Chemistry-化学

ChemistryChemistryisthescientificdisciplineinvolvedwithcompoundscomposedofatoms,i.e.elements,andmolecules,i.e.combinationsofatoms:theircomposition,structure,properties,behaviorandthechangestheyundergoduringareactionwithothercompounds.[1][2]Chemistryaddressestopicssuchashowatomsandmoleculesinteractviachemicalbondstoformnewchemicalcompounds.Therearefourtypesofchemicalbonds:covalentbonds,inwhichcompoundsshareoneormoreelectron(s);ionicbonds,inwhichacompounddonatesoneormoreelectronstoanothercompoundtoproduceions:cationsandanions;hydrogenbonds;andVanderWaalsforcebonds.Seeglossaryofchemistry.Inthescopeofitssubject,chemistryoccupiesanintermediatepositionbetweenphysicsandbiology.[3]Itissometimescalledthecentralsciencebecauseitprovidesafoundationforunderstandingbothbasicandappliedscientificdisciplinesatafundamentallevel.[4]Examplesincludeplantchemistry(botany),theformationofigneousrocks(geology),howatmosphericozoneisformedandhowenvironmentalpollutantsaredegraded(ecology),thepropertiesofthesoilonthemoon(astrophysics),howmedicationswork(pharmacology),andhowtocollectDNAevidenceatacrimescene(forensics).Thehistoryofchemistryspansaperiodfromveryoldtimestothepresent.SinceseveralmillenniaBC,civilizationswereusingtechnologiesthatwouldeventuallyformthebasisofthevariousbranchesofchemistry.Examplesincludeextractingmetalsfromores,makingpotteryandglazes,fermentingbeerandwine,extractingchemicalsfromplantsformedicineandperfume,renderingfatintosoap,makingglass,andmakingalloyslikebronze.Chemistrywasprecededbyitsprotoscience,alchemy,whichisanintuitivebutnon-scientificapproachtounderstandingtheconstituentsofmatterandtheirinteractions.Itwasunsuccessfulinexplainingthenatureofmatteranditstransformations,but,byperformingexperimentsandrecordingtheresults,alchemistssetthestageformodernchemistry.ChemistryasabodyofknowledgedistinctfromalchemybegantoemergewhenacleardifferentiationwasmadebetweenthembyRobertBoyleinhisworkTheScepticalChymist(1661).Whilebothalchemyandchemistryareconcernedwithmatteranditstransformations,thecrucialdifferencewasgivenbythescientificmethodthatchemistsemployedintheirwork.ChemistryisconsideredtohavebecomeanestablishedsciencewiththeworkofAntoineLavoisier,whodevelopedalawofconservationofmassthatdemandedcarefulmeasurementandquantitativeobservationsofchemicalphenomena.Thehistoryofchemistryisintertwinedwiththehistoryofthermodynamics,especiallythroughtheworkofWillardGibbs.[5]EtymologyModernprinciplesMatterAtomElementCompoundMoleculeSubstanceandmixtureMoleandamountofsubstancePhaseBondingEnergyReactionIonsandsaltsAcidityandbasicityRedoxEquilibriumChemicallawsContentsHistoryOfdefinitionOfdisciplinePracticeSubdisciplinesIndustryProfessionalsocietiesSeealsoReferencesBibliographyFurtherreadingExternallinksThewordchemistrycomesfromalchemy,whichreferredtoanearliersetofpracticesthatencompassedelementsofchemistry,metallurgy,philosophy,astrology,astronomy,mysticismandmedicine.Itisoftenseenaslinkedtothequesttoturnleadoranothercommonstartingmaterialintogold,[6]thoughinancienttimesthestudyencompassedmanyofthequestionsofmodernchemistrybeingdefinedasthestudyofthecompositionofwaters,movement,growth,embodying,disembodying,drawingthespiritsfrombodiesandbondingthespiritswithinbodiesbytheearly4thcenturyGreek-EgyptianalchemistZosimos.[7]Analchemistwascalleda'chemist'inpopularspeech,andlaterthesufix"-ry"wasaddedtothistodescribetheartofthechemistas"chemistry".ThemodernwordalchemyinturnisderivedfromtheArabicwordal-kīmīā(الكیمیاء).Inorigin,thetermisborrowedfromtheGreekχημίαorχημεία.[8][9]ThismayhaveEgyptianoriginssinceal-kīmīāisderivedfromtheGreekχημία,whichisinturnderivedfromthewordKemet,whichistheancientnameofEgyptinEgyptian.[8]Alternately,al-kīmīāmayderivefromχημεία,meaning"casttogether".[10]Thecurrentmodelofatomicstructureisthequantummechanicalmodel.[11]Traditionalchemistrystartswiththestudyofelementaryparticles,atoms,molecules,[12]substances,metals,crystalsandotheraggregatesofmatter.Thismattercanbestudiedinsolid,liquid,orgasstates,inisolationorincombination.Theinteractions,reactionsandtransformationsthatarestudiedinchemistryareusuallytheresultofinteractionsbetweenatoms,leadingtorearrangementsofthechemicalbondswhichholdatomstogether.Suchbehaviorsarestudiedinachemistrylaboratory.Thechemistrylaboratorystereotypicallyusesvariousformsoflaboratoryglassware.Howeverglasswareisnotcentraltochemistry,andagreatdealofexperimental(aswellasapplied/industrial)chemistryisdonewithoutit.Achemicalreactionisatransformationofsomesubstancesintooneormoredifferentsubstances.[13]Thebasisofsuchachemicaltransformationistherearrangementofelectronsinthechemicalbondsbetweenatoms.Itcanbesymbolicallydepictedthroughachemicalequation,whichusuallyinvolvesatomsassubjects.Thenumberofatomsontheleftandtherightintheequationforachemicaltransformationisequal.(Whenthenumberofatomsoneithersideisunequal,thetransformationisreferredtoasanuclearreactionorradioactivedecay.)Thetypeofchemicalreactionsasubstancemayundergoandtheenergychangesthatmayaccompanyitareconstrainedbycertainbasicrules,knownaschemicallaws.EtymologyModernprinciplesLaboratory,InstituteofBiochemistry,UniversityofCologneinGermany.Energyandentropyconsiderationsareinvariablyimportantinalmostallchemicalstudies.Chemicalsubstancesareclassifiedintermsoftheirstructure,phase,aswellastheirchemicalcompositions.Theycanbeanalyzedusingthetoolsofchemicalanalysis,e.g.spectroscopyandchromatography.Scientistsengagedinchemicalresearchareknownaschemists.[14]Mostchemistsspecializeinoneormoresub-disciplines.Severalconceptsareessentialforthestudyofchemistry;someofthemare:[15]Inchemistry,matterisdefinedasanythingthathasrestmassandvolume(ittakesupspace)andismadeupofparticles.Theparticlesthatmakeupmatterhaverestmassaswell-notallparticleshaverestmass,suchasthephoton.Mattercanbeapurechemicalsubstanceoramixtureofsubstances.[16]Theatomisthebasicunitofchemistry.Itconsistsofadensecorecalledtheatomicnucleussurroundedbyaspacehostinganelectroncloud.Thenucleusismadeupofpositivelychargedprotonsandunchargedneutrons(togethercallednucleons),whiletheelectroncloudconsistsofnegativelychargedelectronswhichorbitthenucleus.Inaneutralatom,thenegativelychargedelectronsbalanceoutthepositivechargeoftheprotons.Thenucleusisdense;themassofanucleonisappromixately1,836timesthatofanelectron,yettheradiusofanatomisabout10,000timesthatofitsnucleus.[17][18]Theatomisalsothesmallestentitythatcanbeenvisagedtoretainthechemicalpropertiesoftheelement,suchaselectronegativity,ionizationpotential,preferredoxidationstate(s),coordinationnumber,andpreferredtypesofbondstoform(e.g.,metallic,ionic,covalent).Achemicalelementisapuresubstancewhichiscomposedofasingletypeofatom,characterizedbyitsparticularnumberofprotonsinthenucleiofitsatoms,knownastheatomicnumberandrepresentedbythesymbolZ.Themassnumberisthesumofthenumberofprotonsandneutronsinanucleus.Althoughallthenucleiofallatomsbelongingtooneelementwillhavethesameatomicnumber,theymaynotnecessarilyhavethesamemassnumber;atomsofanelementwhichhavedifferentmassnumbersareknownasisotopes.Forexample,allatomswith6protonsintheirnucleiareatomsofthechemicalelementcarbon,butatomsofcarbonmayhavemassnumbersof12or13.[18]Thestandardpresentationofthechemicalelementsisintheperiodictable,whichorderselementsbyatomicnumber.Theperiodictableisarrangedingroups,orcolumns,andperiods,orrows.Theperiodictableisusefulinidentifyingperiodictrends.[19]Solutionsofsubstancesinreagentbottles,includingammoniumhydroxideandnitricacid,illuminatedindifferentcolorsMatterAtomAdiagramofanatombasedontheRutherfordmodelElementStandardformoftheperiodictableofchemicalelements.ThecolorsrepresentdifferentcategoriesofelementsCompoundAcompoundisapurechemicalsubstancecomposedofmorethanoneelement.Thepropertiesofacompoundbearlittlesimilaritytothoseofitselements.[20]ThestandardnomenclatureofcompoundsissetbytheInternationalUnionofPureandAppliedChemistry(IUPAC).Organiccompoundsarenamedaccordingtotheorganicnomenclaturesystem.[21]ThenamesforInorganiccompoundsarecreatedaccordingtotheinorganicnomenclaturesystem.Whenacompoundhasmorethanonecomponent,thentheyaredividedintotwoclasses,theelectropositiveandtheelectronegativecomponents.[22]InadditiontheChemicalAbstractsServicehasdevisedamethodtoindexchemicalsubstances.InthisschemeeachchemicalsubstanceisidentifiablebyanumberknownasitsCASregistrynumber.Amoleculeisthesmallestindivisibleportionofapurechemicalsubstancethathasitsuniquesetofchemicalproperties,thatis,itspotentialtoundergoacertainsetofchemicalreactionswithothersubstances.However,thisdefinitiononlyworkswellforsubstancesthatarecomposedofmolecules,whichisnottrueofmanysubstances(seebelow).Moleculesaretypicallyasetofatomsboundtogetherbycovalentbonds,suchthatthestructureiselectricallyneutralandallvalenceelectronsarepairedwithotherelectronseitherinbondsorinlonepairs.Thus,moleculesexistaselectricallyneutralunits,unlikeions.Whenthisruleisbroken,givingthe"molecule"acharge,theresultissometimesnamedamolecularionorapolyatomicion.However,thediscreteandseparatenatureofthemolecularconceptusuallyrequiresthatmolecularionsbepresentonlyinwell-separatedform,suchasadirectedbeaminavacuuminamassspectrometer.Chargedpolyatomiccollectionsresidinginsolids(forexample,commonsulfateornitrateions)aregenerallynotconsidered"molecules"inchemistry.Somemoleculescontainoneormoreunpairedelectrons,creatingradicals.Mostradicalsarecomparativelyreactive,butsome,suchasnitricoxide(NO)canbestable.The"inert"ornoblegaselements(helium,neon,argon,krypton,xenonandradon)arecomposedofloneatomsastheirsmallestdiscreteunit,buttheotherisolatedchemicalelementsconsistofeithermoleculesornetworksofatomsbondedtoeachotherinsomeway.Identifiablemoleculescomposefamiliarsubstancessuchaswater,air,andmanyorganiccompoundslikealcohol,sugar,gasoline,andthevariouspharmaceuticals.However,notallsubstancesorchemicalcompoundsconsistofdiscretemolecules,andindeedmostofthesolidsubstancesthatmakeupthesolidcrust,mantle,andcoreoftheEartharechemicalcompoundswithoutmolecules.Theseothertypesofsubstances,suchasioniccompoundsandnetworksolids,areorganizedinsuchawayastolacktheexistenceofidentifiablemoleculesperse.Instead,thesesubstancesarediscussedintermsofformulaunitsorunitcellsasthesmallestrepeatingstructurewithinthesubstance.Examplesofsuchsubstancesaremineralsalts(suchastablesalt),solidslikecarbonanddiamond,metals,andfamiliarsilicaandsilicatemineralssuchasquartzandgranite.Oneofthemaincharacteristicsofamoleculeisitsgeometryoftencalleditsstructure.Whilethestructureofdiatomic,triatomicortetraatomicmoleculesmaybetrivial,(linear,angularpyramidaletc.)thestructureofpolyatomicmolecules,thatareconstitutedofmorethansixatoms(ofseveralelements)canbecrucialforitschemicalnature.Carbondioxide(CO2),anexampleofachemicalcompoundMoleculeAball-and-stickrepresentationofthecaffeinemolecule(C8H10N4O2).A2-Dskeletalmodelofabenzenemolecule(C6H6)SubstanceandmixtureExamplesofpurechemicalsubstances.Fromlefttoright:theelementstin(Sn)andsulfur(S),diamond(anallotropeofcarbon),sucrose(puresugar),andsodiumchloride(salt)andsodiumbicarbonate(bakingsoda),whicharebothioniccompounds.Achemicalsubstanceisakindofmatterwithadefinitecompositionandsetofproperties.[23]Acollectionofsubstancesiscalledamixture.Examplesofmixturesareairandalloys.[24]Themoleisaunitofmeasurementthatdenotesanamountofsubstance(alsocalledchemicalamount).Themoleisdefinedasthenumberofatomsfoundinexactly0.012kilogram(or12grams)ofcarbon-12,wherethecarbon-12atomsareunbound,atrestandintheirgroundstate.[25]ThenumberofentitiespermoleisknownastheAvogadroconstant,andisdeterminedempiricallytobeapproximately6.022×1023mol−1.[26]Molarconcentrationistheamountofaparticularsubstancepervolumeofsolution,andiscommonlyreportedinmoldm−3.[27]Inadditiontothespecificchemicalpropertiesthatdistinguishdifferentchemicalclassifications,chemicalscanexistinseveralphases.Forthemostpart,thechemicalclassificationsareindependentofthesebulkphaseclassifications;however,somemoreexoticphasesareincompatiblewithcertainchemicalproperties.Aphaseisasetofstatesofachemicalsystemthathavesimilarbulkstructuralproperties,overarangeofconditions,suchaspressureortemperature.Physicalproperties,suchasdensityandrefractiveindextendtofallwithinvaluescharacteristicofthephase.Thephaseofmatterisdefinedbythephasetransition,whichiswhenenergyputintoortakenoutofthesystemgoesintorearrangingthestructureofthesystem,insteadofchangingthebulkconditions.Sometimesthedistinctionbetweenphasescanbecontinuousinsteadofhavingadiscreteboundary,inthiscasethematterisconsideredtobeinasupercriticalstate.Whenthreestatesmeetbasedontheconditions,itisknownasatriplepointandsincethisisinvariant,itisaconvenientwaytodefineasetofconditions.Themostfamiliarexamplesofphasesaresolids,liquids,andgases.Manysubstancesexhibitmultiplesolidphases.Forexample,therearethreephasesofsolidiron(alpha,gamma,anddelta)thatvarybasedontemperatureandpressure.Aprincipaldifferencebetweensolidphasesisthecrystalstructure,orarrangement,oftheatoms.Anotherphasecommonlyencounteredinthestudyofchemistryistheaqueousphase,whichisthestateofsubstancesdissolvedinaqueoussolution(thatis,inwater).Lessfamiliarphasesincludeplasmas,Bose–Einsteincondensatesandfermioniccondensatesandtheparamagneticandferromagneticphasesofmagneticmaterials.Whilemostfamiliarphasesdealwiththree-dimensionalsystems,itisalsopossibletodefineanalogsintwo-dimensionalsystems,whichhasreceivedattentionforitsrelevancetosystemsinbiology.Atomsstickingtogetherinmoleculesorcrystalsaresaidtobebondedwithoneanother.Achemicalbondmaybevisualizedasthemultipolebalancebetweenthepositivechargesinthenucleiandthenegativechargesoscillatingaboutthem.[28]Morethansimpleattractionandrepulsion,theenergiesanddistributionscharacterizetheavailabilityofanelectrontobondtoanotheratom.MoleandamountofsubstancePhaseDiagramshowingrelationshipsamongthephasesandthetermsusedtodescribephasechanges.BondingAchemicalbondcanbeacovalentbond,anionicbond,ahydrogenbondorjustbecauseofVanderWaalsforce.Eachofthesekindsofbondsisascribedtosomepotential.Thesepotentialscreatetheinteractionswhichholdatomstogetherinmoleculesorcrystals.Inmanysimplecompounds,valencebondtheory,theValenceShellElectronPairRepulsionmodel(VSEPR),andtheconceptofoxidationnumbercanbeusedtoexplainmolecularstructureandcomposition.Anionicbondisformedwhenametallosesoneormoreofitselectrons,becomingapositivelychargedcation,andtheelectronsarethengainedbythenon-metalatom,becominganegativelychargedanion.Thetwooppositelychargedionsattractoneanother,andtheionicbondistheelectrostaticforceofattractionbetweenthem.Forexample,sodium(Na),ametal,losesoneelectrontobecomeanNa+cationwhilechlorine(Cl),anon-metal,gainsthiselectrontobecomeCl−.Theionsareheldtogetherduetoelectrostaticattraction,andthatcompoundsodiumchloride(NaCl),orcommontablesalt,isformed.Inacovalentbond,oneormorepairsofvalenceelectronsaresharedbytwoatoms:theresultingelectricallyneutralgroupofbondedatomsistermedamolecule.Atomswillsharevalenceelectronsinsuchawayastocreateanoblegaselectronconfiguration(eightelectronsintheiroutermostshell)foreachatom.Atomsthattendtocombineinsuchawaythattheyeachhaveeightelectronsintheirvalenceshellaresaidtofollowtheoctetrule.However,someelementslikehydrogenandlithiumneedonlytwoelectronsintheiroutermostshelltoattainthisstableconfiguration;theseatomsaresaidtofollowtheduetrule,andinthiswaytheyarereachingtheelectronconfigurationofthenoblegashelium,whichhastwoelectronsinitsoutershell.Similarly,theoriesfromclassicalphysicscanbeusedtopredictmanyionicstructures.Withmorecomplicatedcompounds,suchasmetalcomplexes,valencebondtheoryislessapplicableandalternativeapproaches,suchasthemolecularorbitaltheory,aregenerallyused.Seediagramonelectronicorbitals.Inthecontextofchemistry,energyisanattributeofasubstanceasaconsequenceofitsatomic,molecularoraggregatestructure.Sinceachemicaltransformationisaccompaniedbyachangeinoneormoreofthesekindsofstructures,itisinvariablyaccompaniedbyanincreaseordecreaseofenergyofthesubstancesinvolved.Someenergyistransferredbetweenthesurroundingsandthereactantsofthereactionintheformofheatorlight;thustheproductsofareactionmayhavemoreorlessenergythanthereactants.Areactionissaidtobeexergonicifthefinalstateislowerontheenergyscalethantheinitialstate;inthecaseofendergonicreactionsthesituationisthereverse.Areactionissaidtobeexothermicifthereactionreleasesheattothesurroundings;inthecaseofendothermicreactions,thereactionabsorbsheatfromthesurroundings.Chemicalreactionsareinvariablynotpossibleunlessthereactantssurmountanenergybarrierknownastheactivationenergy.Thespeedofachemicalreaction(atgiventemperatureT)isrelatedtotheactivationenergyE,bytheBoltzmann'spopulationfactor-thatistheprobabilityofamoleculetohaveenergygreaterthanorequaltoEatthegiventemperatureT.ThisexponentialdependenceofareactionrateontemperatureisknownastheArrheniusequation.Theactivationenergynecessaryforachemicalreactiontooccurcanbeintheformofheat,light,electricityormechanicalforceintheformofultrasound.[29]Ananimationoftheprocessofionicbondingbetweensodium(Na)andchlorine(Cl)toformsodiumchloride,orcommontablesalt.Ionicbondinginvolvesoneatomtakingvalenceelectronsfromanother(asopposedtosharing,whichoccursincovalentbonding)Inthemethanemolecule(CH4),thecarbonatomsharesapairofvalenceelectronswitheachofthefourhydrogenatoms.Thus,theoctetruleissatisfiedforC-atom(ithaseightelectronsinitsvalenceshell)andtheduetruleissatisfiedfortheH-atoms(theyhavetwoelectronsintheirvalenceshells).EnergyArelatedconceptfreeenergy,whichalsoincorporatesentropyconsiderations,isaveryusefulmeansforpredictingthefeasibilityofareactionanddeterminingthestateofequilibriumofachemicalreaction,inchemicalthermodynamics.AreactionisfeasibleonlyifthetotalchangeintheGibbsfreeenergyisnegative,;ifitisequaltozerothechemicalreactionissaidtobeatequilibrium.Thereexistonlylimitedpossiblestatesofenergyforelectrons,atomsandmolecules.Thesearedeterminedbytherulesofquantummechanics,whichrequirequantizationofenergyofaboundsystem.Theatoms/moleculesinahigherenergystatearesaidtobeexcited.Themolecules/atomsofsubstanceinanexcitedenergystateareoftenmuchmorereactive;thatis,moreamenabletochemicalreactions.Thephaseofasubstanceisinvariablydeterminedbyitsenergyandtheenergyofitssurroundings.Whentheintermolecularforcesofasubstancearesuchthattheenergyofthesurroundingsisnotsufficienttoovercomethem,itoccursinamoreorderedphaselikeliquidorsolidasisthecasewithwater(H2O);aliquidatroomtemperaturebecauseitsmoleculesareboundbyhydrogenbonds.[30]Whereashydrogensulfide(H2S)isagasatroomtemperatureandstandardpressure,asitsmoleculesareboundbyweakerdipole-dipoleinteractions.Thetransferofenergyfromonechemicalsubstancetoanotherdependsonthesizeofenergyquantaemittedfromonesubstance.However,heatenergyisoftentransferredmoreeasilyfromalmostanysubstancetoanotherbecausethephononsresponsibleforvibrationalandrotationalenergylevelsinasubstancehavemuchlessenergythanphotonsinvokedfortheelectronicenergytransfer.Thus,becausevibrationalandrotationalenergylevelsaremorecloselyspacedthanelectronicenergylevels,heatismoreeasilytransferredbetweensubstancesrelativetolightorotherformsofelectronicenergy.Forexample,ultravioletelectromagneticradiationisnottransferredwithasmucheficacyfromonesubstancetoanotherasthermalorelectricalenergy.Theexistenceofcharacteristicenergylevelsfordifferentchemicalsubstancesisusefulfortheiridentificationbytheanalysisofspectrallines.Differentkindsofspectraareoftenusedinchemicalspectroscopy,e.g.IR,microwave,NMR,ESR,etc.Spectroscopyisalsousedtoidentifythecompositionofremoteobjects-likestarsanddistantgalaxies-byanalyzingtheirradiationspectra.EmissionspectrumofironThetermchemicalenergyisoftenusedtoindicatethepotentialofachemicalsubstancetoundergoatransformationthroughachemicalreactionortotransformotherchemicalsubstances.Whenachemicalsubstanceistransformedasaresultofitsinteractionwithanothersubstanceorwithenergy,achemicalreactionissaidtohaveoccurred.Achemicalreactionisthereforeaconceptrelatedtothe"reaction"ofasubstancewhenitcomesinclosecontactwithanother,whetherasamixtureorasolution;exposuretosom