rutherford’s alpha-particle scattering experiment

8/9/2019 Rutherford’s Alpha-Particle Scattering Experiment

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Rutherford’s Alpha-ParticleScattering Experiment

By: Muhammad Asif, alha !aisal,Ahmad "ussain, #$rahim Ra%&ue '

Aree$ Shai(h


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)oncept of the Experiment

#f you *anted to (no* more a$out a dar( room that you *ere una$leto go into, you might $e a$le to get an idea of its si+e and contents $ythro*ing $alls into the room and considering their $ehaiour the sortof tas( you might get in an Adenture game made for the computer./you *ould $e a$le to tell *here o$stacles *ere and roughly the si+e of

the room/

Physicists do the same (ind of thing *hen 0loo(ing at0 tiny o$1ectssuch as atoms/ #n 2322 Rutherford *anted to %nd out more a$out thestructure of the atom so he set t*o of his research students 4eigerand Marsden. the tas( of $om$arding gold atoms *ith alpha particles,

gathering data as to *hat happened to the 0missiles0 and ma(ingdeductions a$out the atom0s structure from that data/ #n those daysthey did not hae particle accelerators proiding a ready supply ofprotons or neutrons so they had to use something that could $e usedas a natural pro$e - alpha particles *ere ideal/


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he 5se of Alpha-Particles

Alpha particles are small only t*o protons and t*o neutrons. and yet hae enoughmass 6u. to $e a suita$le missile lots of momentum./

hey are produced naturally $y radioactie nuclides that are alpha emitters smallproton rich nuclei. and so a steady supply *as easy to o$tain/ heir properties had$een under inestigation for a$out a decade much of it done $y Rutherford8. and he

had $een doing research into the fact that if he used high energy alpha particles they*ere a$le to penetrate thin metal foil sheets *hen he came up *ith the idea of thatthey *ould $e ideal as a pro$e to atomic structure/ "e *as expecting tiny changes intra1ectory as they met up *ith atomic su$structure/ Remem$er he thought of the0atom0 as the 0plum pudding0 structure - he hadn0t discoered the one you (no* a$outyet/

he alpha source emits alpha particles randomly in all directions, $ut to study ho*their path is altered *e hae to $e sure that they only hit the foil at a %xed angle39./o achiee this *e must use a collimator/ his a$sor$s all of the alpha particlesexcept those traelling in one direction - unless they are traelling parallel to thesides of the outlet of the collimator they *ill impact on the side and $e a$sor$ed - soonly those traelling parallel to each other and in one direction get through8


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he !oil

A single atom is too small to loo( at/ #t *ould $e impossi$le toget 1ust one to examine/ herefore Rutherford decided to loo(at a metal foil consisting of many atoms in a ery thin sheet/

4old *as the ideal choice as gold can $e rolled out into ery%ne gold leaf sheets/ hese ery %ne sheets are only a fe*atoms deep/ herefore gold foil *ould produce results ofinteractions that could $e $est related to the interaction$et*een a single alpha and a single nucleus

#f the foil *as too thic( the alpha particles *ould 1ust $ea$sor$ed/ Remem$er that he expected most of them to 1ustgo through $ut he (ne* that they could $e a$sor$ed $ythic(er foils and een $y a fe* centimetres of air/


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The Evacuated Chamber:

#t had to $e performed in a acuum $ecausethe air *ould a$sor$ the alpha particles $eforethey hit the foil or $efore they got to the screen8

The Zinc Sulphide Screen:

;inc sulphide


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Measuring the Angle

#n order to %nd out ho* much the alpha particles had $een dehen the angle *as +ero the straight through position. the num$er ofscintillations per minute on the screen *ould $e high/

As the angle increased the num$er of scintillations per minute *ould $emuch lo*er/

he really surprising result *as the num$er than *ere 0$ac(scattered0 -that made angles of greater than 39o *ith their original tra1ectory/Rutherford *as ama+ed, he said, 0#t *as &uite the most incredi$le eentthat has happened to me in my life8 #t *as almost as incredi$le as if you%red a 2?@ shell at a piece of tissue paper and it came $ac( and hit you80


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ess than 2 in ,999 alpha particles $ac( scattered/ hisgies an indication of the tedious *or( inoled in ta(ing achec(ing the results8 hey *ould hae to $e repeatedmany times to $e sure they *ere not due to anomalies8>ould you hae $een tempted to discount totally

inexplica$le results and gie your professor a set of resultshe expectedC r *ould you, li(e 4eiger and Marsden,repeat and repeat and report the strange %ndings toRutherford so that a *hole ne* model of the atom could $edeelopedC #t is the careful and honest8. inestigation into

anomalies that often lead to ne* theories/

Do*adays this could $e done *ith data logging andcomputer analysis/ #n the early 2399s scientists had to do alot of painsta(ing measurements8

he E=orts


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Alpha-Particle Scattering

Experiment


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he )onclusion

>hen Rutherford mathematically inestigated the results he proposed a modelthat explained the results that 4eiger and Marsden o$tained/

he fact that the ast ma1ority of the alpha particles got straight through ledRutherford to propose that the atom *as composed primarily of empty space/

he fact that $ac(scattering occurred in 2 in 999 alpha particles indicatedthat there *as a:

small that *as *hy so fe* *ere a=ected.

massive meaning containing lots of mass - he (ne* the electrons had ery little mass

and the fact that all of the positie charges *ere concentrated into a small area meantthat the mass *as concentrated there too.

positively charged $ecause it repelled the alpha particles. nucleus in the centre of theatom neutrons had not $een discoered at that time - so he made no mention of them8./


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So Rutherford’s picture *as one of the atom $eingli(e the solar system the sun $eing the nucleus

ta(ing a ery small proportion of the olume of thesolar system $ut $eing the ast $ul( of the mass init. and the electrons $eing li(e the planets or$iting

the 0sun0/

his model *as later amended $y Bohr to ta(e intoaccount a couple of points that Rutherford0s atomdid not fully explain the motion of the electrons

and the or$ital paths that could explain *hat the)hemists understood of electron $ehaiour in$onding. to ma(e the model of the atom that youare taught at 4)SE $ut it *as still a magni%centadance to our understanding of atomic structure/

rutherford’s alpha-particle scattering experiment

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