Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
1 EXPERIMENT30A3:LEWISSTRUCTURES&SHAPESOFMOLECULESANDIONS LearningOutcomes Uponcompletionofthislab,thestudentwillbeableto: 1) ConstructLewisstructuresforsimpleionsandmolecules. 2) PredicttheshapesofsimpleionsandmoleculesfromtheirLewis structures. Introduction Lewisstructuresorelectrondotformulasareadiagrammaticrepresentationofthe variouselectrongroups,bothbondingandnon-bonding,aroundacentralatomofan ionormolecule.Lewisstructuresenablethepredictionofthethreedimensional arrangementoftheelectrongroupsbasedontheValenceShellElectronPair Repulsion(VSEPR)theory. Lewisstructurescanbeconstructedfromtheformulaofanionoramoleculeby followingasetofrulesdescribedbelow: 1. Countthetotalnumberofvalenceelectrons.Addoneelectronforeachnegative chargeandsubtractoneelectronforeachpositivecharge. 2. Placetheleastelectronegativeatomthatisnothydrogeninthecenter. 3. Placealltheotheratomsaroundthecentralatom. 4. Connectthecentralatomtoeachouteratombyusingasinglecovalentbond. 5. Completetheoctetforthemoreelectronegativeouteratoms(onlytwoelectrons arerequiredforhydrogen,fourforberyllium,andsixforboron). 6. Placeanyremainingelectronsonthecentralatom. 7. Ifalltheatomsdonothaveanoctetthenborrowelectronsfromanouteratom andconstructadoubleortriplebondbetweentheouteratomandthecentral atom. 8. Step7mayresultinmultiplepossibilities,calledresonancestructures.Incase multiplepossibilitiesexist,thenfindtheformalchargeofeachatominthe structure. a. Thestructurewiththelowestsetofformalcharges(closesttozero)is thebestLewisstructure. 2 b. Forstructureswithequalamountsofchargeseparation,thebest structurewillbetheonewherethenegativechargeisonthemore electronegativeatom. 9. Thereareafewexceptionstotheoctetrule: a. Hydrogen,beryllium,andboron,asalreadydiscusseddonotcomplete theiroctet. b. Somenitrogencontainingionsandmoleculesmayhaveanodd numberoftotalelectronsandwillthereforehaveoneelementwithan incompleteoctet. c. Elementsinperiod3andbelowareabletoexpandtheiroctetand accommodatemorethaneightelectronsintheirvalenceshellsdueto theavailabilityofvacantd-orbitals. AXENotation OncetheLewisstructureforanionormoleculehasbeenconstructed,amoleculeor ion“type”canbeassignedtothestructurethatwillindicatethenumbersofbonding andnon-bondingelectrongroupsaroundthecentralatom.Forinstance,consider NH3.TheLewisstructureforthismoleculeisshownbelow: H H N H Inthisstructurenitrogenisthecentralatomandhydrogenistheouteratom.There arethreebondstotalbetweenthenitrogenandthehydrogen-implyingthereare threebondingelectrongroups.Thereisalsoonelonepairofelectrononthe nitrogen-implyingthereisonenon-bondingelectrongroup. Toclassifyionsormoleculestoatype,thecentralatomisdepictedasA,theouter atomsinthebondinggroupasXandthenon-bondingelectrongroupasE.Since therearethreebondinggroups(so,3ofX)andonenon-bondinggroup(so,1ofE), NH3wouldbeclassifiedasbeingthetypeAX3E1.Oncethetypehasbeenassigned, thefollowingtablecanbeusedtodeterminetheelectrongroupgeometryandthe moleculargeometryofthestructure. 3 TABLE1 Molecule Total orIon number type of electron groups AX2 2 Number of bonding electron groups 2 Number ofnonbonding electron groups 0 Electron group geometry Molecular geometry Bond angle Linear Linear 180° AX3 3 3 0 Trigonal planar Trigonal planar 120° AX2E1 3 2 1 Trigonal planar Bent AX4 4 4 0 Tetrahedral Tetrahedral 109.5° AX3E1 4 3 1 Tetrahedral Trigonal pyramid AX2E2 4 2 2 Tetrahedral Bent AX5 5 5 0 Trigonal Trigonal 180°, bipyramidal bipyramidal 120° AX4E1 5 4 1 Trigonal See-saw bipyramidal AX3E2 5 3 2 Trigonal T-shape bipyramidal AX2E3 5 2 3 Trigonal Linear bipyramidal AX6 6 6 0 Octahedral Octahedral 90° AX5E1 6 5 1 Octahedral Square pyramid AX4E2 6 4 2 Octahedral Square planar NOTE:Structureswitha)lonepairsonthecentralatomb)doublebondswilltendto havecompressedbondangles. 4 ExperimentalDesign Usingamolecularmodelkitconstructthethree-dimensionalarrangementofthe atomsofthelistofmoleculesorionsprovidedbelowinTable2. TABLE2 HCN BeF2 SO2 O3 OF2 2− BF3 CO3 CH4 ClO4 PF3 H2O PF5 SF4 BrF3 XeF2 IOF5 TeF5− XeF4 XeO2F2 SO42- ReagentsandSupplies Molecularmodelkit Procedure 1. DrawtheLewisstructureofeachmoleculeorioninthelistabove. 2. AssigntheLewisstructuretoaspecifictype. 3. Determinetheelectrongroupgeometryandthemoleculargeometry,using Table1. 4. Usingthemolecularmodelkit,constructthemodelofeachstructureinTable2. 5. Alsoindicatethebondanglesandpolarityofeachstructure.