EN
ER
GY
H H
Far apart no attraction
H H
H H
H HAttractionMost Stable
Repulsion
Potential Energy of the Hydrogen Molecular as a Function of Internuclear Distance
104 kcal mol-1
Internuclear Distance
Bond Length of the H-H bond
Pictorial Representation of Bonding and Antibonding Orbitals for the Hydrogen Molecule
addition of orbitals with opposite phases
addition of orbitals with the same phase
Bonding Orbital
Anti-bonding Orbital
-+ +
-
+
-
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!
Wave Characteristics 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
Bonding Molecular Orbital
Antibonding Molecular Orbital
EN
ER
GY Atomic
OrbitalAtomic Orbital
Energy of the isolated hydrogen atom
Energy of the isolated hydrogen atom
Mixing of Atomic Orbitals to Make Hybrid Orbitals
Mix 1 2s with 3 2p orbitals
get
4 sp3 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
C
H
H
H
H
H
C
HH
H
109.5º
bond anglebond length
Structure of Ethane
C2H6
Lewis or Dot Structure Line Bond Structure
C
H
H
H
C
H
H
H
H C C H
H
H
H
H
H
C C
H
H H
HH
Sigma bond caused by
overlap of two sp3 orbitals
C C
C C
H
H
H
H
C C
H
H
H
H
Trivalent carbonnot allowed
C
H
H
CH
H
Line Bond Structure Lewis Structure
Tetravalent carbon
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.
C
Viewed from above
120º
120º
120º
Viewed from sideunhybridized p orbital
C
sp2 hybrid orbitals
sp2 Hybridization
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.
To Apply this idea use the following steps:-
1. Count electron groups on an atom. These are of the following type
Lone Pair - one groupAny pair or set of pairs of electrons - one group.
2. Assume each group moves as far apart as possible.
Valence-Shell Electron Pair Repulsion VSEPR
Acetylene C2H2
The geometry of the molecule is linear and the bond angle = 180°.
H C C HH C C H
Two groups of electrons - one of two and one of 6 electrons.Molecule is linear
CH5N
NC
H
H
H H
H
Geometry around nitrogen ?Geometry around carbon
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 H3C CH2 O H
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.
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
HH
H H
Geometry at this carbon?
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
that each carbon would be planar and sp2 hybridized