Homework
„
Chapter 9
– 11, 21, 25, 27, 29, 31, 35, 39, 45, 51, 65
Chapter 9
Bonding and Molecular Structure:
Orbital Hybridization and
Molecular Orbitals
Chapter 9
„
Broken into two different sections
discussing two different theories on
molecular bonding.
– Valence bond theory
– Molecular orbital theory
Valence Bond Theory
„
„
„
A model of bonding in which a bond
arises from the overlap of atomic
orbitals on two atoms to give a
bonding orbital with electrons localized
between the atoms.
Helps explain the molecular shapes.
Takes the different orbital types (s, p,
d) and creates hybrids of them.
VALENCE BOND THEORY
VALENCE BOND THEORY — Linus
Pauling
„ valence electrons are localized
between atoms (or are lone pairs).
„ half-filled atomic orbitals overlap to
form bonds.
„
Sigma Bond Formation
by Orbital Overlap
Two s orbitals
overlap
1
Sigma Bond Formation
Two s
orbitals
overlap
Types of Bonds
„
Two basic types of bonds.
– Sigma (σ) bonds.
„
„
A bond formed by the overlap of orbitals head to head,
and with bonding electron density concentrated along
the axis of the bond.
Can think of it as a single bond.
– Pi (π) bonds.
„
Two p
orbitals
overlap
„
Components of our known bonds.
– Single bond = 1 sigma bond
– Double bond = 1 sigma bond + 1 pi bond
– Triple bond = 1 sigma bond + 2 pi bonds
Using VB Theory
Bonding in BF3
•• ••
F ••
••••
F
••
B
↑↓
1s
↑↓
2s
↑
2p
planar triangle
angle = 120o
Bonding in BF3
2p
2s
hydridize orbs.
Bonding in BF3
How to account for 3 bonds 120o apart
using a spherical s orbital and p orbitals
that are 90o apart?
„ Pauling said to modify VB approach with
„
Boron configuration
•••
F
•• •
The second (and third, if present) bond in a multiple
bond; results from sideways overlap of p atomic
orbitals.
rearrange electrons
three sp2
hybrid orbitals
unused p
orbital
ORBITAL HYBRIDIZATION
„—
mix available orbitals to form a
new set of orbitals — HYBRID
ORBITALS — that will give the
maximum overlap in the correct
geometry.
Bonding in BF3
• The three hybrid orbitals are made from
1 s orbital and 2 p orbitals → 3 sp2
hybrids.
• Now we have 3, half-filled HYBRID
orbitals that can be used to form B-F
sigma bonds.
2
Bonding in
BF3
Bonding in CH4
An orbital from each F overlaps one of the
sp2 hybrids to form a B-F σ bond.
FF
↑↑↑↑
↑↑↑↑
FF
BB
FF
Bonding in a Tetrahedron —
Formation of Hybrid Atomic
Orbitals
How do we account for 4
C—H sigma bonds 109o
apart?
Need to use 4 atomic
orbitals — s, px, py, and
pz — to form 4 new
hybrid orbitals pointing
in the correct direction.
Bonding in a Tetrahedron —
Formation of Hybrid Atomic
Orbitals
4 C atom orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
Bonding in CH4
o
109
109o
4 C atom orbitals
hybridize to form four
equivalent sp3 hybrid
atomic orbitals.
The Hybridization of the
Orbitals
„
CH4
– How many bonds off of the carbon
center?
„
4
– What is the noble gas notation of carbon?
„
[He] 2s2, 2p2
– So we can take the s and p orbitals of the
carbon and create hybrid orbitals.
3
sp3 Hybridization for
carbon in CH4
Also Works for Other sp3
Hybrid Compounds
„
– Nitrogen has 3 bonds off of it and a lone pair of
electrons.
Electrons available to
form σ bonds
Energy
NH3
2p orbitals
Four sp3 hybrid orbitals
„
Four sp3 hybrid orbitals
H2O
– Oxygen has 2 bonds off of it and two lone pair
of electron.
2s
Giving us the ability to form 4 σ bonds.
Four sp3 hybrid orbitals
How to Determine the
Hybridization of an Atom
1.
2.
3.
4.
5.
6.
Draw the Lewis Structure if not given.
Pick a specific atom if one it not specified.
Count up the number of σ bonds off of it.
Count up the number of lone pairs of
electrons around it.
Add these two numbers up.
Use the s, p, d orbitals (IN THAT ORDER)
to equal the number in part 5.
„
Remember there is only one s orbital, three p
orbitals and five d orbitals.
For Example
(all bonds are single bonds)
„
An atom with 4 bonds off of it.
„
An atom with 3 bonds and one lone electron pair.
„
An atom with 1 bond off of it.
„
An atom with 2 bonds off of it.
– sp3 hybridized
– sp3 hybridized
– s hybridized (which is really not a hybridization)
Multiple Bonds
„
C2H4
– Draw the Lewis Structure.
– What is the hybridization around carbon?
„
sp2
– sp hybridized
„
An atom with 2 bonds and one lone electron pair.
„
An atom with 5 bonds and one lone electron pair.
„
An atom with 5 bond off of it.
– sp2 hybridized
– sp3d2 hybridized
– sp3d hybridized
4
Hybridization Around
Carbon in C2H4
σ and π Bonding in C2H4
Energy
Un-hybridized p orbital
2p orbitals
Three sp2 hybrid orbitals
2s
Giving us the ability to form 3 σ bonds and 1 π bond.
σ and π Bonding in CH2O
σ and π Bonding in C2H2
More Examples
Exploring O2
„
„
„
„
H2CO
C2H2
CSe2
NH4+
„
„
Lewis Dot Structure
Determine the Hybridization around the
oxygens
– How many σ bonds?
„
1
– How many lone pairs?
„
2
– Therefore we need how many orbitals?
„
3
– What is the hybridization?
„
sp2
5
Hybridization Around
Oxygen in O2
The Bonding in O2
Forms the π bond
Un-hybridized p orbital
Energy
Un-hybridized p orbital
2p orbitals
Three sp2 hybrid orbitals
Three sp2 hybrid orbitals
of oxygen
2s
Giving us the ability to form 1 σ bonds and 1 π bond.
Paramagnetic or
Diamagnetic?
„
Based upon the valence bond theory
what would you guess be about the
magnetism of O2?
The Basic Principles of
Molecular Orbital Theory
1.
2.
3.
4.
Un-hybridized p orbital
The total number of molecular orbitals is always
equal to the total number of atomic orbitals
contributed by the atom that have combined.
The bonding molecular orbital is lower in energy
than the parent orbitals, and the anti-bonding
orbital is higher in energy.
The electrons of the molecule are assigned to
orbitals of successively higher energy according
to the Pauli Exclusion Principle and Hund’s Rule.
Atomic orbitals combine to form molecular
orbitals most effectively when the atomic orbitals
are of similar energy
Three sp2 hybrid orbitals
of oxygen
Forms the σ bond
Remember that since the bond between the oxygens is covalent
the electrons are shared equally. This causes the two ½ empty
orbitals to be filled. Therefore oxygen is ___magnetic.
Molecular Orbital Theory
„
„
Molecular orbital (MO) theory is an alternative way
to view orbitals in molecules.
MO theory assumes that pure atomic orbitals of the
atoms in the molecule combine to produce orbitals
that are spread out, or delocalized, over several
atoms or even an entire molecule.
1st Principle of MO Theory
Example of a molecular orbital diagram of H2. Showing that the
number of molecular orbitals that form is equal to the total
number of atomic orbitals that are used.
6
1st Principle of MO Theory
This leads to a increased probability
that the electrons will reside in the
bond region between the two nuclei.
This is called a bonding molecular
orbital and is the same as a chemical
bond in valence bond theory.
The probability of finding
an electron between the
nuclei in the molecular
orbital is reduced and the
probability of finding the
electron in other regions
is higher.
This type of orbital is
called an anti-bonding
molecular orbital. There
is no counterpart in
valence bond theory.
2nd Principle of Molecular
Orbital Theory
The Basic Principles of
Molecular Orbital Theory
1.
2.
3.
4.
The Basic Principles of
Molecular Orbital Theory
1.
What do you think
happens when we
start to populate
the anti-bonding
orbital?
2.
3.
4.
We can see that the bonding orbital is lower in energy
than the parent orbitals, providing stability due to
chemical bond formation.
3rd Principle of MO Theory
The total number of molecular orbitals is always
equal to the total number of atomic orbitals
contributed by the atom that have combined.
The bonding molecular orbital is lower in energy
than the parent orbitals, and the anti-bonding
orbital is higher in energy.
The electrons of the molecule are assigned to
orbitals of successively higher energy according
to the Pauli Exclusion Principle and Hund’s Rule.
Atomic orbitals combine to form molecular
orbitals most effectively when the atomic orbitals
are of similar energy
The total number of molecular orbitals is always
equal to the total number of atomic orbitals
contributed by the atom that have combined.
The bonding molecular orbital is lower in energy
than the parent orbitals, and the anti-bonding
orbital is higher in energy.
The electrons of the molecule are assigned to
orbitals of successively higher energy according
to the Pauli Exclusion Principle and Hund’s Rule.
Atomic orbitals combine to form molecular
orbitals most effectively when the atomic orbitals
are of similar energy
Bond Order
Bond Order = ½ [# bonding e-’s - # antibonding e-’s]
1
0
We can see here how we fill the molecular orbitals based upon
the Pauli Exclusion Principle and Hund’s Rule.
What is the bond order of the two compounds?
7
The Basic Principles of
Molecular Orbital Theory
1.
2.
3.
4.
The total number of molecular orbitals is always
equal to the total number of atomic orbitals
contributed by the atom that have combined.
The bonding molecular orbital is lower in energy
than the parent orbitals, and the anti-bonding
orbital is higher in energy.
The electrons of the molecule are assigned to
orbitals of successively higher energy according
to the Pauli Exclusion Principle and Hund’s Rule.
Atomic orbitals combine to form molecular
orbitals most effectively when the atomic orbitals
are of similar energy.
4th Principle of MO Theory
It is theoretically possible
that the 1s and 2s orbitals
combine to form a molecular
orbital, but since the
energies are so different it
does not occur.
Basic MO Diagram for
Periods 1 & 2
Sigma Bonding from p
Orbitals
π Bonding from p Orbitals
σ & π Bonding from p
Orbitals
Sideways overlap of atomic 2p orbitals that lie in the same
direction in space give π bonding and antibonding MOs.
8
Chemical Reactivity
„
When we talk about the chemistry of
compounds we concern ourselves with
what?
– Valence electrons
„
We do the same thing in MO theory but we
also extend it.
– Highest Occupied Molecular Orbital (HOMO)
– Lowest Unoccupied Molecular Orbital (LUMO)
„
We can use these two two orbitals describe
the chemical reactivity, the shape, and other
important properties of a compound.
Chapter Highlights
1.
2.
Demonstrated more uses of the Lewis
Structure
Valence Bond Theory
„
„
3.
Hybridization of the valence orbitals.
Helps describe the molecular shapes when we
use VESPR Theory
Molecular Orbital Theory
„
„
Improves upon valence bond theory.
Helps explain things where valence bond
theory fails.
„
Ex) paramagnetism of O2
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