Section 3.3 – Molecular Shapes and Dipoles

It is time for these molecules to get in shape!

Nelson: pages 91-104

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Molecular Shape

Lewis structures tell us nothing about how atoms in a molecule are arranged in 3-dimensional space

Could you have predicted the arrangement of atoms on the right from just seeing it’s Lewis structure?

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VSEPR “vesper” THEORYShapes of molecules:

Molecules take on particular shapes depending on the amount of lone pairs and bonding electrons the atom in the middle of the molecule has

Stereochemistry Stereochemistry – study of the 3-D shape of molecules and how it affects their physical and chemical properties

VSEPR VSEPR = “valence- shell-electron-pair-repulsion” theory is based on:Electron pairs try to stay as far as possible from other e

due to repulsion of negative chargesThe number, type and direction of bonds to the central

atom (CA) determine the shape of the molecule3

VSEPR Continued…

VSEPR Theory is a powerful tool that helps us to guess the shape of a molecule. Such shapes are important as they determine the structure and function of the compound. For example, the arrangement of carbon atoms in a diamond help establish its unique hardness and usefulness.

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According to VSEPR theory:

Read and summarize the points on page 91 in the space below:

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Using VSEPR to predict molecular shapes

Bond angles – the angle formed by 2 bonds intersecting at an atom

A few rules to predicting the shape of atomsget electron pairs as far away as possible (like

charges repel)multiple bonds are treated as 1 bondlone pairs take slightly more room than bonding

pairs

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VSEPR Continued…

Let A = Central atom Let X = bonding electrons (bond

pair) Let E = one pair

Remember: the key concept is that all pairs of valence e repel each other and try to get as far from each other as possible

Remember: the key concept is that all pairs of valence e repel each other and try to get as far from each other as possible

There are 6 different arrangements that you will need to memorize!!!memorize!!!

Let’s look at specific examples of molecules to help us come up with general formulas for each of the arrangements

Steps to determining shape:draw a L.D.D and consider the arrangement of valence

e’s:Determine bond pairs and lone pairs around central atom

(CA)7

1. Consider beryllium dihydride• Total pairs = 2• Bond pairs around Be (CA) = 2• Lone pairs around CA = 0

Bond pairs repel each other and try to get as far away as possible = opposite sides of Be• Gives a linear linear orientation with the two bonds at an angleangle

of 180 º180 ºSummary:

General Formula

Bond pairs

Lone pairs

Total pairs

Electron pair arrangement

Stereochemical formula

AX2linear linear

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VSEPR: Linear Arrangement

2. boron trifluoride Lewis dot diagram:

Total pairs = 3 Lone pairs = 0 Bonding pairs = 3 Repulsion of bonding pairs causes 120º120º bond angles

Summary:General Formula

Bond pairs

Lone pairs

Total pairs

Electron pair arrangement

Stereochemical formula

AX3Trigonal Planar

Trigonal Planar

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FBF

F

FBF

F

VSEPR: Trigonal Planar Arrangement

Drawing in 3-D

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AB

C

ED

A

BB

B

B

A

tetrahedral

Coming out toward you

In the plane of the slide

Going back and away from you

3. carbon tetrachloride Lewis dot diagram: Total pairs = 3 Lone pairs = 0 Bonding pairs = 3 Repulsion of bonding pairs causes 109.5 º109.5 º bond angles

Summary:

General Formula

Bond pairs

Lone pairs

Total pairs

Electron pair arrangement

Stereochemical formula

AX4tetrahedral tetrahedral

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VSEPR: Tetrahedral Arrangement

4. nitrogen trichloride Lewis dot diagram: Total pairs = 4 Lone pairs = 1 Bonding pairs = 3 Repulsion of electron pairs causes a tetrahedral shape If we ignore the lone pair, the shape becomes like a 3

sided (triangular) pyramid = trigonal pyramidal We would predict that the bond angles would be 109.5º like

the tetrahedral arrangement. However, lone pairs have a greater repulsion that bond pairs and therefore pushes that bond pair angles to be 107.3º107.3º

General Formula

Bond pairs

Lone pairs

Total pairs

Electron pair arrangement

Stereochemical formula

AX3E tetrahedral trigonal pyramidal

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5. Water Lewis dot diagram: Total pairs = 4 Lone pairs = 2 Bonding pairs = 2 Repulsion of bonding pairs causes a slightly altered (due to

lone pairs) tetrahedral bonding pattern with the bond pairs having a bond angle of 104.5º104.5º

Summary:

General Formula

Bond pairs

Lone pairs

Total pairs

Electron pair arrangement

Stereochemical formula

AX2E2tetrahedral Angular (V-

shaped)

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bonding-pair vs. bondingpair repulsion

lone-pair vs. lone pairrepulsion

lone-pair vs. bondingpair repulsion

> >

6. Hydrogen fluoride Lewis dot diagram: Total pairs = 4 Lone pairs = 3 Bonding pairs = 1 Repulsion of bonding pairs causes a tetrahedral bonding

pattern. Since there is only two atoms held together by one covalent bond, the shape is linear (like all diatomic molecules)

Summary:

General Formula

Bond pairs

Lone pairs

Total pairs

Electron pair arrangement

Stereochemical formula

AXE3tetrahedral Linear

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Table 7: Using VSEPR to predict molecular shape – Nelson Page 95

General Formula

Bond Pairs

Lone Pairs

Total Pairs

Geometry Stereochemical formula

Examples:

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General Formula

Bond Pairs

Lone Pairs

Total Pairs

Geometry Stereochemical formula

Examples:

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Learning Activities:

Read Pages 91-97 in your textbookRead the Learning Tips on pages 92, 94, and 96Try Practice Problems on Page 96 # 2-4

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= _____________________?

= _____________________?

= _____________________?

Practice Problems Page 96- Solutions

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Everyone wants to be the center of the universeIf you have more then one CA you follow three simple stepsstart with a structural or lewis diagramidentity all the CA and treat each one individuallydraw the molecule going from CA to CAExample – C2H6

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More than one CA Continued…

Example: What are the bond pairs and lone pairs around C in the following molecule? Around N? Draw a three dimensional structure for the compound.

H C N

H

H

H

H

C: 4 bond pairs tetrahedral

N: 3 bond pairs, 1 lone pair trigonal pyramidal

VSEPR

For the previous compound, two realistic 3-D structures would be:

C NH

H HH

H

C NH

H

H HH

Multiple bonds and VSEPR

Can we predict stereochemistry for molecules with multiple bonds?

Consider ethylene (used in welding torches): C2H4 (g)

Crystallography indicates that the orientation around the C atoms is trigonal planer

Just as before with multiple CA’s:Step 1: draw L.D.DStep 2: count bond and lone pairs around CA (Recall:

double/ triple bonds count as one)Step 3: Determine general formula for each CA: AX3

Step 4: draw a structural diagram if necessary

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In acetic acid, CH3COOH, there are three central atoms, and one double bond.

3-D molecule 3-D molecule appletapplet

VSEPR

Example: Draw a three dimensional structure for the following compound.

First, determine the bond and lone pairs around each CA

Trigonal planartetrahedral

HCNH2

C

O

H N C

O

HCNH2

C

O

H N C

O

VSEPR

Two possible 3-D structures:

HCNH2

C

O

H N

O

HH

HCNH2

C

O

H N

O

HH

More common way to draw structure.

Learning Activities:

Read Pages 91- 97 from NelsonFinish Practice Problems #2-4 on page 96Try Practice Problems 6-7 on page 98Complete Section 3.3 Questions 1-3 on page 104

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