Chapter 9 Notes
AP CHEMISTRY
Galster
Molecular Geometry
A. VESPR Theory
A. Abbreviation for valence shell electron pair repulsion.
B. Valence electrons will arrange themselves to reduce repulsion
i.e. they will be as far apart as possible
1. 2 e-sets
1. Bond angle:
2. Shape name:
3. AZx designation:
2. 3 e- sets
1. Bond angle:
2. Shape name:
3. AZx designation:
1. 4 e-sets
1. Bond angle:
2. Shape name:
3. AZx designation:
2. 5 e- sets
1. Bond angle:
2. Shape name:
3. AZx designation:
3. 6 e-sets
1. Bond angle:
2. Shape name:
3. AZx designation:
Electron Set
Geometry
Name
Molecular
Geometry
Name
Example
CO2
AZ2
(2 edomains)
Linear
AX2
Linear
BF3
AZ3
(3 edomains)
Trigonal
planar
AX3
Trigonal
planar
SO2
AZ3
(3 edomains)
Trigonal
planar
AX2E
Bent
3-D Shape
Electron Set
Geometry
AZ4
(4 edomains)
AZ4
(4 edomains)
AZ4
(4 edomains)
AZ4
(4 edomains)
Name
Molecular
Geometry
Name
Example
CH4
Tetrahedral
AX4
Tetrahedral
NH3
Tetrahedral
AX3E
Trigonal
pyramidal
H2O
Tetrahedral
AX2E2
Bent
HCl
Tetrahedral
AXE3
Linear
3-D Shape
Electron Set
Geometry
AZ5
(5 edomains)
AZ5
(5 edomains)
Name
Molecular
Geometry
Name
Example
PCl5
Trigonal
bipyramidal
AX5
Trigonal
bipyramidal
SF4
Trigonal
bipyramidal
AX4E
See-Saw
ClF3
AZ5
(5 edomains)
Trigonal
bipyramidal
AZ5
(5 edomains)
Trigonal
bipyramidal
AX3E2
T-shaped
XeF2
AX2E3
Linear
3-D Shape
Electron Set
Geometry
Name
Molecular
Geometry
Name
Example
SF6
AZ6
(6 edomains)
Octahedral
AX6
Octahedral
BrF5
AZ6
(6 edomains)
Octahedral
AX5E
Square
pyramidal
XeF4
AZ6
(6 edomains)
Octahedral
AX4E2
Square
planar
3-D Shape
Effect of non-bonding electrons
on bond angles
A. Order of Repulsion
1.
Nonbonding-nonbonding > nonbonding-bonding > bonding-bonding
B. Trends
A. Angle between nonbonding pairs increase
B. Angles between bonding pairs decrease
C. Examples
1. CH4
tetrahedral
tetrahedral
2. NH3
tetrahedral
trig. Pyramidal
3 Water, H2O
tetrahedral
bent
• 4. Sulfur tetrafluoride, SF4
•
• 5. COCl2
Polarity of Molecules
A. Review of polar bonds
1.
2.
Polar bonds - ∆EN is less than or equal to 0.5 (but >1.7, ionic)
Nonpolar bonds ∆EN is less than 0.5
B. Molecular polarity
1.
Nonpolar bonds
a.
b.
c.
d.
2.
Diatomic molecules or others with nonpolar bonds
Ex: H2, Cl2, I2
No dipole moment
So… molecules with all nonpolar bonds are nonpolar
Cancellation of dipoles
a.
b.
c.
d.
Depends on geometry “tug of war”
Polar bonds – yes
Dipole moment – no
Ex: CCl4, CO2
Examples of Nonpolar
Molecules
• CO2
• SiCl5
• AsCl5
• ICl4-1
Examples of Polar Molecules
• Polar bonds: yes
• Dipole Moment? Yes
• H 2O
SF4
BrF5
Which of these are polar?
1.
2.
3.
4.
5.
6.
SO2
I3-1
CHCl3
PCl3
XeCl4
SeBr4
Atomic Orbitals: Hybridization
• Atomic Orbitals – covalent bonds result when orbitals in
valence level overlap
• **VESPR theory says that the number of unpaired electrons
tells you the number of bonds the atom will form.
• Ex: H2
•
•
HCl
• Cl2
Hybrid Orbitals
• Mixing of orbitals
• Pure orbitals cannot always be used to explain bonding
1. Basics
a.
b.
Atomic orbitals are combined to create new hybrid orbitals
The number of hybrid orbitals formed is equal to the number of
atomic orbitals combined
* We are not increasing the number of orbitals, just rearranging
them.
a. The names of the hybrid orbitals are based on the type and
number of atomic orbitals combined.
b. The hybrid orbitals are degenerate and have the same shape.
Process of hybridization
• A. methane, CH4
• B. BF3
• C. BeCl2
• D. H2O
• E. NH3
• F. XeF4
Hybridization and Electron Set Geometry
•
•
•
•
•
sp = one s & one p = AZ2 linear
sp2= one s & two p = AZ3 trigonal planar
sp3= one s & three p = AZ4 tetrahedral
sp3d = one s, three p, and one d = AZ5 trigonal bipyramidal
sp3d2 = one s, three p, and two d = AZ6 octahedral
Multiple Bonds
• Multiple bonds do not affect the electron set geometry of a
molecule
• Multiple bonds make attractions (i.e. bond) stronger and
shorter
• Types of bonds
• Single bond = σ (sigma) bond
• Double bond = σ (sigma) bond – first bond
π (pi) bond – 2nd or subsequent bonds
Ex: C2H4
• Triple bond
• Ex: C2H2 (Acetylene)
• Sigma bonds are characterized by
• Head-to-head overlap
• Cylindrical symmetry of electron density about the
internuclear axis
• Pi bonds are characterized by
• Side-to-side overlap
• Electron density above and below the internuclear
axis
Delocalized Electrons:
Resonance
• In reality, each of the four
atoms in the nitrate ion has
a p orbital.
• The p orbitals on all three
oxygens overlap with the p
orbital on the central nitrogen.
• This means the electrons are
not localized between the
nitrogen and one of the
oxygens, but rather are
delocalized throughout the
ion.
Resonance
• The organic molecule benzene has six σ bonds and a p orbital
on each carbon atom.
• In reality the π electrons in benzene are not localized but
delocalized
• The even distribution of the π electrons in benzene makes
the molecule unusually stable.
Molecular-Orbital (MO) Theory
• Through valence bond
theory effectively conveys
most observed properties of
ions and molecules, there
are some concepts better
represented by molecular
orbitals.
• In MO theory, we invoke the
wave nature of electrons
• If waves interact constructively
the resulting orbital is lower in
energy: a bond
Molecular-Orbital (MO) Theory
• If waves interact destructively, the resulting orbital is higher in
energy: an antibonding molecular orbital.
MO Theory
• In H2 the two electrons go into the bonding molecular orbital
• The bond order is one half the difference between the
number of bonding and antibonding electrons
MO Theory
• For hydrogen, with two electrons in the bonding MO and
none in the antibonding MO, the bond order is
½ (2 – 0) = 1
MO Theory
• In the case of He2, the bond order would be
½ (2 - 2) = 0
• Therefore, He2 does not exist.
MO Theory
• For atoms with both s
and p, there are two
types of interactions:
• The s and the p orbitals
that face each other
overlap in σ fashion.
• The other two sets of p
orbitals overlap in π
fashion.
MO Theory
• The resulting MO diagram
looks like this :
• There are both σ and π
bonding molecular orbitals
and σ* and π* antibonding
molecular orbitals
MO Theory
• The smaller p-block elements in the second period have a
sizable interaction between the s and p orbitals.
• This flips the order of the σ and π molecular orbitals in these
elements
Second-Row MO Diagrams