Atomic Theories- Part I

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Atomic Theories- Part I

• Chemistry
• Mrs. Coyle

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A) Early Atomic Theories
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Atom

• The word atom comes from the Greek and means
  indivisible.

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Atom

• The smallest particle in an element that retains
  its identity in a chemical reaction.

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Democritus Greek philosopher 4th Century BC

• First to come up with atom
• Matter is composed of tiny particles called atoms
• These atoms are invisible, indestructible
  fundamental units of matter

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• Democrituss ideas were opposed by Aristotle and
  Plato.

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Antoine Lavoisier (France 1782)

• Law of Conservation of Mass
• In a chemical reaction mass is conserved.

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Joseph Proust (France 1799)

• Law of Definite Proportions
• The elements that comprise a compound are
  always in a certain proportion by mass.

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John Dalton (England 1766-1844)

• School teacher
• Studied the ratios in which elements combine in
  chemical reactions
• Formulated first modern Atomic Theory

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Daltons Atomic Theory

1. All matter is made of atoms.
2. Atoms of the same element are identical. The
   atoms of any one element are different from those
   of any other element.

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Daltons Atomic Theory

• Atoms of different elements can chemically
  combine in simple whole number ratios to form
  compounds.
• Example CO2

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Daltons Atomic Theory

• Chemical reactions occur when atoms are
  rearranged.
• Chemical reactions do not change atoms of
  one element to another.

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Daltons Atomic Model
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Atomic Theory

• Much of Daltons theory still holds today
• However, we now know that
• atoms are not indivisible

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Scanning Tunneling Microscope
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Scanning Tunneling Microscope
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Iron Atom Arrangement - STM
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The Size of the Atom

• If you placed 100,000,000 Cu atoms side by side
  they would form a line only 1 cm long.
• Radius of most atoms is about 5x10-11 to 2x10-10m.

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B) The Discovery of the Electron and the Proton
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History

• Electron means amber in Greek
• Properties discovered by the Greek Thales of
  Miletos 600 BC. Rubbed the mineral amber with cat
  fur and attracted feathers.

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Benjamin Franklin (America 1740s)

• Law of conservation of charge.
• Saw electricity as a flowing fluid and called the
  flow direction positive.

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Law of Charges

• Like charges repel
• Opposite charges attract

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J(oseph) J(ohn) Thomson (England 1897)

• He discovered the electron while experimenting
  with cathode rays.

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Cathode Ray
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Deflection of Cathode Ray
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Cathode Ray Tube
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JJ Thomson with the CRT
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Thomsons Discovery

• He determined that the cathode ray was made of
  negatively charged particles electrons.

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Cathode Rays

• Thomson also was able to estimate that the mass
  of the electron was equal to about 1/1840 of the
  mass of a hydrogen atom.
• His discovery of the electron won the Nobel Prize
  in 1906.

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Cathode Rays

• Thompson showed that the production of the
  cathode ray was not dependent on the type of gas
  in the tube, or the type of metal used for the
  electrodes.
• He concluded that these particles were part of
  every atom.

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Charge of the Electron

• Charge of Electron
• 1.6 x 10-19 C (coulombs)
• Mass of Electron
• 9.11 x 10-28 g

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Atoms have no net electric charge.
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Ions

• Positively charge atom (cation)
• Atom lost electrons.
• Negatively charged atom (anion)
• Object gained electrons.

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Electron is the basic quantity of charge.

• Electric charges always exist in whole number
  multiples of a single basic unit, the electron.

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• A particle with a positive charge must be present
  in the atom to balance each negatively charge
  electron.

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Plum Pudding Model (Thomson)
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Application of the CRT
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Why is watching television potentially unsafe?
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The Discovery of the Proton

• Discovered by Eugen Goldstein (German) in 1886.
• He observed Canal rays and found that they are
  composed of positive particles protons.

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Canal Rays

• Canal Rays passed through holes, or channels, in
  the reverse direction as the cathode ray. 

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Canal Rays
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c) Discovery of the Nucleus
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Ernest Rutherford (Born in New Zealand 1871-1937)

• University of Manchester, England
• Tested Thomsons theory of atomic structure with
  the gold foil experiment in 1910.

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Gold Foil Experiment

• Bombarded thin gold foil with a beam of alpha
  particles.
• If the positive charge was evenly spread out, the
  beam should have easily passed through.

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Rutherford's Gold Foil Experiment
Rutherford and coworkers aimed a beam of alpha
particles at a sheet of gold foil surrounded by a
florescent screen.
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Rutherford

• Expected
• Found

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Rutherford's Experiment
Most particles passed through with no deflection,
while some were highly deflected Rutherford
concluded that most particles passed through
because the atom is mostly empty space.
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Rutherfords Conclusions

• All of the positive charge, and most of the mass
  of an atom are concentrated in a small core,
  called the nucleus.

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Size of Nucleus Compared to the Atom is as a Ball
Compares to a Football Field.
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The Discovery of the Neutron

• Discovered in 1932 by James Chadwick (England
  1891-1974).

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The Discovery of the Neutron

• Chadwick bombarded alpha particles(helium nuclei)
  at Beryllium.
• Neutrons were emitted and in turn hit parafin and
  ejected protons from the parafin.

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Discovery of the Neutron
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Neutrons

• Neutrons have mass similar to protons.
• No electrical charge.

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Properties of Subatomic Particles
Particle Symbol Relative Charge Mass Relative to the Proton Mass (g)
Electron e- 1- 1/1840 9.11 x 10-28 g
Proton p 1 1 1.67 x 10-24g
Neutron n0 0 1 1.67 x 10-24g
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Progression of Models
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Daltons Atomic Model
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Plum Pudding Model (Thomson)
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