1. Ch 6.5 Molecular Geometry

2. 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

3. Bonding and Shape of Molecules
Number
of Bonds
Number of
Unshared Pairs
Covalent
Structure
Shape
Examples 2 3 4 1
AB2
Linear
Trigonal planar
Tetrahedral
Pyramidal
Bent or Angular
BeCl2
BF3
CH4, SiCl4
NH3, PCl3
ONF
AB3
AB4
AB3E
AB2E

4. Number of Unshared Pairs
Number of Bonds
Number of Unshared Pairs
Covalent Structure
Shape
Example 2
AB2E2
Bent or Angular
H2O 5
AB5
Trigonal Bipyramidal
PF5 6
AB6
Octahedral
SF6

5. The VSEPR Model .. .. .. O S O S F N O C Linear Bent Trigonal planar
pyramidal F P
AB6 F F S
Students often confuse electron-domain geometry with molecular geometry.
You must stress that the molecular geometry is a consequence of the electron domain geometry.
The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. F C
Tetrahedral
Trigonal bipyramidal
Octahedral

6. Methane CH4 C
109.5o H
Tetrahedral geometry C H

7. N H N H H H Ammonia- NH3 .. N H .. N H Trigonal Pyramidal geometry

8. Water –H2O .. .. .. ..
SO2 .. O H O H H
Bent
geometry

9. Boron trifluoride - BF3 B F B F
120o
Trigonal
planar

10. Carbon dioxide – CO2 O C O O C
Linear

11. N
107o H .. .. C
109.5o H O .. H
104.5o H
Bent
Tetrahedral
Pyramidal

12. Learning Check
Determine the molecular geometry:
A. CCl4
B. H2CO

13. Learning Check Determine the molecular geometry: A. CCl4 Tetrahedral
B. H2CO

14. Learning Check Determine the molecular geometry: A. CCl4 Tetrahedral
B. H2CO Trigonal Planar

15. Intermolecular Forces
Forces of attraction between molecules
Very weak compared to ionic and metallic bonding.
The strongest occur between polar molecules.

16. 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. O
H H H Cl

17. 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

18. 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.

19. 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

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