The Scientific Method

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The Scientific Method
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Scientific Method

• A series of logical steps to follow, including
  experimentation, in order to solve problems or
  answer questions.
• The Process is NOT linear but circular.

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OR
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Hypothesis

• Preceded by a question
• Takes the general form
• If question , then prediction or outcome,
  because tentative explanation.
• Example QUESTION why doesnt my flashlight
  light?
• HYPOTHESIS If a flashlight doesnt light then
  the batteries are exhausted, because the power
  for the flashlight comes from the batteries.
• A scientific hypothesis MUST be testable!!

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THEORYS Explain

• In Science, a Hypothesis is an attempt at an
  explanation for the events that have been
  observed.  A hypothesis has to be testable.
• If a lot of evidence (data) is collected through
  experiments to support the hypothesis, then
  scientists accept the hypothesis as a good
  explanation.
• An accepted hypothesis may, given even more
  testing and agreement, become an accepted theory.
  Usually takes many years. 

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LAWS Describe

• Scientific Laws DESCRIBE observations, they DO
  NOT EXPLAIN them.
• Experiments are not usually associated with Laws.
• Remember this very important idea  Isaac Newton
  knew what gravity did, but he could not explain
  why gravity did it.  Even today the topic of
  what is gravity is an active topic for
  scientific discussion using Einsteins general
  theory of relativity.

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You Can Never Turn A Theory Into A Law!!
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Summary

• Scientific Method is Circular
• Science is Social (scientists talk to scientists)
• A Law Describes
• A Theory explains
• Cant change a law into a theory or vice versa.

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The Modern Atomic Model of the Atom

• Dalton, Thomson and Rutherford

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What do We Know About the Atom?

• and
• How do we know what we know?

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Outline

• Early ideas about the atom
• Discovery of the electron
• Discovery of the nucleus and protons
• Atomic Number, Mass Number
• Atomic Mass

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Greek Model (Democrites)

• Everything is composed of earth, fire, wind and
  water
• Atoms are the smallest building block of matter
• Atoms are indivisible
• All atoms are made of the same thing.
• Atoms of different materials are different
  shapes. (tree atoms look like little trees, rock
  atoms look like little rocks)

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Daltons View of the Atom (1803)

• Elements are made of tiny indivisible particles
  called atoms.
• All atoms of one element are unique from atoms of
  other elements
• All atoms of a given element are identical and
  have the same properties.
• Atoms of one element can combine with atoms of
  other elements in whole number ratios to form
  compounds. (Law of constant composition)
• Atoms are not created or destroyed in a chemical
  reaction they are just rearranged into new
  compounds. (Law of conservation of matter)

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Crookes Tube
Cathode ray
Cathode (-)
Anode ()
MAGNET
DC Power Supply
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Thomsons Cathode Ray Tube (1897)

Second DC Power Supply
Cathode (-)
Anode ()
-
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CRT Tube Video (Click to start)
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Thomsons experiment results

• Beam was made of a particle 2000 times smaller
  than the smallest atom (hydrogen)
• Beam was negatively charged attracted to the
  positive electrode of a second power supply.
  (electron)
• Couldnt find the corresponding positive
  particle.
• Assumed atom had a fog of positive charge in
  which the electrons were dispersed.

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Plum Pudding Model
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Some Information about Radiation

• Alpha particles are large positively charged
  particles they are helium nuclei
• Beta particles are smaller negatively charged
  particles they are electrons
• Gamma rays are electrically neutral, high energy
  electromagnetic radiation.

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Rutherfords Experiment
Radioactive a
Box
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Animation
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Rutherfords Gold Foil Experiment

• Trying to Prove Plum Pudding Model
• Shot pos. () charged alpha particles at thin
  gold foil.
• Expected all to go straight through.
• Most did go straight through but a few (1 in
  20,000) bounced off.

Gold Foil
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Rutherfords Astonishing Conclusion

• Atoms are mostly empty space !
• There is a very dense, positively charged nucleus
  that contains most of the mass.

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SummaryThomson and Rutherford

• Thomson the atom is made up of smaller
  particles
• Thomson one of the smaller particles is
  negatively charged
• Rutherford - The atom has a dense positive
  center.
• Rutherford - The atom is mostly empty space.

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Rutherford Model

• Center positive nucleus with neg. electrons
  dispersed around nucleus
• Atom is mostly empty space.

Large Positive Nucleus
Empty space
Electrons (randomly dispersed)
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Along Comes Neils Bohr
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Bohr Model

• Electrons are in discreet energy levels orbiting
  around the nucleus.

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And Then de Broglie - The Wave Model

• If electrons are particles and they orbit the
  nucleus, why dont they eventually spiral into
  the nucleus?
• Electrons are waves not particles!
  
• Electrons are everywhere around
• The nucleus all at once.
• The Double Slit Experiment

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Wave Model (de Broglie)

• Electron are traveling at near the speed of
  light.
• Electrons are everywhere all at once in their
  energy level or orbital around the nucleus
• Electrons are in standing waves of potential
  around the nucleus at discreet distances from the
  nucleus (Bohr)

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Electrons as Waves The Double Slit Experiment.
(Click to start)
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(No Transcript)
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The Atom and its Parts

• Nucleus
• Protons and Neutrons
• Protons and neutrons are made up of quarks
• Around the Nucleus
• Electrons electrons equals protons when the
  atom has no charge.
• In Between Electrons and Nucleus EMPTY SPACE
  !!!

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• Atomic number Z, the number of protons in the
  atom (Unique to an element, if the atomic number
  is different the element is different)
• Mass number M, the number of protons and
  neutrons in the atom.
• Isotopes are atoms of the same element, ie the
  have the same atomic number, but they have a
  different mass number. They differ in the number
  of neutrons. The chemical properties of the
  isotopes are the same, but they have different
  masses.

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Notation
Mass Number of protons neutrons
Atomic Number of protons
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Practice

• Find the number of protons, neutrons and
  electrons in the following elements

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Answers

• 17 protons
• 17 electrons
• 35-17 18 neutrons
• 95 protons
• 95 electrons
• 243-95 148 neutrons

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The Atom in Summary
Particle Mass Charge Location in Atom
Electron 9.1x10-28g 1/1000 amu -1.6022x10-19C (-1) Outside the nucleus
Proton 1.67 x10-24g 1 amu 1.6022x10-19C (1) Inside the nucleus
Neutron 1.67 x10-24g 1 amu 0 Inside the nucleus
(1amu1.66x10-24g)
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Atomic Mass

• Definition Mass of an atom of a particular
  element in amu (atomic mass unit)
• 1 amu 1.67 x 10-24 g (mass of a proton)
• Average Atomic Mass is the weighted average of
  all of the atomic masses of the isotopes in a
  naturally occurring sample of the element ON THE
  PERIODIC TABLE

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Some Useful Internet Sites

• Cathode Ray Tube http//www.chem.uiuc.edu/clcwebs
  ite/video/Cath.avi
• Gold Foil http//micro.magnet.fsu.edu/electromag
  /java/rutherford/
• Millikan Oil Drop, http//www.physchem.co.za/Stati
  c20Electricity/Millikan.htm

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Law of Constant Composition (or Definite
Proportions)

• Dalton - 1803
• different samples of the same compound always
  contain its constituent elements in the same
  proportion by mass
• Example
• Water always contains hydrogen and oxygen in a
  mass ratio of 18 no matter how it is formed or
  where it is found.

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Law of Multiple proportions

• If two elements can combine to form more than one
  compound, the masses of one element that combine
  with a fixed mass of the other element are in
  ratios of small whole numbers.
• Example
• Water and Hydrogen peroxide both contain
  hydrogen and oxygen
• Water HO by mass is 18
• Hydrogen peroxide is 116

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Cathode Ray Tube ExperimentThomson - 1897