Transcript Shapes of Molecules

Shapes of
Molecules
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]
NH3 Ammonia
Lone Pair
N has Atomic Number 7
Electron pattern 2,5
N
H
H
H
Bonding
Pairs
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
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
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.
Repulsion
Lone Pair
Lone Pair
Lone Pair
Bonding Pair
Bonding Pair
Bonding Pair
Ammonia
NH3
3 bonding : 1 lone
N
H
H
H
• In each slide a three dimensional
representation methane will be left
in the top left hand corner for
purposes of comparison
Methane
Bond angle 109.50
Tetrahedral
Lone Pair Repels more
N
H
H
H
Methane
Bond angle 109.50
Tetrahedral
Lone Pair Repels more
Bond angle 1070
Pyramidal
Group V element at centre
N
H
H
H
BF3
F
B
BF3
F
B
BCl3
F
3 bonding pairs
B
Group III element at centre
Planar
or
Trigonal Planar
Top View
Side View
Bond Angle = 120o
3 bonding
0 lone
Trigonal Planar
O
H
C
H
HCHO
methanal
Bonding Pairs
H2O
O
H
H
H2O
O
H
H2O
O
Methane
Bond angle 109.50
Tetrahedral
Water H2O
O
Methane
Bond angle 109.50
Tetrahedral
Water H2O
2 lone pairs push hard
bond angle = 104.5
O
V shaped or Angular
BeCl2
Beryllium
Atomic Number 4
Electron pattern 2,2
Cl
Be
Cl
BeCl2
Cl
Be
Cl
BeCl2
Cl
Be
Cl
BeCl2
2 bonding Pairs
Cl
Be
Cl
BeCl2
2 bonding Pairs
Bond Angle = 180o
Cl
Be
Linear
Cl
2 bonding pairs
CO2
O
C
O
Bond Angle 180o O = C = O Linear
Rules
•Select the central atom
•Find out how many bond pairs and
lone pairs you have
•Using repulsion rules decide a
shape for the molecule
What shapes are the following
Molecules?
• CCl4
CHCl3
• SiCl4
CO2
CF4
• H3O+
• PH3
• H2S
H2O
• AlH3
NH3
• BF3
SO2
HCHO
BCl3
• BeH2
Examples of Molecular Shapes
Tetrahedral [5atoms]
• CCl4
CHCl3 SiCl4 CF4 NH4+
Pyramidal [4 atoms]
Have 1 lone pair
• H3O+
PH3
NH3
Angular [3 atoms]
Trigonal [4 atoms]
• AlH3 HCHO
• BF3 BCl3
Linear [ 3 atoms]
Have 2 lone pairs
• H2S
H2O
SO2
• BeH2
CO2
[C2H2]