Ch 6.5 Molecular Geometry
Dec 16, 2015
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!!