1 - History of Atomic Theory

7/24/2019 1 - History of Atomic Theory

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Atomos: Not to Be CutAtomos: Not to Be Cut

The History of Atomic TheoryThe History of Atomic Theory


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ATOMIC THEORYATOMIC THEORY

a theory of the nature of matter. Ita theory of the nature of matter. It

states that all matter is composed ofstates that all matter is composed ofatoms. It is credited to John Daltonatoms. It is credited to John Dalton


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Atomic ModelsAtomic Models

This model of theThis model of theatom may looatom may loofamiliar to you. This isfamiliar to you. This isthe !ohr model. Inthe !ohr model. In

this model" thethis model" thenucleus is or#ited #ynucleus is or#ited #yelectrons" $hich areelectrons" $hich arein different ener%yin different ener%y

le&els.le&els. A model uses familiar ideas toA model uses familiar ideas to

e'plain unfamiliar factse'plain unfamiliar factso#ser&ed in nature.o#ser&ed in nature. A model can #e chan%ed asA model can #e chan%ed as

ne$ information is collected.ne$ information is collected.


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The atomic modelThe atomic modelhas chan%edhas chan%edthrou%hout thethrou%hout the

centuries" startin%centuries" startin%in ()) !C" $hen itin ()) !C" $hen it

looed lielooed lie aa

#illiard #all#illiard #all **


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+ho are these men,+ho are these men,

In this lesson" $e-ll learn

a#out the men $hose uests

for no$led%e a#out the

fundamental nature of the

uni&erse helped define our

&ie$s.


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DemocritusDemocritus

This is the /reeThis is the /ree

philosopher Democritusphilosopher Democritus

$ho #e%an the search for$ho #e%an the search for

a description of mattera description of matter

more thanmore than 0())0())years a%o.years a%o. He ased1 Could matterHe ased1 Could matter

#e di&ided into smaller#e di&ided into smaller

and smaller piecesand smaller pieces

fore&er" or $as there afore&er" or $as there alimitlimitto the num#er ofto the num#er of

times a piece of mattertimes a piece of matter

could #ecould #e di&ideddi&ided,,

400 BC


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AtomosAtomos

His theory1 Matter could notHis theory1 Matter could not

#e di&ided into smaller and#e di&ided into smaller and

smaller pieces fore&er"smaller pieces fore&er"

e&entually the smalleste&entually the smallest

possi#le piece $ould #epossi#le piece $ould #e

o#tained.o#tained.

This piece $ould #eThis piece $ould #e

indi&isi#le.indi&isi#le.

He named the smallestHe named the smallest

piece of matter 2atomos"3piece of matter 2atomos"3

meanin% 2not to #e cut.3meanin% 2not to #e cut.3


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AtomosAtomos

To Democritus" atomsTo Democritus" atoms

$ere$ere smallsmall" hard" hard

particles that $ere allparticles that $ere all

made of the samemade of the same

material #ut $erematerial #ut $ere

differentdifferentshapes andshapes and

si4es.si4es.

Atoms $ereAtoms $ere infiniteinfiniteininnum#er" al$ays mo&in%num#er" al$ays mo&in%

and capa#le of 5oinin%and capa#le of 5oinin%

to%ether.to%ether.


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This theory wasThis theory was

ignored and forgottenignored and forgotten

for more thanfor more than20002000

years!years!


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+hy,+hy,

The eminentThe eminentphilosophersphilosophers

of the time,of the time,

AristotleAristotleandand

Plato, had aPlato, had amoremore

respected,respected,

(and ultimately(and ultimately

wrongwrong) theory.) theory.

Aristotle and 6lato fa&ored the earth, fire, air

and water approach to the nature of matter.Their ideas held s$ay #ecause of their

eminence as philosophers. The atomosidea

$as #uried for appro'imately 0))) years.


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Dalton-s ModelDalton-s Model

In the early 78))s"In the early 78))s"

the En%lishthe En%lish

Chemist JohnChemist John

DaltonDalton performed aperformed a

num#er ofnum#er of

e'periments thate'periments that

e&entually led toe&entually led tothe acceptance ofthe acceptance of

the idea of atoms.the idea of atoms.


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Dalton-s TheoryDalton-s Theory

He deduced that allHe deduced that all

elementselementsare composed ofare composed of

atoms. Atoms areatoms. Atoms are

indi&isi#le andindi&isi#le and

indestructi#le particles.indestructi#le particles. Atoms of theAtoms of the samesameelementelement

are e'actly alie.are e'actly alie.

Atoms ofAtoms of differentdifferentelementselements

areare differentdifferent..

CompoundsCompoundsare formed #yare formed #y

the 5oinin% of atoms of t$othe 5oinin% of atoms of t$o

or more elements.or more elements.


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..

This theoryThis theory

became onebecame one

of theof thefoundationsfoundations

of modernof modernchemistry.chemistry.


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Thomson-s 6lum 6uddin% ModelThomson-s 6lum 6uddin% Model

InIn 789:789:" the" the

En%lish scientistEn%lish scientist

J.J. ThomsonJ.J. Thomsonpro&ided the firstpro&ided the first

hint that an atomhint that an atom

is made of e&enis made of e&ensmallersmallerparticles.particles.


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Thomson ModelThomson Model

He proposed a modelHe proposed a modelof the atom that isof the atom that issometimes called thesometimes called the226lum6lum6uddin%6uddin%33

model.model.Atoms $ere madeAtoms $ere made

from a positi&elyfrom a positi&elychar%edchar%edsu#stancesu#stance

$ith ne%ati&ely$ith ne%ati&elychar%ed electronschar%ed electronsscatteredscattereda#out" liea#out" lieraisins in a puddin%.raisins in a puddin%.


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Thomson studiedThomson studiedthethe passa%epassa%e of anof anelectric currentelectric current

throu%h a %as.throu%h a %as.As the currentAs the current

passed throu%hpassed throu%hthe %as" it %a&ethe %as" it %a&eoff rays ofoff rays ofne%ati&elyne%ati&elychar%ed particles.char%ed particles.


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This surprisedThis surprised

Thomson"Thomson"

#ecause the#ecause theatoms of the %asatoms of the %as

$ere unchar%ed.$ere unchar%ed.

+here had the+here had thene%ati&e char%esne%ati&e char%es

come from,come from,

+here did

they comefrom,


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Thomson concluded that the

ne%ati&e char%es came from within

the atom.

A particle smaller than an atom had

to e'ist.

The atom $as di&isi#le;

Thomson called the ne%ati&elychar%ed 2corpuscles"3 today no$n

as electrons.


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= 0))) the mass of the hydro%en atomatom

Discovery of the ElectronDiscovery of the Electron1909 R.A Millikan1909 R.A Millikan

m = 9.11 10-31kg

e = -1.010-19 !

Also confirmed and estimated the negative electric

charge on an electron
http://education.jlab.org/atomtour/fact2.htmlhttp://education.jlab.org/atomtour/fact2.htmlhttp://education.jlab.org/atomtour/fact2.htmlhttp://education.jlab.org/atomtour/fact2.htmlhttp://../zMovies/e)%20millikan%20oil%20drop.mov


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Rutherford-s /old ?oilRutherford-s /old ?oil

E'perimentE'periment In 79)8" the En%lishIn 79)8" the En%lishphysicist Ernestphysicist Ernest

Rutherford $as hardRutherford $as hard

at $or on anat $or on ane'periment thate'periment that

seemed to ha&e littleseemed to ha&e little

to do $ith unra&elin%to do $ith unra&elin%

the mysteries of thethe mysteries of the

atomic structure.atomic structure.


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Rutherford-s e'periment In&ol&edRutherford-s e'periment In&ol&edfirin% a stream of tinyfirin% a stream of tiny positi&elypositi&elychar%edchar%edparticles at a thin sheet ofparticles at a thin sheet of%old foil%old foil@0))) atoms thic@0))) atoms thic


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MostMostof the positi&elyof the positi&elychar%ed 2#ullets3 passedchar%ed 2#ullets3 passedri%ht throu%h the %oldri%ht throu%h the %oldatoms in the sheet ofatoms in the sheet of %old%old

foilfoil$ithout chan%in%$ithout chan%in%course at all.course at all.


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This could only mean that the %old atoms in theThis could only mean that the %old atoms in thesheet $ere mostlysheet $ere mostly open spaceopen space. Atoms $ere. Atoms $ere notnota puddin% filled $ith a positi&ely char%eda puddin% filled $ith a positi&ely char%edmaterial.material.

Rutherford concluded that an atom had aRutherford concluded that an atom had a small"small"dense" positi&ely char%ed centerdense" positi&ely char%ed centerthatthat repelledrepelledhis positi&ely char%ed 2#ullets.3his positi&ely char%ed 2#ullets.3

He called the center of the atom the 2He called the center of the atom the 2nucleusnucleus33 The nucleus isThe nucleus is tinytiny compared to the atom as acompared to the atom as a

$hole.$hole.


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RutherfordRutherford

Rutherford reasonedRutherford reasoned

that all of an atom-sthat all of an atom-s

positi&ely char%edpositi&ely char%ed

particles $ereparticles $ere

containedcontainedin thein the

nucleus. The ne%ati&elynucleus. The ne%ati&ely

char%ed particles $erechar%ed particles $ere

scatteredscatteredoutside theoutside thenucleus around thenucleus around the

atom-satom-s ed%eed%e..


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!ohr Model!ohr Model

In 797B" the DanishIn 797B" the Danishscientist iels !ohrscientist iels !ohr

proposed anproposed an

impro&ement. In hisimpro&ement. In hismodel" he placedmodel" he placed

each electron in aeach electron in a

specificspecific

ener%yener%y

le&el.le&el.


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!ohr Model!ohr Model

Accordin% to !ohr-sAccordin% to !ohr-satomic model"atomic model"electrons mo&e inelectrons mo&e indefinitedefinite or#itsor#its

around the nucleus"around the nucleus"much lie planetsmuch lie planetscircle the sun. Thesecircle the sun. Theseor#its" or ener%yor#its" or ener%y

le&elsle&els" are located at" are located atcertaincertaindistancesdistancesfrom the nucleus.from the nucleus.


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Wave Model


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The +a&e ModelThe +a&e Model

Today-s atomicToday-s atomicmodel is #ased onmodel is #ased onthe principles ofthe principles of$a&e$a&emechanicsmechanics..

Accordin% to theAccordin% to thetheory of $a&etheory of $a&emechanics" electronsmechanics" electronsdo not mo&edo not mo&ea#outa#out

an atom in aan atom in a definitedefinitepath"path"lie the planetslie the planetsaround the sun.around the sun.


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The +a&e ModelThe +a&e Model

In fact" it isIn fact" it is impossi#leimpossi#leto determine the e'actto determine the e'actlocation of an electron. Thelocation of an electron. The pro#a#lepro#a#lelocation of anlocation of anelectron is #ased on ho$ muchelectron is #ased on ho$ much ener%yener%ythe electronthe electronhas.has.

Accordin% to the modern atomic model" at atom hasAccordin% to the modern atomic model" at atom hasaa small positi&ely char%ed nucleussmallpositi&ely char%ed nucleussurrounded #y asurrounded #y alar%e re%ion in $hich there are enou%h electrons tolar%e re%ion in $hich there are enou%h electrons tomae an atom neutral.mae an atom neutral.


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Electron "lo#$%Electron "lo#$%

A space in $hichA space in $hichelectrons are liely to #eelectrons are liely to #efound.found.

ElectronsElectrons $hirl$hirla#out thea#out the

nucleus #illions of timesnucleus #illions of timesin one secondin one second

They are not mo&in%They are not mo&in%around inaround in randomrandompatterns.patterns.

ocation of electronsocation of electronsdepends upon ho$ muchdepends upon ho$ muchener%yener%ythe electron has.the electron has.


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Indi&isi#leIndi&isi#le ElectronElectron ucleusucleus Or#itOr#it ElectronElectron

CloudCloud

/ree/ree

DaltonDalton

ThomsonThomson

RutherfordRutherford

!ohr!ohr

+a&e+a&e

1 - History of Atomic Theory

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