Law of Definite Proportions 1797

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Law of Definite Proportions1797

• A chemical compound always contains the same
  elements in exactly the same proportions by
  weight or mass regardless of source or size of
  sample.

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• This law also states that every molecule of a
  compound is made of the same number and types of
  atoms.
• Joseph Proust, a French chemist, maintained the
  proportions of substances which combined in a
  reaction were always the same. He proposed this
  law in 1797. Also known as Prousts Law.
• Also called the Law of Definite Composition.

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Ethylene Glycol

• Composition by mass
• 51.56 oxygen
• 38.70 carbon
• 9.74 hydrogen

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How is a brownie recipe an analogy for the Law of
Definite Proportions?

• As long as the recipe is followed, the brownies
  will turn out the same way regardless of who
  makes them.

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Law of Conservation of Mass

• Mass cannot be created or destroyed in ordinary
  chemical and physical changes.

Antoine Lavoisier
1743 - 1794
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Law of Multiple Proportions

• When two elements combine to form two or more
  compounds, the mass of one element that combines
  with a given mass of the other is in the ratio of
  small whole numbers.
• Claude-Louis Berthollet had proposed this law
  about 1790 but many scientists did not accept it.

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Daltons Law of Simple Multiple Proportions

• Derived his Atomic Theory by way of meteorology
  his study of gases.
• Unlike the ancient Greek philosophers who merely
  talked about atoms, Daltons theory had the
  weight of careful chemical measurements behind
  it. (During the 1780s Lavoisier ushered in a
  new era in chemistry by making careful
  quantitative measurements.)
• In 1803, Dalton noticed that oxygen and carbon
  combined to make two compounds. Each compound
  has its own particular weight ratio of oxygen to
  carbon (1.331 2.661), but also, for the same
  amount of carbon, one had exactly twice as much
  oxygen as the other.
• Proposed the Law of Simple Multiple Proportions.

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Cathode Ray Tube Animation
Follow the animation from left to right as it
steps through the process of how the CRT tube
operates.
By deflecting the beam of electrons, Thomson was
able to calculate a value representing the
mass/charge ratio of electrons (1.76 X 108
coulombs/gram). His equipment was not
sophisticated enough, however, to find the charge
and mass of a single electron.
http//webclass.cqu.edu.au/Units/81120_FOCT_Hardwa
re/Study_Material/Study_Guide/chap2/sec1p2.html
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Millikans Oil Drop Experiment

• Performed between 1909 and 1913.
• Studied the physical interactions between a
  system of surroundings and a drop of oil.
• Calculated the electric charge on an oil drop
  from this was able to determine the charge on a
  single electron.

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Simulation of Millikans Chamber
Animation of Millikan's Oil Drop Experiment

• Millikan put a charge on a tiny drop of oil
  measured how strong an applied electric field had
  to be in order to stop the electron from falling.
• He was able to work out the mass of the oil drop,
  calculate the force of gravity on one drop, and
  the electric charge the drop must have.
• By varying the charge on different drops, he
  noticed that the charge was always a multiple of
  1.6 x 10-19 coulomb, the charge on a single
  electron. The electrons were always carrying
  this unit of charge.

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Mass of a Single Electron

• Found the mass of a single electron by using his
  value for charge (1.60 X 10-19 C) Thomsons
  charge to mass ratio (1.76 X 108 C/g).
• The Coulomb (C) is the SI unit for electrical
  charge.

Current accepted value is 9.10939 X 10-28 g.
1923 Nobel Prize in Physics
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Animation of Gold Foil Experiment
http//micro.magnet.fsu.edu/electromag/java/ruther
ford
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more models
Electrons are found in a cloud around the
nucleus but not in definite orbits.
Electrons are found in specific orbits
each orbit is associated with a certain amount
of energy.
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Whats the big deal about all these atomic models?

• Our current understanding of matter has led to
  thousands of unpredictable scientific discoveries
  and technologies.
• When Carl Sagan, a Cornell astronomer, said that
  he owed his life to modern science, he might have
  been referring to imaging technology like CAT
  (computerized axial tomography), NMR (nuclear
  magnetic resonance), or PET (Position emission
  tomography).
• These modern diagnostic tools, which save
  countless lives each day, would not be possible
  without our understanding of the atomic structure.