cathode rays anode

8/14/2019 Cathode Rays Anode

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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/


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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.html


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Computer Printout

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