Unit6 Introduction to Metals and Alloys.docx
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Unit6 Introduction to Metals and Alloys.docx
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Unit6IntroductiontoMetalsandAlloys
Deflnition
Inchemistry,ametal(Greek:
Metallon)isdefinedasanelementthatreadilyloseselectronstoformpositiveions,(cations)andformsmetallicbondsbetweenothermetalatoms(formingtonicbondswithnon-metals).
Themetalsoftheperiodictableareformallydefinedaslithium,beryllium,sodium,magnesium,aluminium,potassium,calcium,scandium,titanium,vanadium,chromium,manganese,iron,cobalt,copper,zinc,gallium,rubidium,strontium,yttrium,zirconium,niobium,molybdenum,technetium,ruthenium,rhodium,palladium,silver,cadmium,indium,tin,antimony,caesium,barium,lanthanum,hafnium,tantalum,tungstem,rhenium,osmium,iridium,platinum,gold,mercury,thallium,lead,andbismuth.Metalsaresometimesdescribedasalatticeofpositiveionssurroundedbyacloudofdelocalizedelectrons.Metalsareoneofthethreegroupsofelementsasdistinguishedbytheirionizationandbondingproperties,alongwiththegroupsofmetalloidsandnonmetals.Ontheperiodictable,adiagonallinedrawnfromboron(B)topolonium(Po)separatesthemetalsfromthenonmetals.Mostelementsonthislinearemetalloids,sometimescalledsemi-metals;elementstothelowerleftaremetals;elementstotheupperrightarenonmetals.
Analternativedefinitionofmetalsisthattheyhaveoverlappingconductionbandsandvalencebandsintheirelectronicstructure.Thisdefinitionopensupthecategoryformetallicpolymersandotherorganicmetals,whichhavebetweenmadebyresearchersandemployedinhigh-techdevices.Thesesyntheticmaterialsoftenhavethecharacteristicsilvery-greyreflectiveness(luster)ofelementalmetals.
ChemicalProperties
Metalsareusuallyinclinedtoformcationsthroughelectronloss,reactingwithoxygenintheairtoformoxidesoverchangingtimescales(ironrustsoveryears,whilepotassiumburnsinseconds).Thealkalimetalsarethemostvolatile,followedbythealkalineearthmetals,foundintheleftmosttwogroupsoftheperiodictable.Examples:
4Na+02—2Na2O(sodiumoxide)2Ca+02→2CaO(calciumoxide)4Al+302→2A12O3(aluminiumoxide)
Thetransitionmetals(suchasiron,copper,zinc,andnicked)takemuchlongertooxidize.Others,likepalladium,platinumandgold,donotreactwiththeatmosphereatall.Somemetalsformabarrierlayerofoxideontheirsurfacewhichcannotbepenetratedbyfurtheroxygenmoleculesandthusretaintheirshinyappearanceandgoodconductivityformanydecades(likealuminium,somesteels,andtitanium).Theoxidesofmetalsarebasic(asopposedtothoseofnonmetals,whichareacidic),althoughthismaybeconsideredaruleofthumb,ratherthanafact.
Painting,anodisingorplatingmetalsaregoodwaystopreventtheircorrosion.However,amorereactivemetalintheelectrochemicalseriesmustbechosenforcoating,especiallywhenchippingofthecoatingisexpected.Waterandtwometalsformanelectrochemicalcell,andifthecoatingislessreactivethanthecoatingactuallypromotescorrosion
Physicalproperties
Metalingeneralhavesuperiorelectricandthermalconductivity,highlusteranddensity,andtheabilitytobedeformedunderstresswithoutcleaving.Whilethereareseveralmetalsthathavelowdensity,hardness,andmeltingpoints,these(thealkaliandalkalineearthmetals)areextremelyreactive,andarerarelyencounteredintheirelemental,metallicform.
Density:
Themajorityofmetalshavehigherdensitiesthanthemajorityofnonmetals.Nonetheless,thereiswidevariationinthedensitiesofmetals;lithiumistheleastdensesolidelementandosmiumisthedensest.ThemetalsofgroupsIAandIIAarereferredtoasthelightmetalsbecausetheyareexceptionstothisgeneralization.Thehighdensityofmostmetalsisduetothetightly-packedcrystallatticeofthemetallicstructure.Thestrengthofmetallicbondsfordifferentmetalsreachesamaximumaroundthecenterofthetransitionseries,asthoseelementshavelargeamountsofdelocalizedelectronsinametallicbond.However,otherfactors(suchasatomicradius,nuclearcharge,numberofbondingorbitals,overlapoforbitalenergies,andcrystalform)areinvolvedaswell.
Malleability:
Thenondirectionalnatureofmetallicbondingisthroughtobetheprimaryreasonforthemalleabilityofmetal.Planesofatomsinametalareabletoslideacrossoneanotherunderstress,accountingfortheabilityofacrystaltodeformwithoutshattering.
Whentheplanesofanionicbondisslidpastoneanother,theresultantchangeinlocationshiftsionsofthesamechargeintocloseproximity,resultinginthecleavageofthecrystal.Covalentlybondedcrystalscanonlybedeformedbybreakingthebondsbetweenatoms,therebyresultinginfragmentationofthecrystal.
Conductivity:
Theelectricalandthermalconductivityofmetalsoriginatefromthefactthatinthemetallicbond,theouterelectronsofthemetalsatomsfromagasofnearbyfreeelectrons,movingasanelectrongasinabackgroundofpositivechargeformedbytheioncores.Goodmathematicalpredictionsforelectricalconductivity,aswellastheelectrons5contributiontotheheatcapacityandheatconductivityofmetalscanbecalculatedfromthefreeelectronmodel,whichdoesnottakethedetailedstructureoftheionlatticeintoaccount.
Electriccharge:
Whenconsideringtheexactbandstructureandbindingenergyofametal,isisnecessarytotakeintoaccountthepositivepotentialcausedbythespecificarrangementoftheioncores-whichisperiodicincrystals.Themostimportantconsequenceoftheperiodicpotentialistheformationofasmallbandgapattheboundaryofthebrillouinzone.Mathematically,thepotentialoftheioncoresistreatedinthenearly-freeelectronmodel.
Alloys
Analloyisamixtureoftwoormoreelementsinsolidsolutioninwhichthemajorcomponentisametal.Mostpuremetalsareeithertoosoft,brittleorchemicallyreactiveforpracticaluse.Combiningdifferentrationsofmetalsasalloysmodifiesthepropertiesofpuremetalstoproducedesirablecharacteristic.Theaimofmakingalloysisgenerallytomakethemlessbrittle,harder,resistanttocorrosion,orhaveamoredesirablecolorandluster.Examplesofalloysaresteel(ironandcarbon),brass(copperandzinc),bronze(copperandtin),andduralumin(aluminumandcopper).Alloysspeciallydesignedforhighlydemandingapplications,suchasjetengines,maycontainmorethantenelements.
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