Ch 6.5 Molecular Geometry

VSEPR Theory Use VSEPR theory

Valence-Shell-Electron-Pair Repulsion=the repulsion between electron pairs

causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible.

Determined by number of valence electrons of the central atom

3-D shape is a result of bonded pairs and lone pairs of electrons

Bonding and Shape of Molecules

Number of Bonds

Number of Unshared Pairs Shape Examples

2

3

4

3

2

0

0

0

1

1

Linear

Trigonal planar

Tetrahedral

Pyramidal

Bent or Angular

BeCl2

BF3

CH4, SiCl4

NH3, PCl3

ONF

AB2

CovalentStructure

AB3

AB4

AB3E

AB2E

Number of Bonds

Number of Unshared Pairs

Covalent Structure

Shape Example

2 2 AB2E2 Bent or Angular

H2O

5 0 AB5 Trigonal Bipyramidal

PF5

6 0 AB6 Octahedral SF6

......

The VSEPR Model

O OC

Linear

O OS

BentO O

S

O

Trigonalplanar

FF

F

N

Trigonalpyramidal

F

F

FP

F

F

Trigonal bipyramidalOctahedral

FF

F

S

F

F

F

AB6

SO2

FF

F

C

F

Tetrahedral

Methane CH4

CHH

HH

C109.5o

H

HHH

Tetrahedral geometry

Ammonia- NH3

NH HH

N107o HH

H

....

HH

H

N

TrigonalPyramidalgeometry

NH HH

O

.. ..

Water –H2O

....

H HO

Bentgeometry

SO2

H H

.. ..

BFF

F

Boron trifluoride - BF3

BFF

F

120o

Trigonalplanar

Carbon dioxide – CO2

O OC

Linear

C OO

C109.5o

H

HHH

N107o HH

H

..

O104.5o H

H

..

..

Tetrahedral Pyramidal Bent

Learning Check

Determine the molecular geometry:

A. CCl4

B. H2CO

Learning Check

Determine the molecular geometry:

A. CCl4 Tetrahedral

B. H2CO

Learning Check

Determine the molecular geometry:

A. CCl4 Tetrahedral

B. H2CO Trigonal Planar

Intermolecular Forces

Forces of attraction between molecules

Very weak compared to ionic and metallic bonding.

The strongest occur between polar molecules.

Intermolecular Forces Dipole: One end slightly positive and

one end slightly negative.

Can show the polar nature with an arrow pointing to the more electronegative atom.

OH H H Cl

Just because a molecule has a polar bond does not make it a polar molecule.

The bond polarities cancel if they are in opposite directions.

Example: CO2

Attractions Between Molecules The two weakest attractions between

molecules. Dipole Interactions

When polar molecules are attracted to one another.

Dispersions Forces Caused by the motion of electrons Weakest of all interactions.

Hydrogen Bonds A strong attraction found in hydrogen-

containing molecules. A hydrogen covalently bonded to a

very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom. Example

Water

Homework

6.5 page 210 #33-34, 37-38NOT 37-42!!