1. VSEPR
Pronounced vesper…a vespa for her
A vest purrs???

2. Shapes of Molecules
Determined by number of valence electrons of the central atom
3-D shape a result of bonded pairs and lone pairs of electrons
Use VSEPR theory (valence-shell-electron-pair repulsion

3. Using the VSEPR Model 1. Draw the electron-dot structure
Identify the central atom
Count the total number of electron pairs around central atom
Predict the electron shape
Predict the shape of the molecule using the bonding atoms

4. The AXE system
American General Chemistry textbooks – but for some reason not British ones – adopt the excellent AXmEn system, where A is the central atom, m the number of ligands X, and n the number of nonbonded lone-pairs of electrons, E, about the central atom.
In this system: methane, CH4, is AX4 ammonia, H3N:, is AX3E1 water, H2O, is AX2E2

5. Shapes of Molecules Number of electron pairs 2
(= negative charge clouds)
Number of bonded atoms
Angle °
Name of shape linear

6. BeCl2 Be 2 valence electrons .. .. : Cl : Be : Cl : AX2E0 .. ..
: Cl : Be : Cl :
: Cl Be Cl : linear, 180°
AX2E0

7. More Electron Pairs Electron Angle Bonded Name of Pairs Atoms Shape
3 120° trigonal planar
AX3E0
3 120° angular or bent
AX2E1

8. Electron Shape with 3 Pairs

9. BF3
B has 3 valence electrons °
F trigonal
planar B
F F
AX3E0

10. SO2 :O:: S:O: S S has 2 bonded atoms , 1 lone pair (electron cloud)
120°, angular
:O:: S:O: S ..
O O
AX2E1

11. More Electron Pairs Electron Angle Bonded Name of Pairs Atoms Shape
4 109° tetrahedral
AX4E0
4 <109° 3 pyramidal
AX3E1
4 <<109° 2 bent, angular
AX2E2

12. Tetrahedral CH4 4 electron Pairs, 4 bonded atoms, 109° H H C H C H H
H H H
AX4E0

13. NH3 Pyramidal N has 4 electron pairs, 3 bonded H atoms
Shape is called pyramidal ..
H : N : H N
H H
H H ..
AX3E1

14. H2O Bent or Angular O has 4 electron pairs, 2 bonded H atoms
Shape is called bent or angular, ° ..
: O : H O H
H H .. ..
AX2E2

15. Learning Check
Determine the molecular geometry of each of the following:
A. CCl4
1) tetrahedral 2) pyramidal 3) angular
B. SO3
1) trigonal planar 2) pyramidal 3) angular
C. PCl3
1) trigonal planar 2) pyramidal 3) angular

16. Solution Determine the molecular geometry of each of the following:
A. CCl4
1) tetrahedral
B. SO3
1) trigonal planar
C. PCl3
2) pyramidal

17. More Electron Pairs Electron Angle Bonded Name of Pairs Atoms Shape
5 120°, 90° trigonal bipyramidal
AX5E0
5 <120°, 90° 4 see saw
AX4E1
5 90° T shape
AX3E2 17

18. Trigonal bipyramidal PCl5
5 electron pairs, 5 bonded atoms
AX5E0

19. See saw SF4
5 electron pairs, 4 bonding atoms
AX4E1

20. T shape, ICl3
5 electron pairs, 3 bonding atoms
AX3E2

21. More Electron Pairs Electron Angle Bonded Name of Pairs Atoms Shape
6 90° octahedral
AX6E0
6 90° square pyramid
AX5E1
6 90° Square planar
AX4E2 21

22. Octahedron, SF6
6 electron pairs, 6 bonding atoms
AX6E0

23. Square pyramid, ICl5
6 electron pairs, 5 bonding atoms
AX5E1

24. Square planar, XeF4
6 electron pairs, 4 bonding atoms
AX4E2

25. Now you try!