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).