Atoms Notes Class 12 PDF

Chapt er12-At oms I NTRODUCTI ON: Atomisthefundamental bui ldi ngblockofmat ter havingac...

Chapt er12-At oms I NTRODUCTI ON: Atomisthefundamental bui ldi ngblockofmat ter havingaconf inedposit ivel ychargednucl eusat thecent er,surr oundedbynegat ivel ychar gedel ectr ons.Ev eryi norgani c,organi c,oreven synt heti cobjectismadeupofatoms. Thomson' sAt omi cmodel ThomsonatomicmodelwasproposedbyWi l li am Thomsoninthey ear1900.Thi smodel expl ainedt hedescr ipt ionofaninnerst ruct ureoft heatom theoret icall y. Post ulat esofThomson’ sat omi cmodel  Post ulat e1:Anat om consi stsofaposi ti vel ychar gedspher ewi thel ect ronsembedded i nit  Post ulat e2:Anatom asawhol eisel ect ri cal l yneut ral becauset henegat iveandposi ti ve char gesareequal inmagni tude o Thomsonatomicmodeliscomparedt owat ermel on.Wher eheconsi der ed:Water mel on seedsasnegat ivel ychar gedpart icles.Ther edpartoft hewat ermel onasposi ti vel y charged Li mit ati onsofThomson’ sat omi cmodel  I tf ai l edtoexplaint hestabi li tyofanatom becausehi smodel ofatom fai l edtoexplai n howaposi t ivechar geholdsthenegat ivel ychargedelectr onsinanatom.Theref ore,Thi s theoryal sofail edtoaccountfort heposit ionofthenucleusi nanat om  Thomson’ smodel fai l edt oexpl aint hescat ter ingofal phapar ti clesbyt hinmet alf oil s  Noexper iment alev idencei nit ssuppor t Rutherf ordAtomicModel–Thepl um puddingmodeli sgivenbyJ.J.Thomsonf ailedtoexpl ain cert ainexperi mentalresul tsassoci atedwiththeatomi cstruct ureofel ement s.Ernest Rutherf ord,aBri ti shscienti stconductedanexperi mentandbasedont heobservationsofthi s experi ment,heproposedtheatomicst r uct ureofelement sandgav eRuther fordAt omicModel. Rut her for d’sAl phaScat ter ingExper iment AsperRut herford’s suggest ions, Geigerand Mar sdenconduct edan exper imentby bombar dingat hi nsheet ofgol dwithα- parti cles andt henst udiedthe trajectoryoft hese par ti clesaf tertheir i nt eracti onwi ththegol d foil. Rutherford,inhi sexperiment,di rectedhighener gystr eamsofα-part icl esfr om aradi oacti ve sourceatat hi nsheet(100nm thickness)ofgol d.I nordertost udythedefl ect ioncausedtothe α-part icles,heplacedafluor escentzincsulphi descr eenaroundthethi ngoldf oil.Ruther for d madecer tai nobser v ati onsthatcont r adi ctedThomson’sat omicmodel. Obser vat ionsofRut her for d’sAl phaScat ter ingExper iment Theobser vat ionsmadebyRut her for dledhi mtoconcl udet hat :  Amajorfract ionoftheα-par ti clesbombar dedtowardst hegoldsheetpassedt hroughi t wi thoutanydeflect ion, andhencemostofthespaceinanat om isempty.  Someoft heα-part icl esweredefl ect edbythegol dsheetbyv er ysmallangl es,andhence theposi ti v echargeinanat om isnotuni for mlydi str ibut ed.Theposi ti vechargeinan atom i sconcentratedi naverysmallvol ume.  Veryfewoft heα- part iclesweredef lect edback,thati sonlyafewα-par ticl eshadnear ly 180oangl eofdef lect ion.Sothevol umeoccupiedbyt heposi ti vel ychargedpart icl esi n anatom i sver ysmallascomparedtothet otalvolumeofanatom. Rut her for dAt omi cModel Basedontheaboveobser vat ionsandconcl usi ons, Ruther for dpr oposedt heat omi cst ruct ureof el ements.Accor dingt otheRuther for datomicmodel:  Theposi ti velychar gedpart icl esandmostoft hemassofanatom wasconcent rat edi n anext remelysmallvol ume.Hecall edt hisr egi onoft heatom asanucleus.  Rutherf ordmodelproposedthatthenegati vel ychargedel ect ronssurroundthenucleus ofanatom.Healsoclaimedt hatt heelect ronssur roundi ngthenucleusrevol v ear oundit wit hv eryhi ghspeedincircul arpaths.Henamedthesecir cul arpathsasorbits.  El ectr onsbei ngnegat ivel ychargedandnucleusbei ngadenselyconcentr atedmassof posi ti vel ychar gedpar ticl esareheldt ogetherbyastrongel ect rostat icfor ceofatt ract ion. Li mit ati onsofRut her for dAt omi cModel Alt houghtheRutherf ordat omi cmodel wasbasedonexper iment alobser vat ionsi tfai l edt o expl aincer tai nthings.  Ruther fordproposedthattheelectr onsr evolvear oundthenucl eusinfi xedpat hscal l ed orbits.Accor di ngt oMaxwell ,acceleratedchar gedpar t icl esemi tel ectr omagnet ic radiationsandhenceanel ectronrev ol vingaroundt henucleusshouldemi t electromagnet i cr adi ati on.Thi sr adiati onwoul dcar ryenergyfrom themot i onoft he electronwhi chwoul dcomeatt hecostofshr inkingofor bit s.Ultimatel ytheelect r ons woul dcol l apseint henucl eus.Calculati onshav eshownt hataspert heRutherfor dmodel, anel ectronwoul dcoll apseinthenucl eusinlesst han10- 8seconds.SoRut herford model wasnoti naccordancewi t hMaxwel l ’stheor yandcoul dnotexplainthest abil i tyof anat om.   Oneofthedr awbacksoftheRuther for dmodel wasalsot hathedidnotsayanyt hing aboutt hearr angementofel ectr onsinanat om whi chmadehist heoryincompl ete. I mpactpar amet er Theperpendi cul ardist anceofthev eloci tyvectorofa-part icl efrom t hecent ral li neoft he nucl eus,whenthepart icl eisf arawayfrom thenucleusiscall edimpactpar ameter. where, Z=at omi cnumberoft henucl eus, Ek=ki net icener gyoft hec- par ti cleandθ=angl eof scat ter ing. Al phapar ti clet raj ect ory Thet r ajector ytracedbyanα- particl edependsont heimpactpar amet er, bofcol li sion.The i mpactpar amet eristheper pendiculardist anceoft heini tial velocityvectoroft heα-particle fr om t hecent reoft henucl eus.Agi venbeam ofα- part icl eshasadi stri butionofi mpact paramet ersb, sot hatthebeam i sscat teredi nv ar iousdirect ionswi thdifferentpr obabil ities.(I n abeam, allparti cleshav enearlysameki net i cener gy. )Iti sseent hatanα- particlecloset othe nucleus( smal li mpactpar amet er)suffersl argescat t eri ng.Incaseofhead- oncol l ision,the i mpactpar amet erismi nimum andt heα- particl ereboundsback( θ≅ ).Foral argei mpact paramet er,theα- part iclegoesnear l yundev i atedandhasasmal ldefl ection( θ≅0).Thef actthat onlyasmal lfractionoft henumberofi nci dentparticl esreboundbacki ndicatest hatt henumber ofα-particl esunder goingheadoncol l isioni ssmal l. ATOMI CSPECTRA: Whenanelectr onj umpsamongstener gyl evel sinanatom, ener gyisemit tedorabsor bedi nthe f orm ofel ectr omagnet icr adiat ionsandtheseradi ati onsproduceaspectr all inesof f requenci es( orwavel engt h)associ atedwithanatom,cal ledatomicspect ra Spectr oscopyi sthel earni ngandexami nat ionemi ssi onandabsor pti onspect raassoci atedwi th anatom t odeter minei tsproper ti es Spectralli nesaret hebri ghtanddarkl i neser iesthatconstit utethespect rum associat edwit han atom.Anat om hasadi scretespect ra(wher eexistafixedspecifi cli nesofenergytransi ti onof elect ronwi t hdiscreteenergygaps)alsoknownasquant izedspectr a.Anothert ypei s conti nuousspect ra(wheretherei snospecifi cli nesofener gytransi ti onofelect rons)whichi s thereverseofdiscretespectra SPECTRALSERI ES: Onpassingel ectr icdischar gethr oughhydrogengas, hydrogenmolecul eswoulddissoci ate gi vingri setoexci ted(highl yener geti c)hy drogenatomsthatemitr adiat ionofcer tainspeci fi ed fr equencywhil eretur ningtoit sgroundst ate Hydr ogenspectraisconstit utedof5ser iesofspect rum namedaf tert hei rdi scov erer (Ly man, Bal mer,Paschen,BracketandPfundser ies) SERI ES: Bal merSer ies: Fi rstsci ent istt odi scov eraspect ral ser iesofhy drogenat om wasJohannJakobBal mer. I tconsi stofv isi bler adi ati onspect rum Exper iment all y,hefoundt hatt hesespect ral li nescoul dbeexpr essedmat hemat ical l yint he for m ofwavel engthas: HereR=Rhydber gconstant=109677cm- 1(f oundexperi mental ly ),n=3,4, 5……(higherdi scr ete ener gyst atefrom whi chelect ronj umpsto2ndenergystat ethusemit ti ngradi ati on) λ=wav elengt hofemi tt edr adi ati oni n(cm) o Formaxi mum wav elengt h(λmax)i ntheBal merser ies, n=3( hast obemi nimum) : o Formi nimum wav elengt h(λmi n)i ntheBal merser ies, n=∞( hast obemi nimum) : Ly manSer ies: Spectr alser ieswhenr adi ati onemi tt edi sduet ojumpi ngofel ect ronf rom hi gherener gyst ates togroundst ate Mat hemat ical l yexpr essedas Her en=2, 3,4… Formaxi mum wav elengt hint heLy manser ies, n=2( hast obemi nimum) : λmax=4/ 3R Formi nimum wav elengt hint heLy manser ies, n=∞( hast obemi nimum) : λmi n=1/ R Si mil arl y,al lot herser iescoul dbeexpr essedas: I ntr oduct iont otheBohrModel Bohrmodeloftheatom waspr oposedbyNeil Bohri n1915.I tcamei ntoexi stencewi tht he modif icat ionofRuther ford’ smodelofanat om Bohr ’smodelconsi stsofasmallnucleus(posi ti v el ycharged)sur roundedbynegat iveel ectrons movi ngaroundthenucleusinor bits.Bohrf oundt hatanelect ronl ocatedawayfrom t henucleus hasmoreenergy ,andelectr onsclosetothenucleushaveless energy. Post ulat esofBohr ’sModelofanAt om  I nanat om, el ectr ons(negat ivel ycharged)r evolv ear oundt heposi ti vel ychar gednucl eus i nadef ini teci rcul arpathcal ledorbi tsorshell s.  Eachorbi torshel lhasaf ixedener gyandt heseci rcul aror bit sar eknownasor bit al shel ls.  Theenergyl evelsarerepresent edbyaninteger(n=1,2, 3…)knownast hequantum number.Thi srangeofquant um numberst artsfr om nucleussidewit hn=1hav i ngthe l owestenergylevel.Theor bi tsn=1,2, 3,4…areassignedasK, L,M,N….shel lsandwhen anel ectronatt ainsthelowestenergyl eveli tissaidtobeinthegroundstat e.  Theelect ronsinanatom movef rom alowerenergyl evel toahigherener gylev elby gai ningtherequir edener gyandanelectr onmov esfr om ahi gherenergyl evelt olower ener gylevelbylosi ngener gy. Li mit ati onsofBohr ’sModelofanAt om Itisappli cabletohydr ogeni catoms.I tcannotbeevenextendedt otwoel ectr onatom ashel i um. Ther easonis,i nmanyelect ronatoms,t heelect ronint eract snotonl ywi ththenucl eusbutother electr onsaswel l. Bohr 'smodeli nvol vesel ect ri cal for cebet weenposi ti vel ychar gednucl eusandel ect ronbutnot bet weentheel ectr ons I tcoul dnotexpl aint her elat ivei ntensi ti esoft hedi ff erentl i nesi nthespect rum.