The document discusses valence shell electron pair repulsion (VSEPR) theory, which predicts the shapes of molecules based on electron pair repulsion. It explains that electron pairs are either bonding pairs involved in bonds or lone pairs not involved in bonds. Lone pairs repel other electron pairs more strongly than bonding pairs do. Example molecules like methane, ammonia, water, boron trifluoride, beryllium chloride, carbon dioxide are presented along with their electron arrangements and predicted molecular shapes like tetrahedral, trigonal pyramidal, bent, linear based on the number of electron pairs and lone pairs.

1. 2.5.1
Shapes of Molecules

2. Valence Shell Electron Pair
Repulsion Theory
(VSEPR)
• Electrons can be placed in two
categories
• Bonding pairs [involved in a bond]
• Lone pairs [not involved in a bond]

3. NH3 Ammonia
Lone Pair
N has Atomic Number 7
Electron pattern 2,5
N
H
H
H
Bonding
Pairs

4. Bonding pairs
• Involved in a bond
• usually one e- from each atom in the bond
• Dative bond is a special type of covalent
bond where one atom supplies both the
electrons. This is unusual.
• The electrons in bonding pairs have opposite
spins this is why they can come together.
• Single, double and triple bonds are all
regarded as just one bonding pair

5. Methane CH4
Tetrahedral
Bond Angle
HCH =109.50
H
H
C
• 4 bonding pairs
H
•
•
•
•
H
Bonding
Pairs
All repel equally
Move as far apart as possible
Note they are NOT on the equator
but one at the pole and three below the equator

6. Lone Pairs
• These are pairs of electrons not
involved in a bond.
• Lone pairs have a greater power of
repulsion than bonding pairs.
• They push other electron pairs further
away from themselves than bonding
pairs do.

7. Repulsion
Lone Pair
Lone Pair
Lone Pair
Bonding Pair
Bonding Pair
Bonding Pair

8. Ammonia
NH3
3 bonding : 1 lone
N
H
H
H

9. • In each three dimensional
representation methane will be left
in the top left hand corner for
purposes of comparison

10. Methane
Bond angle 109.50
Tetrahedral
Lone Pair Repels more
N
H
H
H

11. Methane
Bond angle 109.50
Tetrahedral
Lone Pair Repels more
Bond angle 1070
Pyramidal
Group V element at centre
N
H
H
H

12. BF3
F
B

13. BF3
F
B

14. BCl3
F
3 bonding pairs
B

15. Group III element at centre
Planar
or
Trigonal Planar
Top View
Side View
Bond Angle = 120o

16. 3 bonding
0 lone
Trigonal Planar
O
H
C
H
HCHO
methanal
Bonding Pairs

17. H2O
O
H
H

18. H2O
O
H

19. H2O
O

20. Methane
Bond angle 109.50
Tetrahedral
Water H2O
O

21. Methane
Bond angle 109.50
Tetrahedral
Water H2O
2 lone pairs push hard
bond angle = 104.5
O
V shaped or Angular

22. BeCl2
Beryllium
Atomic Number 4
Electron pattern 2,2
Cl
Be
Cl

23. BeCl2
Cl
Be
Cl

24. BeCl2
Cl
Be
Cl

25. BeCl2
2 bonding Pairs
Cl
Be
Cl

26. BeCl2
2 bonding Pairs
Bond Angle = 180o
Cl
Be
Linear
Cl

27. 2 bonding pairs
CO2
O
C
O
Bond Angle 180o O = C = O Linear

28. Other examples of shapes
Tetrahedral [5atoms]
• CCl4
+
CHCl3 SiCl4 CF4 NH4
Pyramidal [4 atoms]
Trigonal [4 atoms]
Has lone pair
+ PH NH
• H3O
3
3
• AlH3 HCHO
• BF3 BCl3
Angular [3 atoms]
Linear [ 3 atoms]
Has 2 lone pairs
• H2S H2O SO2
• BeH2 CO2
[C2H2]