Download Experiment 30A3- Structures

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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.
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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.
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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.
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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.