Evolution of the Atomic Theory

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Evolution of the Atomic Theory
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Democritus (460 - 370BC)

• all things are composed of minute, invisible,
  indestructible particles of pure matter, which
  move forever in empty space
• stated that atoms are made of the same matter,
  yet they differ in shape, size, weight, sequence,
  and position
• Named atomos meaning indivisible

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Aristotle (4BC)

• Claimed there were no such things as atoms.
• Believed everything was uniform

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John Dalton (1766 - 1844)

• Theory
• Atoms are small, spherical objects.
• Each different kind of element is represented by
  a different kind of atom.
• Each different kind of atom has its own distinct
  mass.
• Atoms are indivisible.

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Sir Joseph John Thomson (1856 - 1940)

• Based on the evidence, Thomson came up with his
  "Plum Pudding" or what is more commonly known as
  the Chocolate Chip Cookie Dough model of the
  atom
• Atoms are composed of smaller particles.
• These particles are the same for all different
  types of atoms.
• These particles are negatively charged and are
  called electrons.
• Electrons are embedded throughout the uniform
  sphere of positive charge to make up a neutral
  atom.

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Ernest Rutherford (1871 - 1937)

• Based on the gold foil experiment, Rutherford
  made the following statements about the atom
• Atoms contain a nucleus, or core, that contains
  most of the mass of the atom.
• The nucleus has a positive charge.
• Most of the atom is empty space

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Niels Henrik David Bohr (1885 - 1962)

• Electrons are only allowed to move in certain
  paths, or orbits.
• Each of these orbits corresponds to a specific
  amount of energy, or energy level
• Electrons can travel in one of the allowed orbits
  without losing energy.
• The only way for an electron to change its energy
  level is for it to instantaneously change orbits.

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Niels Henrik David Bohr (1885 - 1962)

• showed that an atom emits electromagnetic
  radiation only when an electron jumps from one
  quantum level to another
• helped lead to the idea that electrons exist in
  shells and that the electrons in the outermost
  shell determine an atom's chemical properties

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Electron Cloud or Quantum Model

• it is impossible to know both the exact position
  and the exact momentum of an object at the same
  time

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What we know so far

• We now know that electrons do not travel in
  distinct orbits
• rather the model on the previous slide shows the
  most common probable location of an electron,
  much like the fan blades of a ceiling fan in
  rotation