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Unit 3 - Atomic StructureUnit 3 - Atomic Structure
Chapter 5 Chapter 5
Addison/Wesley TextbookAddison/Wesley Textbook
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Earliest Model
450 BC – Democritis, a Greek philosopher, first uses the word “atomos” which means indivisible
Definition of atom today – Smallest particle of an element that still retains properties of that element
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Late 1700’s
Lavoisier – Law of Conservation of Matter
Proust – Law of Constant Composition
This says that the same compound from any source always contains the same elements in the same proportion by mass
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First Atomic Theory
John DaltonProposed in 1803Compilation of other people’s
work and a little of his ownStill true except for one partGood biography:
http://www.slcc.edu/schools/hum_sci/physics/whatis/biography/dalton.html
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Dalton’s Atomic Theory
Each element is composed of tiny, indivisible atoms
Each element’s atoms are the same and unique
Atoms are only rearranged in any chemical reaction
A compound has the same number and kind of atom.
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The Atom Today
Since 1981 we have “seen” atoms with a scanning tunneling microscope.
Uses a fine tip and a stable environment to trace the electronic field and image it on a computer
Lots of galleries on the Web:http://physics.nist.gov/GenInt/STM/
fig1.html
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A Good Example
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Discovery Atomic Structure
Early research comes from physicists’ work on electricity
“Electricity” is property of “electron”, which is amber
In ancient Greece, pieces of amber were rubbed and static electricity discharged
Ben Franklin did early research in late 1700’s
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Great Experimenter
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His work
Discovered two kinds of charges, positive and negative
Opposite charges attractLike charges repelObjects pick up chargesThey discharge when touched to ground
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Lightning
New research all the timeGreat photography – Check YouTubehttp://www.pbs.org/wgbh/nova/sciencenow
/3214/02.html
http://www.teachersdomain.org/resource/phy03.sci.phys.mfw.lightning/
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Electricity Research after Franklin
Physicists liked to zap things in the mid 1800’s
Cathode ray tube was device used by many (diagram)
Same device used as TV screen
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Cathode Ray TubeCathode Ray Tube
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How it Works
Metal is electrified in an evacuated tubeAll metals gave a greenish ray going to the
positive electrodeRay could be attracted by a positive
charge, repelled by a negative charge.It could actually make a paddle wheel
move - particle
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Discovery of the Electron
JJ Thomson – Cavendish Lab - 1896Used cathode ray tube to determine amount of
deflectionDetermined that particle has a negative chargeDetermined the charge to mass ratio of the
particleAnimation:
http://highered.mcgraw-hill.com/sites/0072512644/student_view0/chapter2/animations_center.html#
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Finding the Charge of an Electron
American physicist – Robert MillikanFamous Oil Drop Experiment (handout)See animation
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Explanation
Drops of oil are sprayed into a chamber X-rays cause electrons to be formed they cling to oil (in varying numbers) Drops pass through a set of electric plates which have a charge Millikan adjusted charge to balance the charge on each drop Found the greatest common factor
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Conclusion
Charge on an electron is 1.60 X 10-19 Coulombs
Mass of an electron is 9.11 X 10-19 gramsVirtually without mass
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Discovery of Radiation
Henri Becquerel accidentally discovered radiation in 1896
Photographic plate wrapped and put in drawer for weekend gets exposed
Rock was “radiating” somethingRock was pitchblende which contains
radium
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Characteristics of radiationSpontaneously emitted by some elementsStudied by Marie and Pierre CurieThey discovered several elements,
including uranium and poloniumAtom emits radiation and then changesThis gave clues to what atom is actually
made of
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Marie and Pierre Curie
Good sitehttp://www.aip.org/hist
ory/curie/
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Further Research on Radiation
Ernst Rutherford is brought to Cavendish Lab in early 1900’s
Studied radioactivityAnalyzed nature of radiationHandout
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Magic Bullet
Alpha Particle chosenRight sizeCould be detected afterwardsHelium nucleus – 2 protons and 2
neutrons+2 charge
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Gold Foil Experiment
Rutherford got grad students to design set up
Geiger and MarsdenWanted to confirm
Thomson’s “Plum Pudding” model of the atom – electrons stuck in positive pudding
Handout
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Explanation
Find a source of alpha particlesAim them at a piece of gold foilCheck to see where they come out by
counting fluorescent spots
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Results
Most went throughVery small number were deflected almost
straight backOnly explanation was that all matter was
concentrated into a dense nucleus Nucleus had a positive chargeElectrons traveled in empty space around
the nucleusMovie: Empty Space
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Results
Most went throughVery small number were deflected almost
straight backOnly explanation was that all matter was
concentrated into a dense nucleus Nucleus had a positive chargeElectrons traveled in empty space around
the nucleusMovie: Empty Space – next slide
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Atom is Empty SpaceFrom NOVA
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Modern Atomic TheoryThere are 3 major subatomic particles
(protons, neutrons, and electrons).
There are basic particles that make these up but we will not discuss them
The proton also came from the cathode ray tube
The neutron was discovered by Chadwick, a student of Rutherford in 1935.
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Summary of Particles
OUTSIDE
NUCLEUS
NUCLEUSNUCLEUS
VERY SMALLLARGELARGE
-1NONE+1
ELECTRONNEUTRONPROTON
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Planetary Model
Proposed by RutherfordElectrons orbit nucleus like planets around sunAtoms are neutral so
#protons = #electronsCharge on electron: 1.602 X 10-19 C or “1”Mass of proton: 1.67 X 10-24 g or 1 amu (atomic mass unit)
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Atomic Number
Defined by Henry Mosely (1887-1915)Student of RutherfordUnique for each elementNumber of protons in the nucleusWhat is atomic number of
nitrogen? Uranium?
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Isotopes
Means “type or form”All atoms of the same element have the
same number of protonsThere may be different types of the same
elements, called isotopesVary in number of neutrons, massTry Carbon-12 and Carbon-14
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Characteristics of Isotopes
Varying massesSame chemical and
physical propertiesSome may be
unstable, and therefore radioactive
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Symbol
Carbon-12 12 is mass number,
# protons + # neutronsAlso written
126C
Mass # - Atomic # = # of neutrons
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Atomic Mass
Mass of an isotope in amu’s is simply the
Mass numberMost elements have several common
isotopesMass on periodic table must reflect this,
that is why there are decimalsWeighted average calculation (like grades)
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Calculation
Multiply the mass of each isotope by its abundance as a decimal
Add each of these to get weighted average
Try one
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Mass Spectrometer
Inject gaseous form of elementStrip electrons (positive charge)Sort by size with a magnetic fieldComputer counts the isotope and gives
a readout
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Animation
http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html
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