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7/29/2019 Inside the atom - P3, week 8
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Ionization
Electronsarenega-velychargedpar-cles Theyorbittheatomicnucleus Neutralatom:nrofelectrons=nrofprotons Electronsarekeptinorbitbythearac-veforcefromtheprotonsinthenucleus
Toescapetheymustbegivenenergy Ioniza'on:theprocessthatgenerateschargedatoms
e.g.byradia-on
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AtomicExcitation
Excita'on: atomscanabsorbenergywithoutge@ngionized electronsinanexcitedatomjumpintothenextouterorbit deexcita-onandreturntooriginalorbithappensbyemi@nglight(aphoton)
atomsabsorbenergyatcertainvalueswhichdependonthetypeofatom
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EnergyLevels
Electronsarerestrictedtoorbitswithcertainenergies Theorbitsaredifferentforeverydifferenttypeofatom Photonsareabsorbedoremiedonlyatspecificenergies Energyofphotonsdependsontheirwavelength
wherehisthePlackconstant=4.14x1015eVs E= hf =hc
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Example
Whatisthewavelengthofaphotonemiedwhenanelectronjumps
fromthen=3energyleveldowntothen=2energylevelina
Hydrogenatom?(c=3x108m/s,h=4.14x1015eVs)
EnergyLeveln Energy
1 13.6eV
2 3.4eV
3 1.51eV4 0.85eV
5 0.54eV
Thetablebelowshowsthefirstfiveenergylevelsofahydrogenatom.
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Theelectromagneticspectrum
Theelectromagne'cspectrumistherangeofallpossiblefrequenciesofelectromagne-cradia-on
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Wave-particleduality
Wave-par'cleduality:Theamountofenergyaphotonhascancauseittobehavemorelikeawaveormorelikeapar-cle
Wearenottalkingaboutadifferenceinwhataphotonis,butabouthowitbehavesatdifferentenergies
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Thevisiblespectrum
Thevisiblespectrumistheregionoftheelectromagne-cspectrumconsis-ngofphotonswithwavelengthsfrom
approximately400to700nm
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EnergyLevels&Spectra
Spectroscopy:isthestudyoftheinterac-onbetweenmaerandradiatedenergy
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EnergyLevels&Spectra
Spectroscopy:isthestudyoftheinterac-onbetweenmaerandradiatedenergy
Threetypesofspectra: acon'nuousspectrumisasmoothdistribu-onofwavelengthsandhasnogaps
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EnergyLevels&Spectra
Spectroscopy:isthestudyoftheinterac-onbetweenmaerandradiatedenergy
Threetypesofspectra: acon'nuousspectrumisasmoothdistribu-onofwavelengthsandhasnogaps
anemissionlinespectrumisproducedbyexcitedatomsreturningtotheirgroundstate
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Photoelectricity
Aheatedcathode(metalconductor)canbeusedtoproduceasupplyofelectronsbythermionicemission
EnergycanbesuppliedtotheconductoralsointheformofUVlight
Observa-ons: Emissiononlyoccursifthefrequencyofincidentlightexceedsaminimumthreshold
frequency
11
A.Einsteinin1905
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Einsteinandphotoelectricity
In1905Einsteinpublishedapaperexplainingtheeffect Assumedthatlighttravelsinshortpacketscalledwavepackets
Photon=wavepacketoflight=quantumoflight
TheamountofenergyeachphotonhasisgivenbyE=hf=hc/where
f=frequencyofphoton h=Planckconstant=6.63x1034Js c=speedoflight=3x108m/s =wavelengthofphoton
Reminder:EnergymeasuredinJoules(some-mesalsogiveninelectronvolteV)
1eV=1.6x1019Joules12
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EnergyLevels&Spectra
Spectroscopy:isthestudyoftheinterac-onbetweenmaerandradiatedenergy
Threetypesofspectra: acon'nuousspectrumisasmoothdistribu-onofwavelengthsandhasnogaps
anemissionlinespectrumisproducedbyexcitedatomsreturningtotheirgroundstate
anabsorp'onlinespectrumisproducedwhenphotonsfromasourcewithacon-nuousspectrumpassthroughadiffusegas
cloud
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EnergyLevels&Spectra
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TheSolarspectrum
Thehotdensecentreofthesunemitsacon-nuousspectrum Thecoolerouterlayerofthesun,theHeliosphere,absorbssomeofthelightatspecificenergies
Lookingattheabsorp-onspectrumwecanfindoutwhattheSunismadeof
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Modelsoftheatom
TheBohratomForcircularmo-on:
Centripetalforce=Coulombforce +
proton
electron
F
v
r
TheBohrmodeloftheHydrogenatom
F= mv
2
r=e e
40r2= e
2
40r2
v2=
e2
40rm
ClassicalPhysics
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Modelsoftheatom
TheBohratomForcircularmo-on:
Centripetalforce=Coulombforce +
proton
electron
F
v
r
TheBohrmodeloftheHydrogenatom
F= mv
2
r=e e
40r2= e
2
40r2
v2=
e2
40rm
Bohrmodel
n=1
n=2
n=3Allowedorbits:
mvr = nh
2
r =0h2
me2n2
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E= e2
80
me2
0h2n2=
me4
80
2h2n2
Energyoftheelectron
Totalenergyoftheelectron: Kine-cenergy(mo-onaroundnucleus)
ElectricPoten-alenergy(electricfieldof+venucleus)
+
proton
electron
F
v
r
TheBohrmodeloftheHydrogenatom
K.E. =1
2mv
2=
e2
80r
P.E. =e e
40r=
e2
40r
E= K.E.+P.E=
e2
80r
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Wave-particleduality(part2):
deBrogliewavelength Energyofphoton: MassenergyequivalenceE=mc2 DeBrogliehypothesis:Apar-cleofmassmmovingwithvelocityvhasawavelengthrelatedtoitsmomentump=mv
by
Thiswavelength,isknownasthedeBrogliewavelengthofthepar-cle
E= hf = hc
=h
p=
h
mv
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Example
1) Acricketballhasamassof0.15kgandisthrowntowardsabowlerwithaspeedof40m/s.WhatisthedeBroglie
wavelengthofthecricketball?[assumeh=6.631034Js]
2) WhatisthedeBrogliewavelengthofanelectronmovingat2.2106m/s?
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Example
Calculatethewavelengthofelectronswhichhavebeen
acceleratedthroughapoten-aldifferenceof100Volts.Youcan
assumethattheelectronmassme=9.1x1031kg,theelectron
chargee=1.6x1019CandthatthePlanckconstanth=6.6x
1034
Js.
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BeyondBohr
Bohrsmodelisagoodstar-ngpointbuttoosimplis-c WorkswithHydrogenbutnotsowellwithotheratomsbecausetheelectronsinteractwitheachother
Quantummechanics Probabilis-ctreatmentoftheatom mvr = n
h
2
=h
p=
h
mv
hr
= n
h
2 2r = n
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BeyondBohr
Bohrsmodelisagoodstar-ngpointbuttoosimplis-c WorkswithHydrogenbutnotsowellwithotheratomsbecausetheelectronsinteractwitheachother
Quantummechanics Probabilis-ctreatmentoftheatom
Notthepathsof
theelectrons!
Theserepresent
theamplitudesoftheelectrons
probabilitywave
2r= n
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BeyondBohr
Bohrsmodelisagoodstar-ngpointbuttoosimplis-c WorkswithHydrogenbutnotsowellwithotheratomsbecausetheelectronsinteractwitheachother
Quantummechanics Probabilis-ctreatmentoftheatom
2r= n
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BeyondBohr
Bohrsmodelisagoodstar-ngpointbuttoosimplis-c WorkswithHydrogenbutnotsowellwithotheratomsbecausetheelectronsinteractwitheachother
Quantummechanics Probabilis-ctreatmentoftheatom
Bohrmodel
QuantumMechanicalmodelWernerHeisenberg
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ToDo
Readchapter29[p.601608Electronsinsidetheatom],skipXrays(29.3)
HomeworkAssignmentwk8:ques-ons28.14,28.15,28.16,28.17,28.18and28.19
Handitinnolaterthan4:00pmnextWednesday-LATEWORKWILLNOTBEACCEPTED
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