Chemistry - an illustrated guide to science (794128), страница 3
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One typeof particle, alpha radiation, is positivelycharged. Geiger and Marsdeninvestigated how alpha particlesscattered by bombarding them againstthin sheets of gold, a metal with a highatom ic m ass.● They used a tube of radon, aradioactive element, in a metal block(a) as the source of a narrow beam ofalpha particles and placed a sheet ofgold foil in the center of theirapparatus (b). After they bombardedthe sheet, they detected the pattern ofalpha particle scattering by using afluorescent screen (c) placed at thefocal length of a microscope (d).● If the existing model had been correct,all of the particles would have beenfound within a fraction of a degree ofthe beam.
But Geiger and Marsdenfound that alpha particles werescattered at angles as large as 140°.● From this experiment, Rutherforddeduced that the positively chargedalpha particles had come into therepulsive field of a highly concentratedpositive charge at the center of theatom. He, therefore, concluded that anatom has a small dense nucleus inwhich all of the positive charge andmost of the mass is concentrated.Negatively charged electrons surroundthe nucleus—similar to the way theplanets orbit the Sun.aa source of alpha particles(radon tube)b gold foilc screend microscopecd15Investigating theelectron 1ATOMIC STRUCTUREKey wordsanodecathodecathode rays1 Maltese-Cross tubeaelectronfluorescencefInvestigating the electron+● Duringthe last half of the nineteenthcentury, scientists observed that whenan electric current passes through aglass tube containing a small amountof air, the air glowed.
As air wasremoved, a patch of flu orescen ceappeared on the tube, which theycalled cathode rays. Scientists thenbegan investigated these streams ofelectron s traveling at high speed.+––+1 Maltese cross tube● InabcddE.h.t. supplylow voltageheated filament and cathodeanodeheghfgeMaltese-Cross (connected to anode)shadowinvisible cathode raysfluorescent screen2 The Perrin tubepqi+– ––2 The Perrin tube–● In–+––––+–jkli E.h.t.
supplyj 6 V supplyk cathodel anodem track of electron beam in magnetic fieldmnnopqvacuumgold-leaf electroscopeelectrons are collectedinsulated metal cylindero1895 Jean Perrin devised anexperiment to demonstrate thatcathode rays convey negative charge.● He constructed a cathode ray tube inwhich the cathode rays wereaccelerated through the anode, in theform of a cylinder open at both ends,into an insulated metal cylinder calleda Faraday cylinder.● This cylinder has a small opening atone end. Cathode rays enter thecylinder and build up charge, which isindicated by the electroscope. Perrinfound that the electroscope hadbecome negatively charged.● Perrin’s experiments helped toprepare the way for English physicistJ.
J. Thompson’s work on electrons afew years later.© Diagram Visual Information Ltd.cbthe 1880s, William Crookesexperimented on cathode rays using aMaltese cross tube.● The stream of electrons emitted by thehot cathode is accelerated toward thean ode. Some are absorbed, but themajority passes through and travelsalong the tube. Those electrons thathit the Maltese cross are absorbed.Those electrons that miss the crossstrike the screen, causing it tofluoresce with a green light.● The result of this experiment is that ashadow of the cross is cast on thescreen. This provides evidence thatcathode rays travel in straight lines.16ATOMIC STRUCTUREKey wordsanodecathodecathode rayselectronphotoelectriceffectradiationInvestigating theelectron 21 J.J. Thomson’s cathode ray tubeea1 J.J. Thomson’s cathoderay tube1897 J.J.
Thomson devised anexperiment with cathode rays thatresulted in the discovery of theelectron .● Up to this time, it was thought that thehydrogen atom was the smallestparticle in existence. Thomsondemonstrated that electrons (which hecalled corpuscles) comprising cathoderays were nearly 2,000 times smaller inmass than the then lightest-knownparticle, the hydrogen ion.● When a high voltage is placed across apair of plates, they become chargedrelative to each other. The positivelycharged plate is the an ode, and thenegatively charged plate the cathode.● Electrons pass from the surface of thecathode and accelerate toward theoppositely charged anode. The anodeabsorbs many electrons, but if theanode has slits, some electrons willpass through.● The electrons travel into an evacuatedtube, where they move in a straightline until striking a fluorescent screen.This screen is coated with a chemicalthat glows when electrons strike it.–● Incjdhigh voltagecathodegas discharge provides free electronsanode with slity-deflecting platefghijidirection of travel of the cathode raysflourescent screenlightevacuated tubex-deflecting plate2 Evidence of the photoelectric effectklm––––––– –2 Evidence of thephotoelectric effect–++++ +● The© Diagram Visual Information Ltd.photoelectric effect is theemission of electrons from metalsupon the absorption ofelectromagnetic radiation .● Scientists observed the effect in thenineteenth century, but they could notexplain it until the development ofquantum physics.● To observe the effect, a clean zincplate is placed in a negatively chargedelectroscope.
The gold leaf and brassplate carry the same negative chargeand repel each other.● When ultraviolet radiation strikes thezinc plate, electrons are emitted. Thenegative charge on the electroscope isreduced, and the gold leaf falls.h+babcdegf–– –– ––noNegatively chargedelectroscope withzinc plate attachedk mercury vapor lampl ultraviolet lightm brass plateThe leaf falls aselectrons are ejectedfrom the zinc platen gold leafo zinc plate–– ––+ ++ ++If positively chargedthe electroscoperemains charged17Cathode ray oscilloscopeKey words1 The cathode ray oscilloscopeabATOMIC STRUCTUREcedfanodecathodecathode rays1 Cathode ray oscilloscope● Theabcdefjkiheatery-deflection platesy-input terminalx-input terminalx-deflection plateslightghijklghphosphor coatingelectron beamcommon-input terminalaccelerating and focusing anodesgridcathode2 Electron gunmnop+–––● When+–+wm low voltagen heatero cathodep cyclindrical anodeq high speed electronsr accelerated electronss anodevutt cathodeu evacuated tubev electron beamw high voltages2 Electron gunrqa current passes through theheater, electrons are emitted from thesurface of the cathode and attractedtowards an oppositely charged anode.Some will be absorbed by the anode,while others pass through and areaccelerated, forming a stream of highspeed electrons.© Diagram Visual Information Ltd.lcathode ray oscilloscope (CRO) isone of the most important scientificinstruments ever to be developed.
It isoften used as a graph plotter to displaya waveform showing how potentialdifference changes with time. TheCRO has three essential parts: theelectron gun, the deflecting system,and the fluorescent gun.● The electron gun consists of a heaterand cathode, a grid, and severalan odes. Together these provide astream of cathode rays. The grid is atnegative potential with respect to thecathode and controls the number ofelectrons passing through its centralhole.
It is the brightness control.● The deflecting system consists of a pairof deflecting plates across whichpotential differences can be applied.The Y-plates are horizontal but deflectthe beam vertically. The X-plates arevertical and deflect the beanhorizontally.● A bright spot appears on thefluorescent screen where the beamhits it.18ATOMIC STRUCTUREKey wordselectronradiationMeasuring the charge onthe electronMillikan’s apparatusaMeasuring the charge onthe electron● Inthe early part of the 20th century,American physicist Robert Millikanconstructed an experiment toaccurately determine the electriccharge carried by a single electron .● Millikan’s apparatus consisted of twohorizontal plates about 20 cm indiameter and 1.5 cm apart, with asmall hole in the center of the upperplate.● At the beginning of the experiment, anatomizer sprayed a fine mist of oil onto the upper plate.● As a result of gravity, a droplet wouldpass through the hole in the plate intoa chamber that was ionized byradiation .
Electrons from the airattached themselves to the droplet,causing it to acquire a negative charge.A light source illuminated the droplet,making it appear as a pinpoint of light.Millikan then measured its downwardvelocity by timing its fall through aknown distance.● Millikan measured hundreds ofdroplets and found that the charge onthem was always a simple multiple of abasic unit, 1.6 x 10-19 coulomb.
Fromthis he concluded that the charge onan electron was numerically 1.6 x 10-19coulomb.bcdefg© Diagram Visual Information Ltd.hjabcdeisealed containeratomizeroil dropletscharged metal plate (+)charged oil dropletsfghijlight sourceviewing microscopecharged metal plate (–)ionizing radiationpower source19Size and motion ofmoleculesATOMIC STRUCTUREKey wordsBrownian motiondiffusionmolecule1 Estimating the size of a moleculegfbihak1 Estimating the size ofa moleculeh● Scientistscan estimate the size of am olecu le by dividing the volume of asphere by the volume of a cylinder.● In the example in the diagram, thevolume of a spherical oil drop ofradius, r s, is given by:c4 x! x r s33j● Whenethe oil drop spreads across thesurface of water, it takes the shape of acylinder of radius, r c, and thickness, h.The volume of such a cylinder is:dDetermining the radius ofan oil drop! x r c2 x hDetermining the radius of anoil drop spread2 Brownian motion in air● Ifwe assume that the layer of oil isone molecule thick, then h gives thesize of an oil molecule.● When spread on water the drop of oilwill have the same volume therefore:3 Diffusionh= 4 x! x r s3 xr31! x r c2h = 4 r s33 r c2uxv2 Brownian motion in air● Brown ianlwnompqtm otion is the randommotion of particles through a liquid orgas.
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