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Unit 3 - Atomic StructureUnit 3 - Atomic StructureChapter 5Chapter 5
Prof.devender singh RoorkeeProf.devender singh Roorkee
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Earliest Model
450 BC Democritis, aGreek philosopher, firstuses the word atomoswhich means indivisibleDefinition of atom
today Smallestparticle of an elementthat still retainsproperties of thatelement
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Late 1700s
Lavoisier Law of Conservation of Matter
Proust Law of Constant Composition
This says that the same compound from
any source always contains the sameelements in the same proportion by mass
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First Atomic Theory
John DaltonProposed in 1803Compilation of other peopleswork 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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Daltons Atomic Theory
Each element is composed of tiny atomsEach elements atoms are the same anduniqueAtoms are only rearranged in anychemical reaction
A compound has the same number andkind of atom.
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The Atom Today
Since 1981 we have seen atoms witha scanning tunneling microscope.Uses a fine tip and a stableenvironment to trace the electronicfield and image it on a computer Lots of galleries on the Web:
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A Good Example
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Discovery Atomic Structure
Early research comes from physicistswork on electricityElectricity is property of electron, whichis amber In ancient Greece, pieces of amber wererubbed and static electricity dischargedBen Franklin did early research in late1700s
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Great Experimenter
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His work
Discovered two kinds of charges, positiveand negativeOpposite charges attractLike charges repelObjects pick up charges
They discharge when touched to ground
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Lightning
He felt that
lightning wasstaticelectricity on a
larger scale.http://news.nationalgeographic.com/news/2006/05/0504
_060504_lightning_video.html
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Electricity Research after Franklin
Physicists liked to zap things in the mid-1800sCathode Ray Tube was device used bymany (diagram)Same device used as TV screen
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Cathode Ray Tube
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How it Works
Metal is electrified in an evacuated tubeAll metals gave a greenish ray going tothe positive electrodeRay could be attracted by a positivecharge, repelled by a negative charge.
It could actually make a paddle wheelmove - particle
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Discovery of the Electron
JJ Thomson Cavendish Lab - 1896Used cathode ray tube to determine amount of deflection
Determined that particle has a negative chargeDetermined the charge to mass ratio of theparticle
Animation: 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 RobertMillikan
Famous Oil Drop Experiment(handout)See animation
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Millikan Oil Drop Experiment
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Explanation
Drops of oil are sprayed into a chamber X-rays cause electrons to be formed andthey cling to oil (in varying numbers)Drops pass through a set of electric plateswhich can have a charge put on them
Millikan adjusted charge to balance thecharge on each dropFound the greatest common factor
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Conclusion
Charge on an electron is 1.60 X 10 -19 CoulombsMass of an electron is 9.11 X 10 -19 gramsVirtually without mass
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Discovery of Radiation
Henri Becquerel accidentally discoveredradiation in 1896Photographic plate wrapped and put indrawer for weekend gets exposedRock was radiating something
Rock was pitchblende which containsradium
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Characteristics of radiation
Spontaneously emitted by some elementsStudied by Marie and Pierre Curie
They discovered several elements(uranium, radium, and polonium)Atom emits radiation and then changes
This gave clues to what atom is actuallymade of
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Marie and Pierre Curie
Good sitehttp://www.aip.org/hist
http://www.aip.org/history/curie/http://www.aip.org/history/curie/8/14/2019 Cathode Rays Anode
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Further Research on Radiation
Ernst Rutherford is brought to CavendishLab in early 1900sStudied radioactivityAnalyzed nature of radiationHandout
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Three Kinds of Radiation
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Magic Bullet
Alpha Particle chosenRight size
Could be detected afterwardsHelium nucleus 2 protons and 2neutrons
+2 charge
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Gold Foil Experiment
Rutherford got gradstudents to design set upGeiger and Marsden
Wanted to confirmThomsons PlumPudding model of theatom electrons stuck inpositive puddingHandout
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Plum Pudding Model
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Rutherfords Experiment
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Explanation
Find a source of alpha particlesAim them at thin gold foil (like bullets attissue paper)Check to see where they come out bycounting fluorescent spots
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Results
Most went throughVery small number were deflected almoststraight backOnly explanation was that all matter wasconcentrated into a dense nucleus
Nucleus had a positive chargeElectrons traveled in empty space aroundthe nucleus
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Modern Atomic Theory
Three major particles protons, electrons,and neutronsNow there are many subatomic particles(nuclear physics)Proton also discovered with the cathoderay tube in 1900Neutron discovered last by Chadwick in1935
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Summary of Particles
PROTON NEUTRON ELECTRON
+1 NONE -1
LARGE LARGE VERY SMALL
NUCLEUS NUCLEUS OUTSIDENUCLEUS
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Planetary Model
Proposed by RutherfordElectron orbits nucleus like a planetaround the sunAtoms are neutral so
#protons = #electronsCharge on electron: 1.602 X 10 -19 C issimplied to 1Mass of proton: 1.67 X 10 -24 g issimplified to 1 amu (atomic mass unit)
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Atomic Number
Defined by Henry Mosely (1887-1915)Student of Rutherford
Unique for each elementNumber of protons in the nucleusWhat is atomic number of
nitrogen? Uranium?
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Isotopes
Means type or formAll atoms of the same element have thesame number of protonsThere may be different types of the sameelements, 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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Atomic Mass
Mass of an isotope in amus is simply themass number Most elements have several commonisotopesMass on periodic table must reflect this,that is why there are decimalsWeighted average calculation (likegrades)
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Calculation
Multiply the mass of each isotope by itsabundance as a decimalAdd each of these to get weightedaverageTry 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 areadout
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Mass Spectrometer
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Animation
http://www.colby.edu/chemistry/OChem/D
http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.htmlhttp://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html8/14/2019 Cathode Rays Anode
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Computer Printout