Molecular Shapes and Consequences

Molecular Shapes and Consequences The shape of molecules often greatly influences its function. The fact that some molecules have an odor, that dextro-methorphan suppresses coughs and that ethyl alcohol is broken down in the liver alcohol dehydrogenase are a direct consequence of the shape of a molecule and the way it fits into reactive sites in the nose, brain and liver tissue. Change the shape of the molecule and often the molecule no longer functions the same way. Case in point: levo-methorphan will suppress breathing while dextro-methorphan only affects the medullary center controlling coughing.

I.

Predicting the Shape of Molecules

A. The shape of a molecule is a consequence of the way the orbitals in a molecule orient themselves in accordance with Valence Bond Theory. B. However, VB Theory is alone is not sufficient to predict the shape of molecules because it doesn’t differentiate between orbitals that are involved in bonding and those that contain a lone pair of electrons. C. The theory that predicts molecular shape is based built on VB Theory but takes into account the effect of bonding/lone pair orbitals on molecular shape. The theory is known as VSEPR – Valence Shell Electron Pair Repulsion. D. The premise of VSEPR theory is that 1. all regions of electron density orient themselves so as to minimize repulsions between themselves 2. the shape of the molecule is given by the orientation of the bonding orbitals.

By forcing all orbitals in the molecule to orient themselves so as to minimize repulsions, the bonding orbitals are constrained to an orientation that dictates the position of the atoms.

Front Range Community College - General Chemistry 111 - Molecular Shapes and Consequences - Dr. Christine Kelly Created 9.28.04 Page 1 of 9

E. To use VSEPR to predict the shape of a molecule: 1. Begin with a correct Lewis structure. 2. Determine the number of regions of electron density, i.e. all the orbitals whether they are bonding orbitals or lone pair of electron orbitals. 3. Assign an electronic geometry to those orbitals (i.e. the fundamental orientation of those density regions as suggested by VB). 4. Determine how the lone pairs will modify the electronic geometry. 5. Assign a molecular geometry. F. Examples Compound

Lewis Structure

BeCl2 (g)

Cl

Regions of Electron Density at the Central Atom

Electronic Geometry

Bonding and Lone Pr Orbitals

Molecular Geometry

2

Linear

2, 0

Linear

Cl

Be

X

A X S  180°

BF3

3

Trigonal Planar

3, 0

Trigonal Planar

X

F B F NO2-

A X

F 3

..

Trigonal Planar

2, 1

N

O

X Bent

A

-2

O

120o

X

X S  115°

Challenge: What is the relationship between the geometry of the Lewis structure and the actual molecular geometry? Front Range Community College - General Chemistry 111 - Molecular Shapes and Consequences - Dr. Christine Kelly Created 9.28.04 Page 2 of 9

CH4

4

Tetrahedral

4, 0

Tetrahedral

X

H H

C

H

A X

H

X X

S  109° NH3

4

H

Tetrahedral

3, 1

Trigonal Pyramidal

H

N

A

H

X

X X

S  107° H2O

4

H

O

Tetrahedral

2, 2

Bent

H

A X

X S  105° HF

H

F

4

Tetrahedral

1, 3

Linear

X - Y S  180° PCl5

5 Cl

Cl

P Cl

Trigonal Bipyramidal

5, 0


X

Cl Cl

X

A X S  120° S  90° S  180°


X X

SF4

5


4, 1

F

X F

S

F

See-saw

A X

X X

F

ClF3

5 Cl

F


3, 2

T-Shape

X

F

A

X

X

F

I3-

5


2, 3

Linear

-1

X I

I

I

SF6

6

F

F

F

S

F

F

A X

Octahedral

6, 0

Octahedral

X

X

X A

X

F

X

X

S  90° S  180° BrF5

6

Octahedral

5, 1

Square Pyramidal

X

F

X Br

F F

SF4

X A

F

X

F

6

Octahedral

4, 2

X

Square Planar


X

F

F

S

X

F

A X

F

Practice: Determine the molecular geometry of each of the following compounds based on its Lewis Structure and its electronic geometry. Compound

Lewis Structure

Electronic Geometry

Molecular Geometry

NF3 SiF4 CO2 CO

Practice: What are the electronic geometry and the molecular geometry of the thiocyanate anion, SCN-, as predicted by the VSEPR theory? Carbon is the central atom. (Ans: linear; linear) Practice: What are the electronic geometry and the molecular geometry of NOCl as predicted by the VSEPR theory? Nitrogen is the central atom. (Ans: trigonal planar; bent) Practice: What are the electronic geometry and the molecular geometry of ClO3F as predicted by the VSEPR theory? Chlorine is the central atom. (Ans: tetrahedral; tetrahedral) Practice: What is the molecular shape of ClF4- as predicted by the VSEPR theory? (Ans: trigonal bipyramidal; square planar)


X

Practice: What is the molecular shape of the PF3 molecule? Explain your reasoning using VSEPR theory. (Ans: Based on the Lewis structure there are four regions of electron

density around the central phosphorus atom which will orient in the tetrahedral arrangement according to VSEPR. But since 3 regions are bonding and 1 region holds the lone pair, the shape defined by the bonded atoms will be trigonal pyramidal.)

II.

Consequence of Molecular Shape: Molecular Polarity

A. Covalent molecules may carry a partial overall charge and thus be a polar molecule. Challenge: How is this similar to and different from ionic charges? B. The partial charge on a molecule which gives rise to molecular polarity results from the presence of polar bonds and a non-symmetrical geometry. C. Polar bonds result from a net difference in the electronegativies of the atoms in the bond (i.e. ∆EN ≠0) as in HCl: δ+ δ-

H

Cl

Practice: Determine the polarity of the bonds for CO2, CF4, H2O and CH3F. Challenge: An arrow is often used to indicate the dipole moment of the bond: the magnitude and the direction of the shift in electron density. Mark HCl above with the dipole moment arrow.

D. Polar bonds are necessary but not sufficient to create a polar molecule. A nonsymmetrical geometry is required so that the individual bond polarities do not offset/cancel out each other thus leading to a non-polar molecule. The symmetry of the molecule created by the linear orientation of the bonds allows the bond polarities to cancel out, to exactly offset each other. Thus Due carbon dioxide a non-polar to the highissymmetry of molecule. the

1. CO2 2. CF4

δ-

O

Cδ O

δ-

X

tetrahedron and the identical attachments,

Front Range Community College - General Chemistry 111 - Molecular Shapes and Consequences - Dr. Christine Kelly all of the polarities cancel Created 9.28.04 Pageout. 6 of 9This

δ- X

δ

A

X X δ-

molecule is not polar.

3. H2O

δδ+

H

O

H

δ+

Since the water molecule is bent, the individual bond polarities don’t offset/cancel each other. The geometry doesn’t allow the bond polarities to cancel out. Water is a polar molecule.

4. CH3Cl

δX δ- X

δ-

A

X Y δ-

In contrast to CF4, now one of the attachemnts has changed and there are no longer 4 identical attachments. The molecule is no longer symmetrical and the bond polarities don’t cancel out. This molecule is polar.

Therefore the existence of polar bonds does not mean the molecule is polar. Both polar bonds and a non-symmetrical geometry must be present in order for the molecule to be polar. Polar molecules are said to posses a net dipole moment since the individual dipole moments don’t cancel out due to symmetry in the molecule.

Practice: State the necessary requirements for a molecule to have a net dipole moment or to be polar. (Ans: In order for a molecule to have a net dipole moment and be polar, it must possess polar bonds

and have a symmetry such that there is a net dipole moment. Therefore to determine if a molecule is


polar first determine if the bonds are polar and then analyze the geometry to see how the individual dipole moments sum up.)

Practice: Explain what is meant by a “dipole moment”, and give an example of a molecule which has polar bonds but which does not itself have a dipole moment. (Ans: A dipole moment arises in a molecule when the “centers of gravity” of the positive and negative

charges do not coincide. There is thus a separation of charge. The dipole moment is the product of this charge and the distance of separation. Each polar bond in a molecule represents a dipole moment. Carbon dioxide has two polar carbon-oxygen bonds or two dipole moments. However, because the molecule is linear, the two bond dipoles are exactly opposite in direction, and they cancel each other out. Despite having polar bonds, CO2 has no net dipole moment.)

Practice: Which of the following molecule has a net dipole moment? BeCl2

SF2

KrF2

CO2

CCl4

(Ans: SF2.) Practice: Which of the following molecule has no net dipole moment? N2O

NF3

H2Se

TeO3

CH3Cl

(Ans: TeO3.) Practice: BF3 and NF3 have identical formula forms, XY3. One molecule is polar while the other is not. Explain. (Ans: Both molecules will have polar bonds but the geometry of BF3 is trigonal planar while that of

NF3 is trigonal pyramidal due to the presence of the lone pair of electrons on nitrogen. This creates a non-symmetry. Therefore BF3 is non-polar while NF3 is polar.)

E. Why is the polarity of a molecule significant? Molecules that have similar polar properties are able to interact with each other. 1. Polar molecules interact with other polar molecules, e.g. water and ethanol. Front Range Community College - General Chemistry 111 - Molecular Shapes and Consequences - Dr. Christine Kelly Created 9.28.04 Page 8 of 9

2. Non-polar molecules interact with only non-polar molecules, e.g. gasoline and oil. 3. But polar and non-polar molecules don’t interact, e.g. oil and water in salad dressing. “Like dissolves like”


Molecular Shapes and Consequences

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