History of the Atomic Model

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History of the Atomic Model
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Early Greek Theories

• Aristotle (350 B.C.)
• 4 Elements

• Democritus (400 B.C)
• Atoms and a void (empty space)
• Atoms are indivisible

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Daltons Billiard Ball Model (1805)

• All matter is made of atoms.
• Atoms of an element are identical.
• Each element has different atoms.
• Atoms of different elements combine in constant
  ratios to form compounds.
• Atoms are rearranged in reactions, but are not
  created nor destroyed.

• Matter is composed of indestructible, indivisible
  atoms

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Thomsons Raisin Bun Model (1897)

• Materials, when rubbed, can develop a charge
  difference.
• This electricity was called cathode rays
• These rays have a small mass and are negatively
  charged.
• Thomson noted that these negative subatomic
  particles (electrons) were a fundamental part of
  all atoms.

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Rutherfords Nuclear Model

• Rutherford shot alpha (?) particles at gold foil.

Most particles passed through. So, atoms are
mostly empty. Some positive ?-particles deflected
or bounced back! Thus, a nucleus is positive
(protons) holds most of an atoms mass.
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Limitations to Rutherfords Model

• Orbiting electrons should emit light, losing
  energy in the process
• This energy loss should cause the electrons to
  collapse into the nucleus
• However, matter is very stable, this does not
  happen

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Bohrs Planetary Model

• Electrons orbit the nucleus in energy shells
• An electron can travel indefinitely within an
  energy level without losing energy
• The greater the distance between the nucleus and
  the energy level, the greater the energy level
• An electron cannot exist between energy levels,
  but can move to a higher, unfilled shell if it
  absorbs a specific quantity of energy, or to a
  lower, unfilled shell if it loses energy
  (quantized)
• When all the electrons in an atom are in the
  lowest possible energy levels, it is in its
  ground state.

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Bohrs Planetary Model

• An atom becomes excited when one of its
  electrons absorb energy in the form of photons
• If enough energy is absorbed then the electron
  can make a quantum leap to the next energy level,
  if there is room
• When the electron returns to a lower energy state
  the energy is released in the form of a photon,
  which we see as visible light
• The energy of the photon determines its
  wavelength or color
• Each element has its own frequencies of color, so
  it emits its own distinctive glow

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Summary of Atomic Models

• Daltons Billiard ball model (1800-1900)
• Atoms are solid and indivisible.

• Thomsons Raisin bun model (1900)
• Negative electrons in a positive framework.

• Rutherfords Nuclear model (1910)
• Atoms are mostly empty space.
• Negative electrons orbit a positive nucleus.

4) Bohrs Planetary model (1920) Negative
electrons orbit a positive nucleus. Quantized
energy shells
5) Quantum Mechanical model (1930) Electron
probabilities (orbitals)