lecture 11 - vsepr theory, molecular shape

Lecture 11 - VSEPR Theory, Molecular Shape

5 Base e- pair geometries will predict

13 Molecular geometries

STEPS FOR SUCCESS:1. draw Lewis

2. bonding pairs

3. lone pairs

4. multiple bonds count as one bonded pair

Resonance Structures

e.g. O3 (ozone)

O O O O O O..

.... ..

..:.... ..

..:

..

e.g. SO3

O O O

S O S O S O

O O O

e.g. carbonate ion, CO3-2

O O O

C O C O C O

O O O

-2 -2-2

Molecules that don’t obey rules

Case 2. Incomplete octets

F

B F

F

H - Cl

Molecules that don’t 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!

Molecules that don’t obey rules

Case 3. Expanded Octets

Cl FCl F F

Cl P SCl F F

Cl F

10 electrons! 12 electrons!

Molecules that don’t obey rules

Case 3. Expanded Octets

Xe

F

F

F

F

: :

12 electrons!

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

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!

VSEPR

Electron Bonds Lone Notation Shape

Pairs Pairs

2 2 0 AX2 linear

3 3 0 AX3 trigonal

planar

3 2 1 AX2E bent

AX2 e.g. CO2

(Linear)

O = C = O

bond angle = 180o

AX3 e.g. BF3

(trigonal planar)

bond angle = 120o

BF

FF

AX2E e.g. SO2

(bent)

bond angle = 105o

SO

O

:

VSEPR

Electron Bonds Lone Notation Shape

Pairs Pairs

4 4 0 AX4 tetrahedral

4 3 1 AX3E trigonal pyramidal

4 2 2 AX2E2 bent

AX4 e.g. methane

C

All angles 109.5o(tetrahedral)

AX3E e.g. ammonia

..

107o

(trigonal pyramidal)

AX2E2 e.g. water

..

..105o

(bent)

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

AX5

ClCl

Cl PCl

Cl

ideal angles: 90o, 120o

(trigonal bipyramidal)

AX4E

FF

SF

F

:

ideal angles: 90o, 120o

(seesaw)

AX3E2

Cl

Cl I

Cl

:

:

ideal angles: 90o

(t-shaped)

AX2E3

F

Xe :

F

:

:

angle: 180o

(linear)

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

AX6

F

F FS

F F

Fall angles 90o

(octahedral)

AX5E

F

F FI

F F:

all angles 90o

(square pyramidal)

AX4E2

:

F FXe

F F :all angles 90o

(square planar)