Potential Energy of the Hydrogen Molecular as a Function of
Internuclear Distance
Far apart no attraction
Repulsion
H
ENERGY
HH
H
Most Stable
H
Attraction
H H
Internuclear Distance
Bond Length of the H-H bond
H
104 kcal mol-1
Pictorial Representation of Bonding and Antibonding
Orbitals for the Hydrogen Molecule
Anti-bonding Orbital
addition of orbitals with opposite phases
Bonding Orbital
addition of orbitals with the same phase
Wave Characteristics of the Electron
Planar Wave Has Positive and Negative Phases
+
-
+
+
-
-
Electron has wave characteristics and this includes positive and
negative phases
Do not confuse the phase with the charge of the electron!
Overlaping Waves of the
same phase reinforce
+
+
+
Overlaping Waves of
opposite phase produce
a node (zero applitude)
node
Energy Diagram For the Hydrogen Molecule
ENERGY
Antibonding Molecular Orbital
Atomic
Orbital
Energy of the
isolated hydrogen
atom
Atomic
Orbital
Bonding Molecular Orbital
Energy of the
isolated hydrogen
atom
Mixing of Atomic Orbitals to Make Hybrid Orbitals
Mix 1 2s with 3 2p orbitals
get
3
4 sp orbitals
25% s character 75% p character
Overlap of a sp3 hybrid orbital with a 1s orbital to make a
sigma C-H bond.
Methane has 4 C-H sigma bonds.
Four bonds of equal length.
Bond vector separated by 109.5°. This angle also happens to be the angle that places
the substituents around the carbon as far apart as possible. Since each electron pair
of the 4 sigma bonds will be concentrated between the C-H atoms, the bond angle
minimizes the repulsion between the electron pair of one bond and its 3 neighbouring
bonds
H 109.5º
H
H
C
H
bond length
H
H
H
bond angle
C
H
Structure of Ethane
C2H6
Lewis or Dot Structure
H
H
H
C
C
H
H
H
H
H
H
Line Bond Structure
H
H
H
C
C
H
H
H
H
C
C
H
H
Sigma bond caused by
overlap of two sp3 orbitals
C
C
Hybridization: The structure of Ethene or Ethylene - sp2 Hybrid Orbitals
Ethylene C2H4
Since Carbon is tetravalent ethylene must contain a carbon to carbon double bond.
H
H
C
H
H
H
C
C
H
Trivalent carbon
not allowed
H
H
H
C
C
H
H
C
H
Tetravalent carbon
Line Bond Structure
Lewis Structure
sp2 Hybridization
Viewed from above
Viewed from side
unhybridized p orbital
120º
120º
C
C
120º
sp2 hybrid orbitals
Ethylene C2H4
H
H
C
C
H
H
2
Overlap of 2 sp orbitals
gives C-C sigma bond
H
H
C
H
H
H
C
C
H
H
C
H
sp Hybridization:-
Acetylene C2H2
C
C
H
H
Overlap of 2 sp orbitals
gives C-C sigma bond
H
C
C
H
H
C
C
H
Predicting Shapes of Molecules
Valence-Shell Electron Pair Repulsion
VSEPR
Model - electrons in a molecule arrange themselves
to minimize electrostatic repulsion.
CCl4 like methane (CH4) is tetrahedral because this
is how the four groups of electron pairs that make
the bonds between carbon and chlorine can be the
farthest apart.
Valence-Shell Electron Pair Repulsion VSEPR
To Apply this idea use the following steps:1. Count electron groups on an atom.
These are of the following type
Lone Pair - one group
Any pair or set of pairs of electrons - one group.
2. Assume each group moves as far apart as possible.
Acetylene C2H2
The geometry of the molecule is linear and the
bond angle = 180°.
H
C
C
H
H
Two groups of electrons one of two and one of 6 electrons.
Molecule is linear
C
C
H
CH5N
H
H
Geometry around carbon
C
N H
H
H
Geometry around nitrogen ?
CH5N
H
H
Geometry around carbon
C
N H
H
H
Geometry around nitrogen ?
The carbon atom is surrounded by three hydrogen atoms and one N.
The Nitrogen has a lone pair is attached to one carbon and two hydrogens.
Therefore, we would predict that the carbon would have tetrahedral geometry
and is sp3 hybridized, the same for nitrogen.
C2H6O
There are two plausible Lewis or Line Bond structures.
Both are acceptable structures.
H3C
O
CH3
H3C
CH2
O
H
C2H6O
There are two plausible Lewis or Line Bond structures.
Both are acceptable structures.
H3C
O
CH3
The carbon atom is surrounded by three
hydrogen atoms and one O.
The Oxygen has two lone pairs and is
attached to two carbons.
Therefore, we would predict that the
carbon would have tetrahedral geometry
and is sp3 hybridized, the same for
oxygen with two lone pairs and two bonds
to different carbon atoms.
H3C
CH2
O
H
The carbon atoms all surrounded by 4
groups .
The Oxygen has two lone pairs and is
attached to one carbon and one hydrogen
atom.
Therefore, we would predict that the
carbon atoms would have tetrahedral
geometry and are sp3 hybridized, the
same for oxygen with two lone pairs and
two bonds to different carbon atoms.
Benzene C6H6
H
H
H
H
H
H
Geometry at this carbon?
Benzene C6H6
H
H
H
H
H
Geometry at this carbon?
H
Each carbon has an identical set of groups surround it.
one hydrogen one C-C double bond and one C-C single
bond for a total of 3 groups. Therefore, we would predict
2
that each carbon would be planar and sp hybridized
Acetonitrile C2H3N
Lewis structure?
Hybridization?
Shape?