History of the Atom

1
History of the Atom

• Theoretical Chemistry
• Unit 3

2
Early Greek Philosophers

• Democritus matter is made of small, indivisible
  particles (atomos)
• Aristotle matter is continuous and NOT made of
  smaller particles

3
John Dalton (1805)

• Described atoms as small, dense spheres
• Proposed the Billiard Ball model
• Developed Atomic Theory

4
Daltons Atomic Theory

• All matter is made of atoms.
• Atoms of the same element are always the same,
  while the atoms of different elements are
  different
• Atoms join in small whole number ratios to make
  compounds.
• Atoms are rearranged, but do not changed, in
  chemical reactions.

5
Amadeo Avogadro (1811)

• At the same temperature and pressure, equal
  volumes of gases contain the same number of
  particles
• Avogadros Hypothesis

6
J.J. Thomson (1897)

• Performed cathode ray experiment
• Discovered electrons (particles deflected by a
  negative electric field)
• Proposed existence of protons to explain why
  atoms are neutral
• Developed Plum Pudding model

7
Ernest Rutherford (1911)

• Tested Thomsons model by shooting alpha
  particles through a sheet of gold foil
• Discovered positive nucleus due to deflection of
  particles
• Resulted in Nuclear model
• Calculated protons charge (1) and mass (2000x
  greater than electron)

8
Ernest Rutherford
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Robert Millikan (1912)

• Performed the oil drop experiment to determine
  the charge of an electron is -1

10
James Chadwick (1932)

• Discovered the neutron
• Particle with no charge but the same mass as the
  proton

11
Robert Bunsen

• Studied the different colors produced by
  different elements in a flame

12
Niels Bohr (1912)

• Developed Planetary model
• Electrons orbit the nucleus like planets orbit
  the sun
• Explained the emission of colors of light when
  elements are heated
• Due to the release of energy by electrons falling
  from excited states to ground states

13
Werner Heisenberg (1927)

• Heisenbergs Uncertainty Principle
• No experiment can measure both the position and
  the momentum of a quantum particle simultaneously

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Schrödinger and de Broglie

• Proposed that electrons move like waves in the
  Wave-Mechanical model

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Electron Cloud Model

• Tiny nucleus surrounded by cloud of electrons
• Nucleus accounts for nearly all of the mass
  (99.95)
• Chemical properties are based on the arrangement
  of electrons

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Electron Cloud Model