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ASChemistry:Unit1AliceRosen12BP

Module1:AtomsandReactions


The Changing Atom



FifthcenturyBC­Democritus
Democritusdevelopedthefirstideaoftheatom.



Hebelievedanatomcouldnotbesplitanyfurther;thisiswhatyou
wouldendupwithifyoudividedasampleofmattersomanytimes.



Hecalledthisparticle`atamos'­Greekforindivisible.




Early1800s­JohnDalton
Daltonbelievedthatatomsaretinyparticles
thatmakeupelementsandareindivisible.



Hepredictedthatallatomsofagiven
elementarethesame.



Hepredictedthatatomsofoneelementare
differentfromthoseofeveryotherelement.



Daltonusedhisownsymbolstorepresent
atomsofdifferentelementsanddeveloped
thefirsttableofatomicmasses.




1897-1906­JosephJohnThomson
Thomsondiscoveredtheelectronthroughaseriesof
experimentsusingcathoderays.



Hewasabletoshowthatatomsweremadeupofsmallerparticles,
thusdisprovingtheideasproposedbytheancientGreeksandDalton.



Hediscoveredthatcathoderayswereastreamofelectronsand
concludedthattheymusthavecomefromwithintheatomsofthe
electrodesthemselves.



Thomsonproposedthatanatomshouldbethoughtofasbeingmade
upofnegativeelectronsmovingaroundina`sea'ofpositivecharge.



This`plum-pudding'modelisneutralwithnooverallcharge.

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ASChemistry:Unit1AliceRosen12BP

Module1:AtomsandReactions


1909-1911­ErnestRutherford
Rutherfordandtwoofhisstudentscarriedoutanexperimentthat
ledtothedownfalloftheplum-puddingmodel.



Theydirectedalphaparticlestowardsasheetofverythingoldfoil
andmeasuredanydeflectionoftheparticles.



Asmallpercentageofparticlesweredeflectedsignificantly,
indicatingasmall,concentratedpositivecharge.



Heproposedthemodelofthenuclearatom:





o Thepositivechargeofanatomandmostofits
massareconcentratedinanucleus,atthecentre.





o Negativeelectronsorbitthisnucleus.






o Mostofanatom'svolumewouldbeinthespace
betweenthetinynucleusandtheorbitingelectrons.





o Theoverallpositiveandnegativechargesmustbalance.



In1918,Rutherforddiscoveredtheproton.





1913­NeilsBohr
TheDanishphysicistalteredRutherford'smodel
toallowelectronstofollowonlycertainpaths,
otherwise,electronswouldspiralintothenucleus.



Thiswastheplanetaryatom,inwhichelectrons
orbitedacentralnuclear`sun'in`shells'.



1926­ErwinSchrödinger
Schrödingersuggestedthatanelectronhaswavepropertiesinan
atomandintroducedtheideaofatomicorbitals.




1932­JamesChadwick
TheEnglishphysicistbombardedlightelementswithhigh-energy-
particles.Hefoundthatanewtypeofradiationwasemitted,much
morepenetratingthananythathadbeendetectedbefore.



Heshowedthatthisnewtypeofradiationwasmadeupofuncharged
particleswithapproximatelythesamemassasaproton,knownas
neutrons.

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