Electron Affinity : Clearing The Misconception

ElectronAffinityElectronAffinityisoneofthetopicsonwhichchemistsdonttalkmuch.However,chemistslinkelectronaffinitywithElectronegativitytodevelopabetterunderstanding.Thatswhymostoftextbooksdontprovideenoughunderstandingofthisconcept.ThepurposeofthisresearchpaperistoeliminatetheconfusionregardingthesignconventionofElectronaffinity.InthisresearchpaperIwilltrytoeliminatethemisconceptionaboutthesignconventionofElectronaffinitybycitingthemostauthenticauthorsbooksonInorganicChemistry.SignConvention:Generally,weusesignconventionsinscience(chemistry,physicsetc.)forourconvenienceandthechoiceofsignconventionisarbitrary.Insignconventionplus(+)andminus(-)signsareusedwithvariousphysicalandchemicalprocesses.Forexample:Inthermodynamics,negativechangeinenthalpy(-H)indicatesanexothermicreactionwhereaspositivechangeinenthalpy(+H)indicatesanendothermicreaction.SignConventionofElectronAffinity:Thesignconventionofelectronaffinitydependsuponthewaywedefineit.Therearetwosignconventionsofelectronaffinityandtheseconventionsdifferduetothedefinitionofelectronaffinity.Theseare:TheHistoricalsignconvention(ornon-Thermodynamicsignconvention)TheThermodynamicsignconventionTheHistoricalsignconvention(ornon-Thermodynamicsignconvention):Accordingtothissignconventiontheelectronaffinityisdefinedas:Theelectronaffinity,Eea,ofanelementistheenergyreleasedwhenanelectronisaddedtoagas-phaseatom.Apositiveelectronaffinitymeansthatenergyisreleasedwhenanelectronattachestoanatom.Anegativeelectronaffinitymeansthatenergymustbesuppliedtopushanelectronontoanatom.Thisconventionmatchestheeverydaymeaningofthetermaffinity.Moreformally,theelectronaffinityofanelementXisdefinedas:X(g)+e-(g)X-(g)Eea(X)=E(X)-E(X-)whereE(X)istheenergyofagas-phaseXatomandE(X-)istheenergyofthegas-phaseanion.Forinstance,theelectronaffinityofchlorineistheenergyreleasedintheprocessCl(g)+e-(g)Cl-(g)energyreleased=Eea(3.62eV,349kJmol-1)Becausetheelectronhasalowerenergywhenitoccupiesoneoftheatomsorbitals,thedifferenceE(Cl)-E(Cl-)ispositiveandtheelectronaffinityofchlorineispositive.Likeionizationenergies,electronaffinitiesarereportedeitherinelectronvoltsforasingleatomorinjoulespermoleofatoms.(1)FigureshowingthetrendofelectronaffinityacrossthePeriodicTableTheThermodynamicsignconvention:AccordingtothissignconventionElectronAffinityisdefinedas:Theenergychangethatoccurswhenanelectronisaddedtoagaseousatomiscalledtheelectronaffinitybecauseitmeasurestheattraction,oraffinity,oftheatomfortheaddedelectron.Formostatoms,energyisreleasedwhenanelectronisadded.Forexample,theadditionofanelectrontoachlorineatomisaccompaniedbyanenergychangeof-349kJ/mol,thenegativesignindicatingthatenergyisreleasedduringtheprocess.(2)WethereforesaythattheelectronaffinityofClis-349kJ/mol.Electronaffinity:Cl(g)+e-Cl-(g)E=-349kJ/molFigureshowingthetrendofelectronaffinityacrossthePeriodicTableAnotherdefinitionis:Theelectronaffinity(EA)istheenergychange(kJ/mol)accompanyingtheadditionof1molofelectronsto1molofgaseousatomsorions.(3)Thefirstelectronaffinity(EA1)referstotheformationof1molofmonovalent(1-)gaseousanions:Atom(g)+e-ion-(g)E=EA1FigureshowingthetrendofelectronaffinityacrossthePeriodicTableConclusions:So,wecansummarizeourdiscussionas:SignofElectronaffinitymaydifferdependinguponthedefinitionofElectronAffinityusedThermodynamicapproachdefinestheElectronaffinityasenergychangeinvolvedduringgainofelectron(s)Non-ThermodynamicorHistoricalapproachdefinesElectronaffinityasenergyreleasedduringthegainofelectron(s)Note:InmostofthefamousuniversitiesandtheauthenticbooksofInorganicchemistryHistoricalsignconventionisusedinsteadofThermodynamicconvention.References:(1)ChemicalPrinciples:TheQuestforInsight(5thedition)byPeterAtkinsandLorettaJones(2)Chemistry:Thecentralscience(11thedition)byTheodoreL.Brown,H.EugeneLeMay,Jr,BruceE.Burstenandothers(3)PrinciplesofGeneralChemistry(3rdedition)byMartinS.SilberbergResearchedby:MudassirBinQasimMTBSchoolsandCollegesSadiqAbad