Atoms, Molecules & Ions · “Leucippo è l’iniziatore di quel famigerato sistema atomistico,...
Transcript of Atoms, Molecules & Ions · “Leucippo è l’iniziatore di quel famigerato sistema atomistico,...
Atoms,Molecules&Ions
“Leucippo è l’iniziatore di quel famigerato sistema atomistico,che, risuscitato modernamente, è passato come principiofondamentaledell’indaginerazionalesullanatura.Esaminatoquestosistemapersè, lotroviamobensìassaipoveroetaledaoffrirepocoallaricerca;madeveesserericonosciutoaLeucippo il grande merito di aver distinto, come si dicecomunemente nella nostra fisica, le qualità generali dei corpisensibili,o leprimariedalle secondarie,o leessenzialidallenonessenziali.”
GeorgWilhelmFriedrichHegel“LezionidiStoriadellaFilosofia”LaNuovaItaliaEditriceVol.1,p.231,1963
GeorgWilhelmFriedrichHegelJena1805-1806
Elements
ThesearefoundandlistedonthePeriodicTable
Puresubstancescomposedofonlyonetypeofatom.
Atoms
The smallest particle of an element thatretainsallofthepropertiesoftheelement.
Molecules
A sufficiently stable electrically neutral group of at least twoatoms in a definite arrangement held together by chemicalbonds.
Compounds
Apuresubstanceconsistingoftwoormoredifferentelements chemically bonded together in a fixedproportionbymass.
Pnictogens
ThePeriodictableAlkaliMetals
AlkalineEarths
TransitionMetals
Halogens
Nob
leGases
LanthanidesandActinides
MainGroup
MainGroupCalchogens
A given compound always contains thesameproportionofelements,bymass.
Dalton,John1766-1844Lithograph
Dalton’sAtomicTheory1. Eachelementismadeupofatoms;
2. Atoms of a given element are identical while atoms ofdifferentelementsdiffer;
3. Chemical compounds are made up of specific wholenumberratiosofatoms;
4. Reactions involve reorganizationofatoms.Atomsdonotchange.
Essay on a Manner of Determining theRelative Masses of the ElementaryMoleculesofBodies,andtheProportionsin Which They Enter Into TheseCompoundLorenzoRomanoAmedeoCarloAvogadroJournaldePhysique,1811,73,58-76
AtomicMasses:
Arelativecomparisonoftheaveragemassesofatoms.
Avogadro’sHypothesis(1811):
EqualvolumesofagascontainthesamenumberofatomsunderthesameconditionsofTandP.
AtomicStructureUntiltheendofthe19thcentury,notmuchwasknownabouttheatom.
BUT,WHATWASTHEATOM?
Peopleknew:i) Thereweredifferentelementsii) Eachhadauniquemassandreactedin
aspecialway.
Introductionto
AtomicTheory
Theatomwasablackbox.Peopleknewtherewere different substances (different boxes),eachwithuniqueproperties,but“why”wasaquestion.
SirJosephJohnThomson(1856- 1940)was awarded the 1906NobelPrize.“… in recognition of the great merits of histheoreticalandexperimental investigationsonthe
conductionofelectricitybygases.”
TheearlyexperimentsofJ.J.Thompsoncastlightonthequestion.Anevacuatedtube,containingasmallamountofagaswasattachedtoapowersupply
LVIII.OntheMassesoftheIonsinGasesatLowPressures.ByJ.J.Thomson,M.A.,F.R.S.,
CavendishProfessorofExperimentalPhysics,CambridgePhilosophicalMagazine,December1899
Series5,Vol.48,No.295,p.547-567
https://www.youtube.com/watch?v=UUpD62r2wq8
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Thesamecathoderaywasseennomatterwhatthetubewasfilledwith.
Conclusion:Thecathoderaywasafundamental
unitofallmaterials.
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Throughanumberofexperiments,J.J.wasabletodeterminethecharge(e)tomass(m)
ratioofthecathoderay.
e/m=somenumbere/m=-1.75881962x1011C/Kg.
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By similar analysis he was also able tolook at the canal Ray, the positive piece“left over” after the Cathode ray hadbeenejected.
e/mwasdifferentforeachgasthatwasintheevacuatedtube.
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Startingwithtwodifferentatomic“Boxes”
Same
Positivepieces“leftover”aredifferent
qm
=ν0
2dEaL
= EdB2aL
x =ν0t
tanα = dy
dx⎛⎝⎜
⎞⎠⎟ x=d
= qm
⎛⎝⎜
⎞⎠⎟
Edν
02 = Δy
L
qm
=ν
02Δy
EdL
y = 1
2qm
⎛⎝⎜
⎞⎠⎟Et 2 = 1
2qm
⎛⎝⎜
⎞⎠⎟E x
ν0
⎛
⎝⎜⎞
⎠⎟
2
ν = E
B
Eq= qνB
qm
=ν
02Δy
EdL = EΔyB2dL
qm
=ν
02Δy
EdL
AtthispointJ.Jwasreadytotalkabouttheconstitution of the atom: a large “sea” ofpositivecharge,imbeddedwithelectrons.
ThePlumPuddingModel
Thee/meratio(-1.75881962x1011C/Kg)
Whatdoesittellus?
Millikan’sOilDropExperiment
Smalldropletsofoil,eachgivenanegativecharge,arepushedupwardby an electric forcewhile they simultaneously falldownward undertheinfluenceofgravity.Controlledvariationsoftheelectricfieldshowthatthenegativechargeoneachdropisalwaysamultipleofacertainvaluee.
Smalldropletsofoil,eachgivenanegativecharge,arepushedupwardbyanelectricforcewhiletheysimultaneouslyfalldownwardundertheinfluenceofgravity.Controlledvariationsoftheelectricfieldshowthatthenegativechargeoneachdropisalwaysamultipleofacertainvaluee.
Zapwithradiation
Experimentaldataindicatethat:e =-1.60217733x10-19C;e/me =-1.75881962x1011C/Kgme =9.1093897x10-31kg.
Millikan’sOilDropExperiment
BacktoPlumPudding
A “sea of positivecharge” containingembeddedelectrons
They shot a particles through the foil. This should beequivalenttoshootingahowitzershellthroughpapertissue.
Thegoldfoilexperiment:
They shot a particles through the foil. Thisshouldbeequivalenttoshootingahowitzershellthroughpapertissue.
Thegoldfoilexperiment:
United Kingdom and New Zealand Victoria University Manchester, U. K. b. 1871 (in Nelson, New Zealand) d. 1937 Nobel prize in Chemistry 1908
Asolarsystemmodeloftheatom.
Electronscirclethenucleus.
mass atomicmassunit uorDa 1u/1Da =1.6605402✕10-27kg
TheConceptofAtomicNumberZ=numberofprotons
YouwillnoticethatmostofthePeriodicTableiswell-arrangedaccordingtoAtomicMass.
XZ
Z=atomicnumber=#protons
A=atomicmass=#protons+neutrons
A
Differentatomicmasses
Isotopes:Atomswith thesamenumberofprotons (Z)butdifferentnumberofneutrons(A-Z)
XZ
A
CompositionoftheAtom
But,thenucleusismadeofprotonsandneutrons
Electrons
Nucleus
Li
2e- 1e-
Be
2e- 2e- ......
Ne
2e- 8e-
2e-
Na
2e- 8e-
Mg
2e- 8e- ......
Ar
2e- 8e-
1e- 8e-
K
2e- 8e-
8e-
Ca
2e- 8e-
8e-1e-
......
2e-
LawofDefiniteProportionAgivencompoundalwayscontainsthesameproportionofelements,bymass.
LawofMultipleProportionsWhen two elements form twodifferent compounds, the mass ratioof theelements inonecompound isrelated to the themass ratio of theelementsinthelattercompoundbyasmallwholenumber.
MassofOxygencombiningwith1gramofNitrogen
CompoundA 1.1428gCompoundB 2.2857g
Dividebysmallestnumbertogetratios:
1.1428/1.1428=12.2857/1.1428=2
NON2O2
Wereallydonotknowtherealformula
NO2N2O4