Atomic Models Over Time Section 51

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Atomic Models Over TimeSection 5-1
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5-1 notesThe atomic model

• Based on indirect evidence by how stuff behaves
• Allows you to create a mental picture (model)
  because you can not see an atom it is too tiny

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Democritus

• Greek over 2,000 years ago
• Matter at some point has to be made of one
  particle
• Named that particle the atom
• Small and hard
• Comes in different shapes and sizes

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Dalton

• Early 1800s
• All elements are made of matter
• Atoms of the same element are identical
• Atoms of different elements are different
• Compounds are made by joining atoms of 2 or more
  elements together

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Dalton's wooden atomic models, c early 19th
century.Image number 10312949

• These wooden balls, were the first models made to
  represent atoms and were used by John Dalton
  (1766-1844) to demonstrate atomic theory. He
  stated that matter is made up of small
  indivisible particles, or solid atoms. Dalton
  first published, in 1808, in a small book
  entitled A new system of Chemical Philosophy,
  Part 1, by John Dalton. The book had an enormous
  influence on the development of both chemistry
  and physics.

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Thompson

• The atom is made of smaller particles
• Inside the atom there is a pudding like positive
  charge with a scattering of negative electrons

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J.J. Thomson

• One of the first atomic models was that proposed
  by J.J. Thomson after he discovered the existence
  of the electron as a result of his work with
  cathode rays. Since each atom was electrically
  neutral, Thomson thought that it consisted of a
  relatively large, uniformly distributed, positive
  mass with negatively charged electrons  embedded
  in it like "raisins in a plum pudding."
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Rutherford

• 1911
• Atoms have a small positive center (nucleus)
• Negative electrons are scattered around the
  outside of the nucleus
• The negative electrons are held in place by the
  attraction of the positive nucleus (like a
  magnet)

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Bohr the model we will use in class

• 1913
• Positive nucleus is in the center
• Electrons move in definite orbits around the
  nucleus called energy levels

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Wave Model

• Most up to date model.
• Small positive nucleus is surrounded by a large
  region that has electrons enough to make the
  atom neutral.
• There is no specific orbit for the electron but
  there are regions where the electron is most
  likely to be found.