Lecture 11 VSEPR Theory, Molecular Shape

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Lecture 11 - VSEPR Theory, Molecular Shape

• 5 Base e- pair geometries will predict
• 13 Molecular geometries
• STEPS FOR SUCCESS
• draw Lewis
• bonding pairs
• lone pairs
• multiple bonds count as one bonded pair

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Resonance Structures

• e.g. O3 (ozone)

..
..
..
..
..
..

O O O O O O

..
..
..
..
3

• e.g. SO3

O O O S O S O S O O O O
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• e.g. carbonate ion, CO3-2

-2
-2
-2
O O O C O C O C O O O O
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Molecules that dont obey rules

• Case 2. Incomplete octets

..


F B F F
..
..

H - Cl

..
..


..
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Molecules that dont obey rules

• Case 3. Expanded Octets
• e.g. Phosphorus Ne 3s2 3p3
• e.g. Sulfur Ne 3s2 3p4
• e.g Xenon Ne 3s2 3p6
• 3d sublevel is close in energy to 3p!

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Molecules that dont obey rules

• Case 3. Expanded Octets

Cl F Cl F F Cl P S Cl F F Cl F
10 electrons!
12 electrons!
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Molecules that dont obey rules

• Case 3. Expanded Octets

F
F

Xe

F
F
12 electrons!
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Predicting Molecular Shapes

• VSEPR Theory
• (Valence Shell Electron Pair Repulsion)
• - Electron clouds repel one another
• - Terminal atoms move as far apart as possible
• - Distinctive geometry results

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VSEPR

• Step 1.
• - count the number of total electron pairs around
  the central atom
• - count Bonding Pairs
• count Nonbonding or Lone Pairs
• Step 2. Predict the shape!

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VSEPR

• Electron Bonds Lone Notation Shape
• Pairs Pairs
• 2 2 0 AX2 linear
• 3 3 0 AX3 trigonal
• planar
• 3 2 1 AX2E bent

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AX2 e.g. CO2

• (Linear)
• O C O
• bond angle 180o

13
AX3 e.g. BF3

• (trigonal planar)
• bond angle 120o

F
B
F
F
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AX2E e.g. SO2

• (bent)
• bond angle 105o

O
S

O
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VSEPR

• Electron Bonds Lone Notation Shape
• Pairs Pairs
• 4 4 0 AX4 tetrahedral
• 4 3 1 AX3E trigonal
  pyramidal
• 4 2 2 AX2E2 bent

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AX4 e.g. methane
(tetrahedral)
All angles 109.5o
H
C
H
H
H
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AX3E e.g. ammonia
(trigonal pyramidal)
..
N
H
H
107o
H
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AX2E2 e.g. water
(bent)
H
H
..
105o
O
..
H
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VSEPR

• Electron Bonds Lone Notation Shape
• Pairs Pairs
• 5 5 0 AX5 trigonal
  bipyramidal
• 5 4 1 AX4E see-saw
• 5 3 2 AX3E2 T-shaped
• 5 2 3 AX2E3 Linear

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AX5
(trigonal bipyramidal)
Cl Cl Cl P Cl Cl
ideal angles 90o, 120o
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AX4E
(seesaw)
F F S F F

ideal angles 90o, 120o
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AX3E2
(t-shaped)
Cl Cl I Cl


ideal angles 90o
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AX2E3
(linear)
F Xe F


angle 180o
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VSEPR

• Electron Bonds Lone Notation Shape
• Pairss Pairs
• 6 6 0 AX6 octahedral
• 6 5 1 AX5E square
  pyramidal
• 6 4 2 AX4E2 square
  planar

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AX6
(octahedral)
F F F S F F F
all angles 90o
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AX5E
(square pyramidal)
F F F I F F
all angles 90o
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AX4E2
F F Xe F F
(square planar)
all angles 90o