Molecular GeometryBackground Information

●We’ve done Lewis Dot representations of molecules: 2D●We’ve got to go to 3D!●But first, some terms....

Molecular Geometry

Some Terms:● Electron Domains: The spaces electrons occupy

around the nucleus of an atom

- Bonding domains: shared pairs- Any bond type = 1 bonding domain- Non-bonding domains: lone pairs

Molecular GeometrySome Terms - Electron Domains

CH HH

H

OCO

Molecular Geometry

●VSEPR Theory

Valence Shell Electron Pair Repulsion Theory

- Electron domains repel each other

-They push as far from each other as possible

●VSEPR theory + electron domains = 3D structures

Molecular Geometry

One Last Thing to Keep in Mind●Not all electron domains are the same size:

+ +-

+-

-Bonded electron domains are long and skinny-Due to the pull of the two nuclei

-Non-bonding electron domains are short and fat-Due to the pull of one nuclei

Molecular Geometry

Central Atoms with 4 Electron Domains●When four objects surround a central point,

what angles are created if these objects are as far apart as possible?

109.5º angles

Tetrahedral

Molecular Geometry4 Electron Domains: Tetrahedral domain geometry

Molecule LDMS Bonding Non-Bonding Angle Shape Name

CH4

NH3

H2O

HF

C HHH

H

N HHH

O HH

FH

4 0 109.5º Tetrahedral

3 1 <109.5º TrigonalPyramid

2 2 <109.5º Bent

1 3 Undefined Straight

Let’s see it!http://phet.colorado.edu/en/simulation/build-a-molecule

Molecular Geometry

Central Atoms with 4 Electron Domains

●All of these molecules have tetrahedral domain geometries

-They all have 4 electron domains●Their molecular geometries depend on the number

of bonding and non-bonding domains

Molecular Geometry

Central Atoms with 3 Electron Domains●When three objects surround a central point,

what angles are created if these objects are as far apart as possible?

120.0º anglesTrigonal Planar

Molecular Geometry3 Electron Domains: Trigonal Planar domain geometry

●Both of these molecules have trigonal planar domain geometries

-They both have 3 electron domains

Molecule LDMS Bonding Non-Bonding Angle Shape Name

C2H4

NO2-1

CHH

C HH

N OO

3 0 120.0º TrigonalPlanar

2 1 < 120.0º Bent

Let’s see it!http://phet.colorado.edu/en/simulation/build-a-molecule

Molecular GeometryCentral Atoms with 3 Electron Domains

● Their molecular geometries depend on the number of bonding and non-bonding domains

● Notice that we left off the example of 1 bonding pair- Anytime there is only one bonding pair, the angle

is undefined and the name is straight, regardless of the number of electron domains

Molecular Geometry

Central Atoms with 2 Electron Domains●When two objects surround a central point, what angles are created if these objects are as far apart as possible?

180.0º anglesLinear

Molecular Geometry2 Electron Domains: Linear domain geometry

● Any molecule with 2 electron domains will have a linear domain geometry

● Notice, again, that we didn’t include an example with 1 bonding domain

Molecule LDMS Bonding Non-Bonding Angle Shape Name

CO2 CO O 2 0 180.0º Linear

Let’s see it!http://phet.colorado.edu/en/simulation/build-a-molecule

Molecular GeometryLet’s Practice

●Draw the LDMS for the following molecules. Then…○Identify the domain and molecular geometries○Determine the bond angle of the central atom

PBr3 O2

HCNBr BrP

Br

DG =MG =Angle =

TetrahedralTrigonal pyramid107.0º

O O

DG =MG =Angle =

Trigonal pyramidStraightUndefined

H CDG =MG =Angle =

LinearLinear180.0º

N