (-)
• mass of p is 1840 x mass of e- (1.67 x 10-24 g)

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Rutherford Model

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James Chadwick1891-1974

• Worked with Rutherford
• Interpreted work of the Curies
• Discovered Neutron 1932
• Nobel Prize in Physics 1935

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Chadwicks Experiment (1932)(1935 Noble Prize in
Physics)
H atoms - 1 p He atoms - 2 p mass He/mass H
should 2 measured mass He/mass H 4
neutron (n) is neutral (charge 0) n mass p
mass 1.67 x 10-24 g
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The Neutron

• Chadwick 1932 - nuclear bombardment
• No charge
• n0
• Actual Mass 1.67 x 10-24g
• Relative Mass 1

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Niels Bohr1885-1962

• Planetary Model 1913
• Nucleus surrounded by orbiting electrons at
  different energy levels
• Electrons have definite orbits
• Utilized Plancks Quantum Energy theory
• Worked on the Manhattan Project (US atomic bomb)

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Bohrs Model

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Bohr Model for Nitrogen

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Ernst Schrödinger 1887-1961
Werner Heisenberg 1901-1976

• Quantum Mechanical Model 1926
• Electrons are in probability zones called
  orbitals, not orbits and the location cannot be
  pinpointed
• Electrons are particles and waves at the same
  time
• Developed quantum numbers based on theories of
  Einstein and Planck

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Orbitals

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Quantum Mechanical TheoryElectron in a Hydrogen
atom

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Atomic Theories Timeline
Name Democritus Dalton Thomson Rutherford Bohr Schrödinger Heisenberg
Time Frame
Key Points
Historical Events
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