Transcript VSEPRE theory-lesson 4 - Science-with

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” THEORY
 Shapes 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 – study of the 3-D shape
of molecules and how it affects their
physical and chemical properties
 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 charges
 The number, type and direction of bonds to the central
atom (CA) determine the shape of the molecule
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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 atoms
 get electron pairs as far away as possible (like
charges repel)
 multiple bonds are treated as 1 bond
 lone pairs take slightly more room than bonding
pairs
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VSEPR Continued…
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There are 6 different arrangements that you will need to
memorize!!!
Let’s look at specific examples of molecules to help us come
up with general formulas for each of the arrangements
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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
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)
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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 orientation with the two bonds at an angle
of 180 º
 Summary:
General
Formula
AX2
Bond
pairs
Lone
pairs
Total
pairs
Electron pair
arrangement
Stereochemical
formula
linear
linear
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VSEPR: Linear Arrangement
2. boron trifluoride
Lewis dot diagram:
F
Total pairs = 3
 Lone pairs = 0
F B F
 Bonding pairs = 3
 Repulsion of bonding pairs causes 120º bond angles
 Summary:

General
Formula
AX3
Bond
pairs
Lone
pairs
Total
pairs
Electron pair
arrangement
Stereochemical
formula
Trigonal
Planar
Trigonal Planar
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VSEPR: Trigonal Planar Arrangement
A E
Drawing in 3-D
B
D
In the plane of the
slide
tetrahedral
B
B
Going back and away
from you
A
B
B
Coming out toward
you
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3. carbon tetrachloride
Lewis dot diagram:
Total pairs = 3
 Lone pairs = 0
 Bonding pairs = 3
 Repulsion of bonding pairs causes 109.5 º bond angles
 Summary:

General
Formula
AX4
Bond
pairs
Lone
pairs
Total
pairs
Electron pair
arrangement
Stereochemical
formula
tetrahedral
tetrahedral
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VSEPR: Tetrahedral Arrangement
4. nitrogen trichloride
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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 107.3º
General
Formula
AX3E
Bond
pairs
Lone
pairs
Total
pairs
Electron pair
arrangement
Stereochemical
formula
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 104.5º
 Summary:

General
Formula
AX2E2
Bond
pairs
Lone
pairs
Total
pairs
Electron pair
arrangement
Stereochemical
formula
tetrahedral
Angular (V-shaped)
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lone-pair vs. lone pair >
repulsion
lone-pair vs. bonding >
pair repulsion
bonding-pair vs. bonding
pair 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
AXE3
Bond
pairs
Lone
pairs
Total
pairs
Electron pair
arrangement
Stereochemical
formula
tetrahedral
Linear
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Table 7: Using VSEPR to predict molecular
shape – Nelson Page 95
General
Formula
Bond Lone
Pairs Pairs
Total
Pairs
Geometry
Stereochemical
formula
Examples:
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General
Formula
Bond Lone
Pairs Pairs
Total
Pairs
Geometry
Stereochemical
formula
Examples:
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Learning Activities:
 Read Pages 91-97 in your textbook
 Read the Learning Tips on pages 92, 94, and 96
 Try 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
universe
 If you have more then one CA you follow three simple steps
 start with a structural or lewis diagram
 identity all the CA and treat each one individually
 draw the molecule going from CA to CA
 Example – 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
H C N 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:
H
C N
H
H
H
H
H
H
C N
H
H
H
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.D
 Step 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
applet
VSEPR
H
Example: Draw a three dimensional structure for the
following compound.
O
O
HCNH2
C
First, determine the bond and lone pairs around each H
CA
N C
Trigonal planar
O
HCNH2
tetrahedral
VSEPR
 Two possible 3-D structures:
H
O
C
H
N H
O
H
O
C
H
N H
More common way to draw
structure.
Learning Activities:
 Read Pages 91- 97 from Nelson
 Finish Practice Problems #2-4 on page 96
 Try Practice Problems 6-7 on page 98
 Complete Section 3.3 Questions 1-3 on page 104
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