119
Lab11:MolecularModels
Introduction
Whycan’tyouplaybasketballwithafootball?
The obvious answer is because a football isn’t the right
shape.Afootballcan’tbedribbledandwouldbeverydiffi
culttoshoot.Ontheotherhand,afairlylargesphericalball
can be easily dribbled and is needed to successfully play
basketball.Theshapeofamoleculeisalso
extremelyim
portantinhowitcanbeused.Justlikesportsneedobjects
ofaparticularshape,certainreactionsorapplicationsneed
moleculesofaparticularshape.
The shapes ofthe different objectsused in sports can be
described in a few words. For example, an arrow can be
describedasstraightorlinear,ahockeypuckasaflatdisc,
a football as oblong, and a basketball as spherical. Mole
culesalso have definiteshapesthatcanbe describedina
few words. A few molecular shape examples are linear,
bent,trigonalplanar,pyramidal,andtetrahedral.Someof
these
words might already give you an idea of how such
moleculesareshaped.
But how do you know the shape of a particular molecule
sinceyoucan’tseeit?Fortunatelyamodelhasbeendevel
oped that helps us predict the shapes of molecules.It is
called the VSEPR model.
VSEPR stands for valenceshell
electronpairrepulsion.
Whenyouplayateamsportoneofthefirstthingsyouneedtodecideiswhoisgo
ingtobe the captain. Whenusingthe VSEPR modelacentralatomhasto first be
decided.Thisisusuallytheatomin
themoleculewiththemostbonds.Asinglebond
countsasonebond, adoublebondcountsastwobonds,etc.Forexample,thecar
bonatominmethane,CH
4
,isthecentralatomsinceithas4bonds andeachhydro
genatomhasjustonebond.Inwater,H
2
O,theoxygenhastwobondssoitis chosen
asthecentralatom.Incarbondioxide,CO
2
,thecarbonhas2doublebondsor4total
bonds so it is chosen as the central atom. In the case where two atoms have the
samenumberofbonds,onecanbechosen.Forexample,theoxygenatomsinhy
drogenperoxide,HOOH,eachhave2bondstothem
whereasthehydrogenatoms
each have only one bond. In this case either one of the oxygen atoms can be de
pictedasthecentralatom.
Figure1:Molecularstructureofacaffeinemolecule.Canyou
identifyinstanceswhereatomsactasthecentralatomoftheir
grouping?
Figure2:Molecularmodelformeth
ane,CH
4
.Notethefourregionsof
electrondensity—oneforeachsingle
bondtothecentralcarbonatom
Conceptstoexplore:
 Understandwhymoleculeshaveaparticularshape
 DeterminetheshapesofmoleculesusingtheVSEPRmodel
120
ExampleAtom
(centralatomis
underlined)
Numberof
BondsonCen
tralAtom
(multiplebonds
countas1)
Unsharedelec
tronpairson
centralatom
Regionsof
ElectronDen
sity
MolecularGe
ometry
Structure
CO
2
2 0 2
Linear
COO
AlCl
3
3 0 3
TrigonalPlanar
Al
Cl
Cl
Cl
H
2
O 2 2 4
Bent
O
H
H
:
..
NH
3
3 1 4
Trigonalpyrami
dal
..
N
H
H
H
CCl
4
4 0 4
Tetrahedral
C
Cl
Cl
Cl
Cl
PCl
5
5 0 5
Trigonalbipyra
midal
P
Cl
Cl
Cl
Cl
Cl
SF
6
6 0 6
Octahedral
S
F
F
FF
FF
Table1:MolecularModels
Lab11:MolecularModels
121
Afterthecentralatomisdecided,thenumberofregionsofelectrondensity
arounditisdetermined.Thisisaregionthatgenerallydescribesthelocation
of negative charge due to surrounding electrons. A single, double, or triple
bondora pair ofunshared electronscounts asoneregionof
electronden
sity.Thecarboninmethane,CH
4
,hasfoursinglebondsso ithasfourregions
of electron density. However, the number of regions can be different than
thenumberofbonds:in H
2
O,theoxygenmoleculehastwosinglebondsand
two pairs of unshared electrons—resulting in four regionsof 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
modelis basedona
similarconcept.Valenceelectrons,sharedorunshared,
createanegativeregionthatrepelstheothernegativeregions.Forthisrea
son, the regions of electron density are placed the same distan ce from the
centralatom,butasfarawayfromeachotheraspossible.
Thelastthingtobedone
istodescribetheshapetheatomsmake.Theun
sharedelectrons influencetheshape,butonlytheresultingshapeoftheatomsisdescribed.Certainterminology isused
forthis.Theshapeiscalledthemoleculargeometry.ListedinTable1aresomecommonmolecularshapesandthewords
usedtodescribethem.
Allofthiscanbesummarizedin4steps.
1.Determinethecentralatom.
2.Determinetheregionsofelectrondensityaroundthecentralatombyaddingthenumberofbondedatoms
andthenumberofunsharedelectrons.
3.Place the regions the same distance away from the central atom and as far as possible away from each
other.
4.
Describetheshapeonlytheatomsmake.
PrelabQuestions
1.Identifythecentralatomanddeterminehow manyregionsofelectrondensitytherearearoundthecentral
atomineachofthefollowingmolecules:
Figure3:Regionsofelectrondensityfora
watermolecule(H
2
O).
Lab11:MolecularModels
Molecule CentralAtom #RegionsofElectronDensity
BeCl
2
BH
3
CBr
4
Table2:Prelabexercise
Be
2
B
3
C
4
122
2.ExplainwhytheshapeofH
2
Oisbentandnotlinear.
Experiment:MolecularStructures
Procedure
Part1:Magnets
1.Bringtogetherthesamepolesoftwobarmagnets.
2.Observeanddescribewhathappens.
3.Explore the linear shape of molecules by bringing the
samepolesasclosetogetherasyoucaninastraightline.
RecordyourobservationsintheDatasection.
4.Explorewhyamoleculemakes
atrigonalplanarshapeby
placingthesamepoleof3barmagnetsasthepointsof
anequilateraltrianglewiththerestofthemagnetbehind
them.
5.Slowlybringthesamepoleofthe3magnetsascloseasit
ispossiblewhilemaintainingtheequilateraltriangularappearance.
Nowtrytofind anothershapethatwillbe
asstablewithallofthemagnetsatleastascloseastheywereintheequilateraltriangleposition.Recordyour
observationsinTable3.
Figure4:MagnetsetupforexperimentProcedure1.
Lab11:MolecularModels
Materials
3Barmagnets
Toothpicks
Modelingclay
Protractor
H2O is bent because it has 2 pairs of unshared electrons and 4 regions of electron density.
123
Part2:ModelingClay
1.MakeamodelofamoleculemadeupoftwoatomssuchasH
2
byattachingballsofmodelingclaytoboth
endsofatoothpick.DescribetheshapeinTable3.
2.Makeamodelofalinearmoleculethatismadeupofthreeatoms(suchasCaCl
2
)
usingthefollowingsteps:
a. Firstattachtwotoothpickstoalargerballofmodelingclayrepresentingthecentralatom.
Locatethemsothattheyareasfarawayfromeachotheraspossible.
b. Nextattachtwosmallerballsofmodelingclaytotheendsofthetwotoothpicks.
c.
DescribetheshapeinTable3.
d. Determinethebondanglefromoneofthesmallerballsofmodelingclaytothelargeballto
anothersmallballusingaprotractor.RecordthebondangleinTable3.
3. MakemodelsoftheothermolecularshapeslistedinTable4bythefollowing:
a. Firstattachatoothpicktoalargeballofmodelingclay(centralatom)foreachregionof
electrondensityfromTable1.Placethemsothattheyareasfarawayfromeachotheras
possible.
b. Nextattach a smallballofmodelingclayfor each bond in the
molecule. Youshouldhave
oneballforeachbondlistedinTable1.
c. Removethetoothpicksthatdonothavesmallballsattached.
d. DescribeandrecordtheshapeinTable4.
e. Useaprotractortodeterminethebondanglefromoneofthesmallballstothe
largeballto
anothersmallball.RecordthebondangleintheDatasection.
Lab11:MolecularModels
Data
ProcedureStep Observations
Step1
Step2
Step3
Table3:ObservationsforProcedure1
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
MolecularShape Observations BondAngle
Linear(2atoms)

Linear(3atoms)

TrigonalPlanar

Bent

TrigonalPyramidal

Tetrahedral

TrigonalBipyrami
dal

Octahedral

Table4:ObservationsanddataforProcedure2
Lab11:MolecularModels
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
PostlabQuestions
1.InPart2oftheprocedure,didyourmodelsfitthemolecular shapedescriptionormatchthegeometryinthe
introduction?Explainwhyorwhynot.
2.Predicttheshapesandbondanglesofthefollowingmolecules:
a. BeCl
2
b. BH
3
c. CBr
4
Lab11:MolecularModels
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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