Molecular Geometry

• Lewis structures show the number and type of bonds between atoms in a molecule.– All atoms are drawn in the same plane (the

paper).– Do not show the shape of the molecule.

1

Molecular Shapes

• The shape of a molecule plays an important role in its reactivity.

• The shape of a molecule is determined by the bond angles and the bond lengths.

• By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.

2

Molecular Geometry

• Bond length: the distance between two atoms held together by a chemical bond– Bond length decreases as the number of

bonds between two atoms increases.• Single bond is the longest.• Triple bond is the shortest.

3

Molecular Geometry

• Bond angle: the angle made by the “lines” joining the nuclei of the atoms in a molecule

H

O

H

104.5o

4

Molecular Geometry

• Many of the molecules we have discussed have central atoms surrounded by 2 or more identical atoms:

ABn

where A = central atom

B = outer atoms

n = # of “B” atoms

Examples: CO2, H2O, BF3, NH3, CCl4

5

Molecular Geometry

• The shapes that ABn molecules can have depend, in part, on the value of n.

• For a specific value of n, only a few general shapes are observed.

• AB2 molecules– linear

– bent

6

Molecular Geometry

O C O

H

O

H

AB2 molecules can either be linear or bent.

CO2

H2O

linear

bent

7

Molecular Geometry

F

B

F

F

AB3 molecules can either be trigonal planar, trigonal pyramidal, or T-shaped.

Trigonal planar: “A” atom in the center and “B” atoms at each corner of an equilateral triangle. All atoms in the same plane.

8

Molecular Geometry

• Trigonal pyramidal: “A” atom in the center with “B” atoms in the corners of an equilateral triangle. – “A” is above the plane of the triangle formed

by “B” atoms

NH

H

H

9

Molecular Geometry

• Why are some AB2 molecules linear while others are bent?

• Why are some AB3 molecules trigonal planar while others are trigonal pyramidal or T-shaped?

• How can we accurately predict the shape of various ABn molecules?

10

Molecular Geometry

• If “A” is a main group element, the valence-shell electron-pair repulsion model (VSEPR) can be used to predict the shape of an ABn molecule.

• VSEPR counts the number of regions around the central atom where electrons are likely to be found and uses this number to predict the shape.

11

Molecular Geometry

• Electron domains: regions around the central atom where electrons are likely to be found.

• Two types of electron domains are considered:– bonding pairs of electrons– nonbonding (lone) pairs of electrons

12

Molecular Geometry

• Bonding pairs of electrons: electrons that are shared between two atoms

Cl

Cl C Cl

Cl

Bonding pairs

Bonding pairs

CCl4 has 4 bonding pairs, C has 4 electron domains

13

• Nonbonding (lone) pairs of electrons: electrons that are found principally on one atom, not in between atoms= unshared electrons

H N H

H

Molecular Geometry

Nonbonding pair

14

• N in Ammonia (NH3) has 4 electron domains:

H N H

H

Molecular Geometry

3 bonding pairs

1 nonbonding pair

15

Valence Shell Electron Pair Repulsion Theory (VSEPR)

“The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

16

Molecular Geometry

• By considering the arrangement that minimizes repulsions between electron domains, we can determine the

electron domain geometry– The arrangement of electron domains around

the central atom

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Molecular Geometry

3 electron domains

2 electron domains

Trigonal planar

e- domain geometry

Linear

electron domain geometry

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Molecular Geometry

4 electron domains

5 electron domains

Tetrahedral electron domain geometry

Trigonal bipyramidal

e- domain geometry

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Molecular Geometry

6 electron domains

Octahedral

electron domain geometry

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Electron-Domain Geometries

• All one must do is– draw the Lewis structure– count the total number of

electron domains around the central atom

• double and triple bonds count as 1 electron domain

• The geometry will be that which corresponds to the number of electron domains.

21

Molecular Geometry

Determine the electron domain geometry of CO2.

Valence electrons: 16

O C OLewis structure:

# of electron domains of C: 2

Electron domain geometry: linear22

Molecular Geometry

Determine the electron domain geometry of PCl3.

Valence electrons: 26

Lewis structure:

# of electron domains of P: 4

Electron domain geometry: tetrahedral

Cl

Cl P Cl

23

Molecular Geometries

• The electron-domain geometry is often not the shape of the molecule, however.

• The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.

• Molecular geometry is a consequence of electron-domain geometry.

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Molecular Geometry

• H2O has 4 electron domains--

– electron-domain geometry = tetrahedral

H

HIf you ignore the lone pairs of electrons, however, the atoms are arranged in a bent shape.

O

25

Molecular Geometry

• The molecular geometry is a consequence of electron domain geometry because the lone pairs of electrons take up space around the central atom.– This forces the atoms in the molecule to

occupy positions around the central atom in a way that minimizes repulsion between the electron domains.

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Molecular Geometry

• Electron domain geometry and molecular geometry are the same only if there are no non-bonding electron domains.

• See tables 9.2 and 9.3 for the relationship between electron domain geometries and molecular geometries.

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Linear Electron Domain

• In the linear domain, there is only one molecular geometry: linear.

• NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

28

Trigonal Planar Electron Domain

• There are two molecular geometries:– Trigonal planar, if all the electron domains are

bonding,– Bent, if one of the domains is a nonbonding pair.

29

Tetrahedral Electron Domain

• There are three molecular geometries:– Tetrahedral, if all are bonding pairs,– Trigonal pyramidal if one is a nonbonding pair,– Bent if there are two nonbonding pairs. 30

Trigonal Bipyramidal Electron Domain

• There are four distinct molecular geometries in this domain:– Trigonal bipyramidal– Seesaw– T-shaped– Linear

31

Octahedral Electron Domain

• All positions are equivalent in the octahedral domain.

• There are three molecular geometries:– Octahedral– Square pyramidal– Square planar

32

Molecular Geometry

AB

B

B

A

B

BB

B

Trigonal planarTetrahedral

33

Molecular Geometry

A

B

BBB

B

A

B

BB

B

B

B

Trigonal bipyramidal

octahedral

34

Molecular Geometry

• In order to determine the actual molecular geometry:– draw the Lewis structure– count the total # of electron domains

• multiple bonds = 1 electron domain

– determine the electron-domain geometry– describe the molecular geometry in terms

of the arrangement of the bonded atoms

35

Molecular Geometry

What is the molecular geometry of NH3?

Lewis Structure:

# of electron domains = 4

36

Molecular Geometry

• Electron domain geometry: tetrahedral

• Molecular geometry: trigonal pyramidal

NH

H H

NH

HH

37

Molecular Geometry

Example: Predict the molecular geometry of IF5.

Lewis structure:

# electron domains:

38

Molecular Geometry

• Electron domain geometry = octahedral

• Molecular geometry = square pyramidal

N

I

HH

H

FF

FF

F

N

I

HH

H

FF

FF

F

39

Larger Molecules

In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

40

Polarity of Molecules• Consider the carbon dioxide molecule:

– contains two polar covalent bonds– nonpolar molecule

• Just because a molecule contains polar covalent bonds does not mean the molecule as a whole will be polar.

41

Polarity of Molecules

• Polar Molecules– contain polar covalent bonds which are

asymmetrically distributed within the molecule• contain a “positive” end and a “negative”end

– Examples:• HCl

• H2O

• CH3OH

-+

OH H

-

+

+

OHC

H

H H

-

++42

Polarity of Molecules

• Polar molecules have large dipole moments– A measure of the separation between the positive and

negative charges in polar molecules.

+ -

H – F

+ -

43

Polarity

By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

• The overall polarity of a molecule is determined by doing a vector addition of the individual bond dipoles:– add both the magnitude

and direction of the dipole moments

• must consider the molecular geometry!

44

Polarity of Molecules

• Examples:

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