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119



Lab11:MolecularModels

Introduction

Whycan’tyouplaybasketballwithafootball?

The obvious answer is because a football isn’t the right

shape.Afootballcan’tbedribbledandwouldbeverydiffi‐

culttoshoot.Ontheotherhand,afairlylargesphericalball

can be easily dribbled and is needed to successfully  play

basketball.Theshapeofamoleculeisalso

extremelyim‐

portantinhowitcanbeused.Justlikesportsneedobjects

ofaparticularshape,certainreactionsorapplicationsneed

moleculesofaparticularshape.

The shapes ofthe different objectsused in sports can be

described in a few words. For example, an arrow can be

describedasstraightorlinear,ahockeypuckasaflatdisc,

a football as oblong, and a basketball as spherical. Mole‐

culesalso have definiteshapesthatcanbe describedina

few words. A few molecular shape examples are linear,

bent,trigonalplanar,pyramidal,andtetrahedral.Someof

these

 words might already give you an idea of how such

moleculesareshaped.

But how do you know the shape of a particular molecule

sinceyoucan’tseeit?Fortunatelyamodelhasbeendevel‐

oped that helps us predict the shapes of molecules.It is

called the VSEPR model.

VSEPR stands for valence‐shell

electron‐pairrepulsion.

Whenyouplayateamsportoneofthefirstthingsyouneedtodecideiswhoisgo‐

ingtobe the captain. Whenusingthe VSEPR modelacentralatomhasto first be

decided.Thisisusuallytheatomin

themoleculewiththemostbonds.Asinglebond

countsasonebond, adoublebondcountsastwobonds,etc.Forexample,thecar ‐

bonatominmethane,CH

,isthecentralatomsinceithas4bonds andeachhydro ‐

genatomhasjustonebond.Inwater,H

O,theoxygenhastwobondssoitis chosen

asthecentralatom.Incarbondioxide,CO

,thecarbonhas2doublebondsor4total

bonds so it is chosen as the central atom. In the case where two atoms have the

samenumberofbonds,onecanbechosen.Forexample,theoxygenatomsinhy‐

drogenperoxide,HOOH,eachhave2bondstothem

whereasthehydrogenatoms

each have only one bond. In this case  either one of the oxygen atoms can be de‐

pictedasthecentralatom.

Figure1:Molecularstructureofacaffeinemolecule.Canyou

identifyinstanceswhereatomsactasthecentralatomoftheir

grouping?

Figure2:Molecularmodelformeth‐

ane,CH

.Notethefourregionsof

electrondensity—oneforeachsingle

bondtothecentralcarbonatom

Conceptstoexplore:

 Understandwhymoleculeshaveaparticularshape

 DeterminetheshapesofmoleculesusingtheVSEPRmodel



120

ExampleAtom

(centralatomis

underlined)

Numberof

BondsonCen‐

tralAtom

(multiplebonds

countas1)

Unsharedelec‐

tronpairson

centralatom

Regionsof

ElectronDen‐

sity

MolecularGe‐

ometry

Structure

 2 0 2

Linear 





COO

AlCl

 3 0 3

TrigonalPlanar





O 2 2 4

Bent





 3 1 4

Trigonalpyrami‐

dal





CCl

 4 0 4

Tetrahedral





PCl

 5 0 5

Trigonalbipyra‐

midal





 6 0 6

Octahedral





Table1:MolecularModels

Lab11:MolecularModels

121



Afterthecentralatomisdecided,thenumberofregionsofelectrondensity

arounditisdetermined.Thisisaregionthatgenerallydescribesthelocation

of negative charge due to surrounding electrons. A single, double,  or triple

bondora pair ofunshared electronscounts asoneregionof

electronden‐

sity.Thecarboninmethane,CH

,hasfoursinglebondsso ithasfourregions

of electron density. However, the number of regions can be different than

thenumberofbonds:in H

O,theoxygenmoleculehastwosinglebondsand

two pairs of unshared electrons—resulting in four regionsof ele ctron den‐

sity.

If you have played with magnets, you probably know that when the same

poles of magnets are brought together they repel each other. The VSEPR

modelis basedona

similarconcept.Valenceelectrons,sharedorunshared,

createanegativeregionthatrepelstheothernegativeregions.Forthisrea‐

son, the regions of electron density are placed the same distan ce from the

centralatom,butasfarawayfromeachotheraspossible.

Thelastthingtobedone

istodescribetheshapetheatomsmake.Theun‐

sharedelectrons influencetheshape,butonlytheresultingshapeoftheatomsisdescribed.Certainterminology isused

forthis.Theshapeiscalledthemoleculargeometry.ListedinTable1aresomecommonmolecularshapesandthewords

usedtodescribethem.

Allofthiscanbesummarizedin4steps.

1. Determinethecentralatom.

2. Determinetheregionsofelectrondensityaroundthecentralatombyaddingthenumberofbondedatoms

andthenumberofunsharedelectrons.

3. Place the regions the same distance away from the central atom and as far as possible away from each

other.

 Describetheshapeonlytheatomsmake.

Pre‐labQuestions

1. Identifythecentralatomanddeterminehow manyregionsofelectrondensitytherearearoundthecentral

atomineachofthefollowingmolecules:

Figure3:Regionsofelectrondensityfora

watermolecule(H

O).

Lab11:MolecularModels

Molecule CentralAtom #RegionsofElectronDensity

BeCl



CBr



Table2:Pre‐labexercise



122

2. ExplainwhytheshapeofH

Oisbentandnotlinear.

Experiment:MolecularStructures

Procedure

Part1:Magnets

1. Bringtogetherthesamepolesoftwobarmagnets.

2. Observeanddescribewhathappens.

3. Explore the linear shape of molecules by bringing  the

samepolesasclosetogetherasyoucaninastraightline.

RecordyourobservationsintheDatasection.

4. Explorewhyamoleculemakes

atrigonalplanarshapeby

placingthesamepoleof3barmagnetsasthepointsof

anequilateraltrianglewiththerestofthemagnetbehind

them.

5. Slowlybringthesamepoleofthe3magnetsascloseasit

ispossiblewhilemaintainingtheequilateraltriangularappearance.

Nowtrytofind anothershapethatwillbe

asstablewithallofthemagnetsatleastascloseastheywereintheequilateraltriangleposition.Recordyour

observationsinTable3.

Figure4:MagnetsetupforexperimentProcedure1.

Lab11:MolecularModels

Materials

3Barmagnets

Toothpicks

Modelingclay

Protractor

H2O is bent because it has 2 pairs of unshared electrons and 4 regions of electron density.

123



Part2:ModelingClay

1. MakeamodelofamoleculemadeupoftwoatomssuchasH

byattachingballsofmodelingclaytoboth

endsofatoothpick.DescribetheshapeinTable3.

2. Makeamodelofalinearmoleculethatismadeupofthreeatoms(suchasCaCl

)



usingthefollowingsteps:

a. Firstattachtwotoothpickstoalargerballofmodelingclayrepresentingthecentralatom.

Locatethemsothattheyareasfarawayfromeachotheraspossible.

b. Nextattachtwosmallerballsofmodelingclaytotheendsofthetwotoothpicks.

 DescribetheshapeinTable3.

d. Determinethebondanglefromoneofthesmallerballsofmodelingclaytothelargeballto

anothersmallballusingaprotractor.RecordthebondangleinTable3.



3. MakemodelsoftheothermolecularshapeslistedinTable4bythefollowing: 

a. Firstattachatoothpicktoalargeballofmodelingclay(centralatom)foreachregionof

electrondensityfromTable1.Placethemsothattheyareasfarawayfromeachotheras

possible.

b. Nextattach a smallballofmodelingclayfor each bond in the

molecule. Youshouldhave

oneballforeachbondlistedinTable1.

c. Removethetoothpicksthatdonothavesmallballsattached.

d. DescribeandrecordtheshapeinTable4.

e. Useaprotractortodeterminethebondanglefromoneofthesmallballstothe

largeballto

anothersmallball.RecordthebondangleintheDatasection.

Lab11:MolecularModels

Data

ProcedureStep Observations

Step1



Step2



Step3



Table3:ObservationsforProcedure1

The same poles will repel each other and not come together. I have to use pushing

force to get them close to each other.

When I bring the poles together to form a straight line, the poles repel each other and

make the ends either go in the direction of up or down.

The equilateral triangle is the shape that will let them come the closest compared to

all the rest of the shapes.



124

MolecularShape Observations BondAngle

Linear(2atoms)



Linear(3atoms)



TrigonalPlanar



Bent



TrigonalPyramidal



Tetrahedral



TrigonalBipyrami‐

dal



Octahedral



Table4:ObservationsanddataforProcedure2

Lab11:MolecularModels

The small and large atoms form a line in the same plane.

180 degrees

The small and large atoms are on the same plane.

180 degrees

The atoms form a triangle around a central atom, they are all on the

same plane.

120 degrees

The large and small atoms form a triangle.

104 degrees

Three atoms form a triangle at the bottom of the central atom.

107 degrees

The atoms forms a triangle on four planes.

109 degrees

3 atoms form triangle around central atom at 120 degrees and 2 more

atoms at 90 degrees, one above and below the plane.

90 degrees and

120 degrees

4 atoms form a square around the central atom, in the same plane

with 2 or more atoms, one above and below the plane.

90 degrees

125



Post‐labQuestions

1. InPart2oftheprocedure,didyourmodelsfitthemolecular shapedescriptionormatchthegeometryinthe

introduction?Explainwhyorwhynot.



2. Predicttheshapesandbondanglesofthefollowingmolecules:

a. BeCl



b. BH



c. CBr



Lab11:MolecularModels

Not all of them matched the description, but they were close. I think the reason was the fact that the

atoms models could not repel each other like real atoms can do.

Linear and 180 degrees

Trigonal planar 120 degrees

Tetrahedral and 109.5 degrees

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