The Structure of the Atom

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• The Structure of the Atom

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Early Research on Atomic Particles

• Eventually exceptions were discovered to Daltons
  Atomic Theory..

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Cathode Ray Studies of the mid 1800s
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See Figure 3-4 p.70

• Cathode negative electrode
• Cathode ray glowing ray that comes from cathode
  when electric current was passed through a tube
  of gas under low pressurewhat was the source?

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Cathode tube experiments showed

• See top page 71
• Object in path of ray casts a shadow
• Paddle wheel on rail inside cathode tube would
  roll
• Rays were deflected by magnetic fields
• Rays were deflected by negatively charged objects

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What did these observations mean?

• Glowing ray in cathode tube was made of particles
• Particles that composed cathode rays are
  negatively charged

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1897 J.J. Thomson

• Further studied cathode rays and concluded
• cathode ray particle had a very large charge for
  its tiny mass
• All cathode rays are composed of identical
  negatively charged particles, regardless of metal
  used as cathode
• Particles later called ELECTRONS
• first to say cathode ray was a building block
  of the atom ATOM WAS NO LONGER INDIVISIBLE!

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"At first there were very few who believed in the
existence of these bodies smaller than atoms."
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Thomson (cont)

• PLUM PUDDING Model because atom was thought to
  consist of electrons moving about in a sphere of
  positive charge

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Robert Millikan

• OIL DROP EXPERIMENT

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Robert Millikan

• Determined the charge on an electron in his
  famous OIL DROP EXPERIMENT

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Combining the work of Thomson and Millikan
-------gt

• Thomson charge/mass ratio of an electron
• Millikan charge on an electron
• Together determined the mass of an electron
• 1/1837 lightest atom (H)

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• Work with modified cathode ray tubes found rays
  moving in the opposite direction that had a
  positive charge - now called PROTONS (early
  l900s)
• 1930s verification of a 3rd atomic particles
  with high energy,no charge and mass about the
  same as a proton - NEUTRONS

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• Discovery of these 3 subatomic particles called
  for major revisions in Daltons Atomic Theory..
  (atoms could be broken into smaller particles!)

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Rutherford, 1912

• Gold FOIL experiment

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Observations of Rutherfords Gold Foil experiment

• Most positive particles passed straight through
  the gold foil
• About 1 in 8,000 particles bounced almost
  straight back

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Rutherford NUCLEAR MODEL

• As a result of his gold foil experiments, he
  concluded that
• atom is mostly empty space
• atom consists of a small, positively-charged
  core (nucleus) surrounded by electrons
• Radii of most atoms 40 --gt 270 pm (10-12m)
• Radii of nuclei only 0.001 pm
• Density of nuclei 2 x 108 tons/cm3
  !!!!!!!!!!!!!!!

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Good analogy..

• If the nucleus were the size of a ping pong ball,
  the electron would be found.
• 0.8 mile away ! (Homestead Rd 322 intersection
  to Dons Pier)

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Composition of Nucleus

• Proton
• Positive charge (equal in magnitude but opposite
  in charge to an electron)
• Mass 1.673 x 10-27 kg
• Neutron
• Electrically neutral
• Mass 1.675 x 10-27 kg (slightly larger than
  proton)

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• The number of protons in an atoms nucleus
  determines that atoms identity
• (Atomic number, Z)

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• Good summary table of subatomic particles on page
  74

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• NUCLEAR FORCE short-range proton-proton,
  neutron-neutron and proton-neutron forces that
  hold the nuclear particles together