Ch 17. Radioactivity and Nuclear Chemistry
Chem 30A
Nuclear Activity
• Thenucleusdoesnotstronglyaffectthechemistryoftheatom(bondmaking/breaking).
• HOWEVER,thenucleuscanundergochanges,whichusuallycauseanelementtochangeintoanotherelement.
Implications of Nuclear Activity for Humans
• HazardouseffectsofradiaDon• MedicalapplicaDonsofradioacDvity• DaDngbyradioacDvity• Nuclearenergy• Nuclearweapons
Radioactivity
Radioactivity (Radioactive Decay)
RadioacDvity:thespontaneousdecomposiDonofatomicnuclei,whichreleaseshigh-energyparDclesorrays• RadioacDvedecayresultsinadifferentnucleus.• Nuclide:anucleuswithaspecifiednumberofprotonsandneutrons(=aspecificisotope)
Isotope Symbol
ZAX ElementSymbol
AtomicNumber(numberofprotons)IdenDfieselement
MassNumber(numberofprotons+neutrons
Similarnota+onforsubatomicpar+clesProton NeutronElectron1
1p 01n −1
0e
Radioactivity
• Everyelementhasatleastoneisotopewhosenucleusspontaneouslydecomposes(radioisotope).
• AcertainlevelofradiaDonnaturallyoccursallaroundus,eveninourbodies(potassium-40).
Nuclear Equations
InabalancednuclearequaDon,boththeatomicnumberandthemassnumbermustbeconserved.
Types of Radioactivity
1. AlphaParDcle(α)Emission
• αparDcle=heliumnucleus• Neteffect:lossof4inmassnumber,lossof2inatomicnumber
Types of Radioactivity
2. BetaParDcle(β)Emission
• βparDcle:anelectron• Neteffect:Aneutronischangedintoaproton.
01n→ 1
1p+ −10e
Types of Radioactivity
3. GammaRay(γ)Emission
• Gammaray:high-energyphotonoflight(nochargeormass)
• O^enaccompaniesdifferenttypesofnucleardecays
• Neteffect:nochangeinmassoratomicnumber
Types of Radioactivity
4. PositronEmission
• Positron(β+):aparDclewithsamemassaselectronbutoppositecharge
• Neteffect:Aprotonischangedintoaneutron.
11p→ 0
1n+ 10e
Types of Radioactivity
ElectronCapture
• Oneoftheinner-orbitalelectronsiscapturedbythenucleus.
• Netresult:Changesprotonintoneutron.• Awaytomakegold?ImpracDcal
Summary: Radioactivity
ExProbs
Nuclear Transmutation
• NucleartransmutaDon:thechangeofoneelementintoanotherbyparDclebombardment(neutron,αparDcle,proton,nucleus)
• Mostofthe~3300knownradioisotopesaremadethrougharDficialnucleartransmutaDoninparDcleaccelerators.
714N + 0
1n→ 614C + 1
1H
92238U + 2
4He→ 94241Pu+ 0
1n
Nuclear Transformation, cont’d
• Allelementswithatomicnumbersgreaterthan92(transuraniumelements)havebeenarDficiallymade.
Half Life
Detection of Radioactivity
• Geiger-Müllercounter:High-energyparDclesionizeargon;argonionscanconductelectricity
• ScinDllaDoncounter:Usesasubstancethatgivesofflightwhenstruckbyhigh-energyparDcles
• Thermoluminescentdosimeters:Electronsexcitedbyhigh-energyparDclesaretrappedincrystals,thenheatedtorelaxintogroundstateandemitlight
Half-Life
Half-life:Timerequiredforhalfoftheoriginalsampleofnucleitodecay 1min 1min
1mil1/2mil1/4mil 4.5bilyrs 4.5bilyrs1mil1/2mil1/4mil
86220Rn
86220Rn 86
220Rn
92238U
92238U 92
238U
Half Life of Th-232
Half-lives of Radium Nuclides
ExProbs
U-238 Decay Series
Inmanycases,daughternuclidesarealsoradioacDve.
Application: Radiocarbon Dating (C-14 Dating)
C-14Made
C-14Brokendown
Half-lifeof14C=5715years• Atmospherehasconstantamountof14C.• 14Ccontentinalivingplantisconstantdueto
constantuptakeof14CO2byplant.• Atmomentofdeath,plant14Cdecayswithout
replenishment.• So14CcontentoffossiltellshowmuchDmehas
passedsincetheplantdied(age).
14 7N 1
014 6
11n C H+ +
14 6C 0
-1e + 14 7N
Effect of Radiation on Life
Damaging Effects of Radiation
• Ionizingpower:abilityofradiaDontoionizemoleculesandatoms.MoremassiveparDcleshavemorepotenDaltointeractwithandionizeothermolecules.
• PenetraDngpower:abilityofradiaDontopenetratemaier.SmallerparDclescanpenetratemoredeeply.
Effects of Different Types of Radiation StoppedBySheetofpaper,clothingSheetofmetal,thickwoodThickleadorconcreteSheetofmetal,thickwood(alsoforgammaray)
γray
Types of Radiation Damages
• AcuteradiaDondamage:damagetocellsresulDngfromexposurestolargeamountsofradiaDoninshortperiodofDme
• Increasedcancerrisk:LowerradiaDondosesoveralongerperiodofDmecancausedamagetosomaDccellsandincreasecancerrisk.
• GeneDcdefects:damagetoDNAinreproducDvecellsthatshowseffectinoffspring
Effects of Short-Term Radiation Exposure
Rem:AunitthatindicatesbiologicaldamageofradiaDon
Biological Effects of Radiation
TypicalRadiaDonExposureinU.S.
Nuclear Medicine Application: Radiotracers
• Radiotracers:radioacDvenuclidesthatcanbeputintoorganismsthroughfoodordrugs,andbetracedformedicaldiagnosDcs
Radiotracers in PET Scans
• PET:positronemissiontomography• Examplesofpositron-producingisotopes:18F,
11C• TheseradioacDveisotopesareaiachedtoglucose.
• Areasofbrainrapidlyconsumingtheseradiotracers“lightup”onPETscreen.
• Shorthalf-lives(110minfor18F,20minfor11C)requirespeedysynthesis.
Nuclear Medicine Application: Radiotherapy
• Gammaraysareusedtokillrapidlydividingcancercells.
• PaDentsundergoingradiotherapydevelopsymptomsofradiaDonsickness(vomiDng,hairloss,skinburns).