Chapter 9 Jan13

8/12/2019 Chapter 9 Jan13

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Chapter 9:

Molecular Geometry


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Learning Outcomes

Student should be able to use Lewis structure

to determine the electron domain geometry and

the molecular shape of a compound.


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Molecular Shapes

Lewis electron-dot structures:- give us the connectivity between atoms.- is drawn with atoms in the same plane.

e.g. Lewis structure of CF4shows 4F atomsbonded to a central C atom.

C

F

F

F

F


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The actual 3D arrangement of the atoms has Fatoms at the corners of atetrahedron.

(4 corners, 4 faces, each which is an equilateraltriangle)

The overall shape of a molecule is determined byits bond angle.

In CF4: all 6 C-F angles = 109.5o- characteristicof tetrahedron.

109.5o


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The VSEPR Model

Valence Shell Electron Pair Repulsion Theory(think in 3D)

Developed by Ron Gillespie and IstvnHargittai in 1957.Assumption: electron pairs repel, so the

bonding pairs and lone pairs attached to acentral atom are located as far apart fromeach other as possible.

Bonding pair of electron: a region/domain in

which it is most likely to find electrons.

Nonbonding pair/lone pair: an electron domainthat is located principally on 1 atom.


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e.g.

Electron domain geometry: arrangement ofelectron pairs/domainsaround the central atom.

Molecular geometry: arrangement of atomsaround the central atom.

When describing shapes of molecules - give themolecular geometry rather than electron domaingeometry.

N

HH

H

Non bonding pair/lone pair

electron

All the N-H bonds involve

bonding electron pairs


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The VSEPR Model

5 Basic Arrangements of Electron Pair s/Domain:

No. of electrondomain

2

3

Electron-domain Geometry

Linear

Trigonalplanar

Bond angles

180o

120o


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No. of electron

domain

4

5

6

Electron-domain Geometry

Tetrahedral

Trigonal

bipyramidal

Octahedral

Bond angles

109.5o

120oand 90o

90


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The VSEPR Model Trigonal bipyramid:

the electron domainscan point toward 2

distinct type of positions.

2 positions arecalled axial positions,

remaining 3 -

equatorial

positions.

120o

90o

120o


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The VSEPR Model

Axial (ax)

Axial (ax)

Equatorial (eq)


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Effects of Lone Pair Electrons on Bond Angle

E.g. All 3 compounds below have tetrahedral

electron-domain geometries, but their bond anglesdiffer slightly. Why?

Note that the bond angle decreases as no. of lonepair electrons increases.

C

H

HH

H

NHH

H

O

H

H

107.643o

105.611o

109.471o


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Reasons: Lone pairs repel more than bonding pairs. Therefore, the lone pairs cause the bond angles

to close up.

Bonding pair of electrons is attracted by both

nuclei of the bonded atoms whereas lone pairelectronis attracted by only 1 nucleus. Because lone pair electrons experiences less

nuclear attraction, its electron domain is spread

out more in space/exert greater repulsion onadjacent electron domains (compress bondangles).


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Electron domain geometry: Trigonal Planar

:

Molecular

Geometry: Trigonal Planar Bent

:

120o

Eg: BF3

B

F

F F

Eg: NO2-

N

O O

115.4o


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Electron domain geometry: Tetrahedral

:

Molecular

Geometry::

Tetrahedral

:

Trigonal

pyramidal:Bent

109.5o

N

H

H

H

C

H

H

H

H

O

H

H


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Molecules with Expanded Valence Shells When the central atom of a molecule is from the

3rd period and beyond of the periodic table, theatoms may have 4 electron pairs around it.

Molecules with 5/6 electron domains display avariety of molecular geometries.

E.g.: Trigonal bipyramidal: will lone pair electrondomain occupy axial or equatorial positions?

Repulsions between domain is greater if they aresituated 90ofrom each other than when they are

120o. Hence equatorial domain experiences lessrepulsion than axial domain.

Thus, lone pair electrons always occupy equatorialpositions in a trigonal bipyramidal.


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The VSEPR Model

120o

90o

120o


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Octahedron - solid object with 6 vertices and8 faces, each of which is an equilateraltriangle.

All angles are 90o, and all 6 positions areequivalent, thus lone pair electrons may

occupy any of the 6 vertices.

However, if there are 2 lone pair electron

domains, their repulsions are minimized bypointing them toward opposite sides of theoctahedron.


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Steps to predict molecular geometries usingVSEPR model1. Sketch Lewis structure of the molecule/ion.

2. Count total no. of electron domains and arrange themin the way that minimizes repulsions among them.

3. Describe molecular geometry.

4. A double/triple bond is counted as 1 electron domain.Note:

# of electron domains

= (# of atoms bondedto the central atom) +

(# of nonbondingpairson the central atom)


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Determine the molecular geometry in ammonia, NH3.

Valence Electrons:

N = 5

3H = 3 1 = 3

Total = 8

Example 1:

N

H

HHN

H

HH N HH

H


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4 electron pairs around N:

Electron domain geometry istetrahedral.

3 atoms around N with 1 lone pair:

Molecular geometry is trigonal pyramidal.

H

H

H

N

107.6


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Example 2:

Determine the molecular geometry and bond angles in

the ammonium ion, NH4+.

Valence Electrons:

N = 5

4H = 4 1 = 4

Charge = -1

Total = 8

N

H

HH

H

N

H

HH

H

N

H

H

H

H


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N

H

H

H

H

4 pairsof electrons around N:Electron domain geometry is tetrahedral.

4 atomsaround N:Molecular geometry istetrahedral.

No lone pairs: Bond angles are 109.5o

.

N

H

HHH

109.5o


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water.Valence Electrons:

O = 6

2H = 2 1 = 2

Total = 8

Example 3:

O HH O HH O HH


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4 pairsof electrons around O:Electron domain geometry istetrahedral.

2 atomsaround O:Molecular geometry isbent.

Two lone pairs:Bond angle is less than 109.5o.

O HH O

H

H105.6

o


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CO2.Valence Electrons:

C = 4

2O = 2 6 = 12

Total = 16

Example 4:

O C O O C O O C O C OO


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In VSEPR, we count multiple bonds as a singleelectron domain.2 pairsof electrons around C:

Electron domain geometry is linear.

2 atomsaround C: Molecular geometry is linear.

No lone pairs on C: Bond angle is 180o.

C OO CO O

180o


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Determine the molecular geometry and approximate bond

angles in XeOF4.Valence Electrons:

Xe = 8

O = 6

4F = 4 7 = 28

Total = 42

Example 5:

Xe

O

F

FF

F

O

F

FF

F

XeXe

OF

F F

F

6 f l d


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6 pairsof electrons around Xe:Electron domain geometry is octahedral.

5 atomsaround Xe:Molecular geometry is square pyramidal.

One lone pair on Xe:O-Xe-F bond angle is less than 90o.

Xe

O

F

F F

F O

XeF F

FF

E i


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Exercises:

Determine the molecular geometry in each of the

following:

(a)SF4

(b)CH2O

F S F

F F

C

O

HH

See-

saw

Trigonal

planar


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End of Chapter 9


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The Lewis structure of phosphine, PH3

has

_____________ of electrons.

A. four bonding pairs

B. three bonding pairs and one lone pair

C. one bonding pair and three lone pairs

D. two bonding pairs and two lone pairs


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The Lewis structure of phosphine, PH3

has

_____________ of electrons.

A. four bonding pairs

B. three bonding pairs and one lone pair

C. one bonding pair and three lone pairs

D. two bonding pairs and two lone pairs


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Use Valence Shell Electron Pair Repulsion (VSEPR)

theory to predict the molecular geometry of CO32

ion.

A. Linear

B. Tetrahedral

C. Trigonal planar

D. Trigonal pyramidal


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Use Valence Shell Electron Pair Repulsion (VSEPR)

theory to predict the molecular geometry of CO32

ion.

A. Linear

B. Tetrahedral

C. Trigonal planar

D. Trigonal pyramidal

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Chapter 9 Jan13