Development of Atomic Theory

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Development of Atomic Theory

• The Scientific Method in Action

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Democritus vs. Aristotle

• Matter is composed of discrete particles.
  Democritus named these atoms.

Matter is continuous. You can break it into
smaller and smaller particles indefinitly.
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John Dalton - Early 1800s

• Conservation of Matter
• Mass is conserved in a reaction. The amount you
  begin with is equal to the amount you end up
  with.

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John Dalton - Early 1800s

• Conservation of Matter
• Law of Definite Proportions
• Types of Matter combine in given ratios. For
  example
• H2 O2 -------gt H2O
• 100g 793g ----gt 893g
• 50g 793g ----gt 445g

Water
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John Dalton - Early 1800s

• Conservation of Matter
• Law of Definite Proportions
• Law of Multiple Proportions
• Matter combines in Whole Number Ratios
• C O2 -------gt CO2
• 2C O2 --------gt CO

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Daltons TheoryThe Pool Ball Model

• All matter is made up of atoms.
• Atoms are tiny, indivisible, indestructible,
  fundamental particles.
• Atoms cannot be created or destroyed.
• Atoms of a particular element are alike.
• Atoms of different elements are different.
• A chemical change involves the union or
  separation of individual atoms.

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Problems with Pool Ball Model

• Doesnt explain bonding.
• Doesnt explain ions
• Michael Farraday demonstrated that some matter
  can be charged or can carry a charge.

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Michael Farraday

• When certain substances are dissolved in water,
  they conduct electricity.
• Certain compounds decompose into their elements
  when current is passed through them. Atoms of
  some elements are then attracted to the negative
  electrode, others to the positive electrode.

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J. J. Thompsons Plum Pudding Model

• Cathode Ray Tube Experiments
• A charge was applied across a tube filled with
  various gases or a vacuum. Tiny charges flowed
  out of the cathode (-) and traveled towards the
  anode(). These particles were negatively
  charged. Where were they coming from?

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J.J.Thompsons Plum Pudding Model
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• Tiny, negatively charged particles which Thompson
  called corpuscles and are now known as
  electrons were imbedded in a positive mass, like
  raisins in a plum pudding.

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Rutherfords Scattering Experiment

• Rutherford shot alpha particles ( charged, 7000x
  the mass of an electron) at gold foil.
• Prediction - They would go straight through.
• Results - The particles were scattered, sometimes
  at wide angles.

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• Rutherford wrote -
• -----Scattered! It was as if you fired a 15
  inch shell at a piece of tissue paper, and the
  shell bounced back and hit you.

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Rutherfords Planetary Model

• Something small and massive inside the atom.
• Rutherford called it the nucleus
• Nucleus contains positive protons
• Negative electrons orbit around

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The Neutron

• Discovered by James Chadwick in 1932
• About the same mass as a proton
• No charge (neutral)

Proton Neutron
Electron(-)
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Line Spectra of Elements

• Gas discharge tubes filled with gas of one
  element produce light.
• Light contains discrete lines

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Bohrs Planetary Model

• Electrons exist in quantized orbitals
• Certain wavelengths of light result when
  electrons jump or fall from one level to another.

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Quantum Mechanic Model

• Electrons can be written as an equation
• Solving equation gives orbitals
• These orbitals are areas where electrons can exist