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The atom and its nucleus

The atom and its nucleus. By the end of this chapter you should be able to : appreciate that atomic spectra provide evidence for an atom that can only take discrete values in energy ;

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The atom and its nucleus

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  1. Theatom and itsnucleus Bytheend of thischapteryoushouldbeableto: appreciatethatatomicspectraprovideevidenceforanatomthat can onlytakediscretevalues in energy; explainwhatisotopes are and howtheirexistenceimpliesthatneutrons are presentinsidethenucleus; statethemeaning of thetermsnuclide, nucleon, massnumberand atomicnumber(protonnumber); outlinetheproperties of theforcesthatoperatewithinthenucleus.

  2. Thediscovery of the nuclear atom In 1909, Geiger and Marsden, workingunderRutherford’sdirection, performed a series of experiments in whichtheystudiedthescattering of alphaparticlesshot at a thingoldfoil.

  3. Consequences of theRutherfordexperiment Theverylargedeflectionwasindicative of anenormousforce of repulsionbetweenthealphaparticle and thecarrier of the positive charge of theatom. Rutherfordcalculatedtheoreticallythenumber of alphaparticlesexpected at particular scatteringanglesbasedonCoulomb’sforcelaw. He foundagreementwithhisexperimentsifthe positive atomicchargewasconfinedto a region of linear sizeapproximatelyequalto 10-15m.

  4. TheRutherfordmodel of theatom Themaindifficultywasthatanacceleratedchargeshouldradiateelectromagneticwaves and thus lose energy. The time requiredfortheelectrontofallintothenucleusis of theorder of nanoseconds. Thus, theRutherfordmodelcannotexplainwhymatterisstable.

  5. TheBohrmodel Bohrexaminedthesimplestatom, that of hydrogen, and realizedthattheelectroncouldexist in certainspecificstates of definiteenergy, withoutradiatingawayenergy, if a certainconditionwasmetbytheorbitradius. Theelectronenergyisthusdiscrete as opposedto continuos. Theelectron can only lose energywhenitmakes a transitionfromonestatetoanother of lowerenergy. Theemittedenergyisthenthedifference in energybetweentheinitial and final states.

  6. Spectra Under normal conditionstheelectron in eachhydrogenatomoccupiedthelowestenergystate. Iftheatoms are somhowexcitedtheelectronsleavethegroundstate and occupyone of thehigherenergy, excitedstates. As soon as they do so, however, themake a transition back downtolowerenergystatesradiatingenergy in theprocess. Forexample, hydrogenemits light of wavelength 656 nm, 486 nm and 410 nm.

  7. Spectra Conversely, consideratoms of hydrogenthat are in theirgroundstates and imagine sending light of a specificwavelengththrough a givenquantity of hydrogen. Ifthewavelength of light doesnotcorrepondtoanty of thewavelengths in theemissionspectrum of hydrogen, the light istransmittedthroughtheatoms of hydrogenwithoutanyabsorption. If, however, itmatchesone of theemissionspectrumwavelengths, thenthis light is absorbed. Thus, ifwhiteligthissentthroughthe gas and thetransmitted light isanalysedthrough a spectrometer, darklineswillbefound at the position of the absorbed wavelengths.

  8. Nuclear structure Nuclei are made up of smallerparticles, calledprotons and neutrons. Thewordnucleonisusedto denote a protonor a neutron. Thenumber of protons in a nucleusisdenotedbyZ, and iscalledtheatomic (orproton) number. The total number of nucleons (protons + neutrons) iscalledthemass (ornucleon) number, and isdenotedbyA. Theelectriccharge of thenucleusisZ|e|. Thenumber of neutrons in thenucleusisN = A – Z. A nucleuswith a specificnumber of protons and neutronsisalsocalled a nuclide.

  9. Isotopes Nucleithathavethesamenumber of protonsbutdifferentnumber of neutrons are calledisotopes of eachother. Sinceisotopeshavethesamenumber of protons, theiratomshavethesamenumber of protons, theiratomshavethesamenumber of electrons as well. Thismeansthatisotopeshaveidenticalchemicalbutdifferentphysicalproperties. Theexistence of isotopesisevidencefortheexistence of neutronsinsideatomicnuclei.

  10. Theforceswithinthenucleus Thenucleons are boundtogether in thenucleusbythestrong nuclear force. Itisnecessarybecauseotherwisetheelectricalrepulsionbetweenthepositivelychargedprotonswould break thenucleusapart. Thestrong nuclear forceisanattractiveforce and muchstrongerthantheelectricalforceiftheseparationbetweentwonucleonsisverysmall (about 10-15 m orless). Forlargerseparations, the nuclear forcebecomes so small as tobenegligible.

  11. Theforceswithinthenucleus Experimental evidence show thatthe nuclear radiusRisgivenby whereAisthemassnumber. Thisimpliesthatthe nuclear densityisthesameforallnuclei. The short range of theforceimpliesthat a givennucleon can onlyinteractwith a few of itsimmediateneighbours and notwithall of thenucleons in thenucleus.

  12. Forcesacting in thenucleus

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