Intersection 2

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Intersection 2
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• 9/12/06
• Reading 1.8-1.11 p17-28
• 2.1-2.2 p 40-44

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Outline
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• Demos, News
• Law, Theories, Models
• Scientific Inquiry
• History of the atom
• Dalton
• Thompson (Plum Pudding)
• Millikan
• Rutherford
• Representation and Scale in Chemistry

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Demos!

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News bit
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• From CE News July 3, 2006
• Copper topped 8000/metric ton (the highest price
  since 1870)
• Pre 1992 1p and 2p coins (UK) 97 copper
• 1 metric ton of coins 5400
• Smelted into copper, 1 metric ton of coins
  8700
• Post 1992 are steel electroplated with copper
  (magnetic)

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According to the National Academy of Science, a
Law is
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A descriptive generalization about how some
aspect of the natural world behaves under stated
circumstances How are a fact and a law
related? Can you think of any scientific laws?
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Some Examples of Laws as stated by Dalton
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• Law of Conservation of Matter
• In an ordinary chemical reaction matter is
  neither created nor destroyed.
• The sum of the masses of the reactants equals the
  sum of the masses of the products.

• Law of Constant Composition
• A chemical compound always contains the same
  elements in the same proportions by mass.

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Laws Can Be Overturned !

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Which Law from previous page is now known to be
incorrect ?
Law of Conservation of Matter
Law of Constant Composition
The Law of Constant Composition also has an
older name given by Joseph Proust. The Law of
Definite Proportions Once a Law has become
accepted, it is very difficult to get it convince
the scientific community to discard it. Hence,
this one still appears in your textbook despite
its limitations. This law is only true for
simple, small molecules.
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Law of Definite Proportions
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compounds composed of two or more elements in
definite proportions
Joseph Proust 1754-1826
Opposed Law But conceded to Proust
Supported Law
For most compounds discussed in Chem 130, Proust
was correct !
However, more complex materials violate this Law
(for example, LEDs used later in course) Today,
Berthollet is believed to be correct! He was
vindicated 160 years after his death!
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According to the National Academy of Science, a
Theory is
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In science, a well-substantiated explanation of
some aspect of the natural world that can
incorporate facts, laws, inferences, and tested
hypotheses. Theories must be falsifiable.
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According to the National Academy of Science, a
Model is
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• A description or analogy used to help visualize
  something (as an atom) that cannot be directly
  observed

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Theory vs. Model
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• Theory based on facts, evidence
• Model is the picture, an analogy, a way of
  describing a theory

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NAS Building in Washington, DC
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• The investigation of truth is in one way hard
  and in another way easy. An indication of this
  is found in the fact that no one is able to
  attain the truth entirely, but everyone says
  something true about the nature of things, and by
  the union of all a considerable amount is
  amassed.
• -Aristotle in Metaphysics

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History of the Atom
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• A Swiss alchemy lab from
• http//www.rosicrucians.org/salon/swiss/swiss.html

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Early history of the atom
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• The word atom dates to 420 BC.
• Democritus and his teacher Leucippus proposed the
  idea that space was either empty (as in a vacuum)
  or occupied by atoms that were eternal,
  invisible, and so small that they could not be
  further diminished.  

Rumis poem about atoms
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• Aristotle and Plato disagreed, claiming that the
  four basic elements of earth, wind, fire, and
  water made up the material world and that all
  things could be derived of some combination of
  the four

Picture from http//www.npp.hu/tortenelem/atomosok
-e.htm
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Alchemy
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• 300 BC-1650
• Based on the idea that everything was made by
  some combination of earth, air, fire, and water
• Attempts to transmute material to gold the
  perfect substance
• Sought universal solvent
• Looking for an Elixir to extend life
• Aristotles and Platos incorrect ideas held sway
  for almost 2000 years. Even though correct
  concepts were proposed and discussed! Another
  example of how Scientific Laws evolve.

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17th Century Elements
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• Element a substance homogenous in its
  properties and not be divisible into even more
  fundamental elements
• Only 12 elements were known in 1700
• Carbon, copper, gold, iron, lead, mercury,
  silver, sulfur, tin, zinc
• Water, potash, air believed to be elements
• Note data gathered and evaluated and fact
  changed

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Phlogiston
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• Late 17th century phlogiston was put forth by
  Beecher and advocated by Stahl.  They claimed
  when wood is burned, it releases "phlogiston"
  into the air.  If the wood is burned in a jar,
  the flame eventually goes out when the air is
  saturated with "phlogiston."  
• Once scientists were better able to study gases
  and carry out quantitative research, they
  discovered that oxygen was the critical component
  of all of the reactions involving phlogiston.
     Phlogiston became the opposite of oxygen.  

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1800s Daltons Theory

• In the early 1800s, Dalton began to formulate his
  theory and model. His theory had five main
  principles
• 1. Chemical elements are made of atoms.
• 2. The atoms of an element are identical in their
  masses
• 3. Atoms of different elements have different
  masses
• 4. Atoms only combine in small, whole number
  ratios such as 11, 12, 23 and so on.
• 5. Atoms can be neither created nor destroyed

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Dalton on Elements
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• Atoms are the smallest units of matter
• Chemical elements are made of atoms.

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Dalton on Compounds
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• Compounds are combinations of different elements,
  and that in these compounds there is a constant
  ratio of atoms
• Changing their physical state could not separate
  these compounds
• Chemical reactions occurred due to a
  rearrangement of combinations of atoms

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Daltons Model Leaves Unanswered Questions
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• What makes the atoms of each element different?
• Why do atoms combine to form compounds?
• Why do they combine only in integer ratios?
• Why are specific ratios of atoms observed in
  compounds?
• Why do groups of elements have such similar
  properties and reactivities?

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1890s Electrical Charge is Key to Atomic
Structure
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• Charges of the same type repel one another
  charges of the opposite type attract one another
• 1891 -G. Johnstone Stoney term "electron" coined
  for the unit of electrical charge found when
  current was passed through chemicals
• 1896 -Henry Bacquerel discovers that uranium ore
  emits rays that exposed a photographic plate
  through protective black paper
• 1898 -Marie and Pierre Curie isolate polonium and
  radium which emit the same rays.  Radioactive
  elements emit three types of radiation (alpha
  (), beta (-), and gamma (neutral) rays) which
  can be separated by passing them through
  electrically charged plates.  Alpha and beta rays
  have mass.  
• Conclusion  Radioactive elements have atoms
  which are made of something smaller (alpha and
  beta particles.)

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Radioactivity
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1897 JJ Thomson the Cathode Ray Tube
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Picture from www.chem.uiuc.edu/
clcwebsite/cathode.html
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Thompsons Experiments
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1. Cathode rays were generated across an evacuated
   tube by heating up the cathode and applying a
   high voltage across the tube.
2. Thompson showed that rays mass and negative
   charge using magnetic and electric fields
3. Thompson calculated the ratio of the mass of the
   particles to their electric charge 5.6x10-9
   g/coulomb
4. Using different metals as cathodes, all cathode
   rays consisted of particles with the same mass to
   charge ratio.
5. This estimate of mass implied that electrons were
   was 1000 times less massive than a hydrogen
   atom.  This estimate was confirmed by multiple
   independent experiments supporting its surprising
   conclusion.

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Thompsons Addition to Atomic Theory
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• Cathode rays are charged particles called
  corpuscles (now called electrons)
• 2) Corpuscles are constituents of the atom
• 3) Corpuscles are the only constituents of the
  atom

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Plum Pudding Model of an Atom
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• Thompson described an atom as consisting of
  small, negatively charged corpuscles situated
  inside a positively charged field by electric
  static forces.

http//nobelprize.org/physics/educational/quantise
d_world/structure-1.html
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1909 Millikan Finds the Charge of e-
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• In 1909 Robert Millikan determined the charge of
  the electron in his "oil drop experiment
• Charge was always a multiple of -1.6 x 10-19 C.
   He proclaimed that this value was the correct
  value for the charge of an electron.

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Millikans Experiment
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• Question How did Millikan determine the charge
  on one electron if he didn't know how many
  electrons an oil drop acquired?
• Your challenge to determine the number of
  pennies in this beaker without counting the
  pennies.
• The rules1. Pennies may be removed from the
  beaker, but they cannot be counted at any
  time.2. All pennies have to be returned to the
  beaker.
• Procedure
• Need 15-20 volunteers to grab a handful of
  pennies and weigh them

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A
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Plum Pudding Model of an Atom
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• Thompson described an atom as consisting of
  small, negatively charged corpuscles situated
  inside a positively charged field by electric
  static forces.

http//nobelprize.org/physics/educational/quantise
d_world/structure-1.html
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Rutherfords Hypothesis
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Alpha (a) particles are positively charged
particles emitted by certain radioactive atoms.
If particles are shot at a thin gold foil
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Rutherfords Data
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Flash demo
In Rutherford's own words, "It was almost as if
you fired a 15-inch shell into a piece of tissue
paper and it came back and hit you."
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Rutherfords Conclusions
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• Most of his a particles passed through the foil
  without encountering the atomic nucleus, but a
  few came near enough to the nucleus to be
  deflected by the repulsion of a like charged
  nucleus.
• Nucleus was only 1/10,000th the size of the
  entire atom, but contained nearly all the mass.  

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Question How did Rutherford see the nucleus of
an atom?
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• Your challenge to determine the location and
  size of an object in a closed box.
• The rulesYou should observe slits cut in four
  sides of your box with white paper covering three
  of the four slits. This design will allow you to
  use a laser pointer as a light emitting probe to
  collect data.
• Rulers are available for data collection.
• It goes without saying that the laser
  pointer is for answering the questions posed
  above, not for pointing at people or in anyones
  eyes

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Object in a Box
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1. Describe the manner in which you collected data.
2. Sketch a figure or diagram that would be most
   useful in reporting your data.
3. What percent of the volume of the box is occupied
   by the object?
4. The object in a box is analogous to Rutherford's
   experiment. What components of the experiment
   correspond to which parts of the object in the
   box?

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Why do chemists use models/analogies?
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• macroscale
• physical properties that can be observed by the
  unaided human senses
• microscale
• samples of matter that have to be viewed with a
  microscope
• nanoscale
• samples that are at the atomic or molecular scale
  where chemical reactions occur

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Macroscale, Microscale, and Nanoscale
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Models and representations
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• How are atoms represented?
• How are elements represented?
• http//www.webelements.com/

            exactly 12 amu
12C
               
6 protons 6 neutrons
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Models of Molecules
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H2O
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Chemical Formula for water
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• Chemistry Teacher Johnny, what is the chemical
  formula of water?

Johnny HIJKLMNO.
Chemistry Teacher Thats wrong!
Johnny But yesterday you said it was H to O
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Nanoscale representations of the three states of
matter
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Concept Question 1
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• The circle on the left shows a magnified view of
  a very small portion of liquid water in a closed
  container.
• What would the magnified view show after the
  water evaporates?

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Summary
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• Atomic models have evolved as scientists
  discovered more facts
• Models help to chemists with macroscopic eyesight
  to conceptualize a nano- and microscopic world

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Reminders
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• No sandals or contacts in studio
• HW 2 Due Monday 9/18
• Pennies lab report is due next Friday 9/22