Chemical Bonding II:Molecular Geometry andHybridization of Atomic Orbitals
Chapter 10
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Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostaticrepulsions between the electron (bonding and nonbonding) pairs.
10.1
• Theory: electron domains keep as far away as
possible from one another
• 2 types of electron domains
– Bonding domains: electron pairs that are involved in
bonds between two atoms
(single,double,triple bond==1 bond domain)
– Nonbonding domains: contain electron pairs that are
associated with a single atom
(called Lone Pairs)
10.1
Molecules in whichthe central atomhas no lone pairs
No Lone Pairs on CentralAtom
• Arrangement of electrondomains is very symmetrical
• The molecular geometry is thesame as the electron domaingeometry
Steps to Assign Molecular Geometry
• Draw Lewis structure
• Count # of electron domains around the Centralatom(treat double and triple bonds as single domain)
• Assign electron domain geometry
• Finally assign the molecular shape
– based on the arrangement of the atoms
– not the arrangement of the domains
No Lone Pairs on Central Atom
• Arrangement of electron domains is very symmetrical
• The molecular geometry is the same as the electron domaingeometry
Cl
Cl
Be
2 atoms bonded to central atom
0 lone pairs on central atom
10.1
AB2
2
0
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
linear
linear
B
B
AB2
2
0
linear
linear
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB3
3
0
trigonalplanar
trigonalplanar
10.1
BF3
AB2
2
0
linear
linear
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB3
3
0
trigonalplanar
trigonalplanar
10.1
AB4
4
0
tetrahedral
tetrahedral
CH4
AB2
2
0
linear
linear
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB3
3
0
trigonalplanar
trigonalplanar
10.1
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
PCl5
AB2
2
0
linear
linear
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB3
3
0
trigonalplanar
trigonalplanar
10.1
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB6
6
0
octahedral
octahedral
SF6
Molecules in which the central atom has one or morelone Pairs
• Arrangement of electron domains is
Not symmetrical (usually)
• The molecular geometry is different from
the electron domain geometry
10.1
bonding-pair vs. bonding
pair repulsion
lone-pair vs. lone pair
repulsion
lone-pair vs. bonding
pair repulsion
>
>
Repulsive force
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB3
3
0
trigonalplanar
trigonalplanar
AB2E
2
1
trigonalplanar
bent
10.1
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB3E
3
1
AB4
4
0
tetrahedral
tetrahedral
tetrahedral
trigonalpyramidal
10.1
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
AB4
4
0
tetrahedral
tetrahedral
10.1
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
H
O
H
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
10.1
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB4E
4
1
trigonal
bipyramidal
distortedtetrahedron
SF4
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
10.1
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB4E
4
1
trigonal
bipyramidal
distortedtetrahedron
AB3E2
3
2
trigonal
bipyramidal
T-shaped
Cl
F
F
F
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
10.1
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB4E
4
1
trigonal
bipyramidal
distortedtetrahedron
AB3E2
3
2
trigonal
bipyramidal
T-shaped
AB2E3
2
3
trigonal
bipyramidal
linear
I
I
I
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
10.1
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
squarepyramidal
Br
F
F
F
F
F
Class
# of atoms
bonded tocentral atom
# lone
pairs oncentral atom
Arrangement ofelectron pairs
Molecular
Geometry
VSEPR
10.1
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
squarepyramidal
AB4E2
4
2
octahedral
squareplanar
Xe
F
F
F
F
Predicting Molecular Geometry
1.Draw Lewis structure for molecule.
2.Count number of lone pairs on the central atom andnumber of atoms bonded to the central atom.
3.Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
S
O
O
AB2E
bent
S
F
F
F
F
AB4E
distorted
tetrahedron
10.1
10.1
Dipole Moments and Polar Molecules
10.2
H
F
electron rich
region
electron poor
region
= Q x r
Q is the charge
r is the distance between charges
1 D = 3.36 x 10-30 C m
Predicting Polarity of Molecules:
• A measure of the polarity of a molecule
• The dipole moment can be determinedexperimentally
• Measure in Debye units
• Predict polarity by taking the vector sum ofthe bond dipoles
Molecules containing net dipole moments are called polarmolecules. Otherwise they are called nonpolar moleculesbecause they do not have net dipole moments.
Diatomic molecules: Determined by the polarity of bond
Polar molecules:HCl, CO,NO
Nonpolar molecules: H2,F2,O2
Molecules with three or more atoms: determined by thepolarity of the bond and the molecular geometry
Dipole moment is a vector quantity, which has bothmagnitude and direction.
• Symmetric molecules such as these are nonpolar becausethe bond dipoles cancel
• All of the basic shapes are symmetric, or balanced, if all thedomains and groups attached to them are identical
O C O
C
O O
Linear molecule
Nodipolar moment
bent molecule
Net dipolar moment
Chloroform, CCl3H, is
asymmetric. The vector
sum of the bond dipoles
is nonzero giving the
molecule a net dipole.
• A molecule will be nonpolar if: (a) the bonds are nonpolar,or (b) there are no lone pairs in the valence shell of the
central atom and all the atoms attached to the central atomare the same
10.2
Which of the following molecules have a dipole moment?
H2O, CO2, SO2, and CH4
O
H
H
dipole moment
polar molecule
S
O
O
C
O
O
no dipole moment
nonpolar molecule
dipole moment
polar molecule
C
H
H
H
H
no dipole moment
nonpolar molecule
Covalent Bonding Theories
• Valence bond (VB) theory
– Bonding is an overlap of atomic orbitals
• Includes overlap of hybrid atomic orbitals
• Molecular orbital (MO) theory
– Bonding happens when molecular orbitals are formed
Molecular Geometry & Hybridization
10.3
• Lewis structures and VSEPR do not tell us whyelectrons group into domains as they do
• How atoms form covalent bonds in molecules requiresan understanding of how orbitals interact
Hybridization – mixing of two or more atomicorbitals to form a new set of hybrid orbitals.
1.Mix at least 2 nonequivalent atomic orbitals (e.g. sand p). Hybrid orbitals have very different shapefrom original atomic orbitals.
2.Number of hybrid orbitals is equal to number ofpure atomic orbitals used in the hybridizationprocess.
3.Covalent bonds are formed by:
a.Overlap of hybrid orbitals with atomic orbitals
b.Overlap of hybrid orbitals with other hybridorbitals
10.4
Formation of sp Hybrid Orbitals
10.4
10.4
Formation of sp3 Hybrid Orbitals
# of Lone Pairs
+
# of Bonded Atoms
Hybridization
Examples
2
3
4
5
6
sp
sp2
sp3
sp3d
sp3d2
BeCl2
BF3
CH4, NH3, H2O
PCl5
SF6
How do I predict the hybridization of the central atom?
1.Draw the Lewis structure of the molecule.
2.Count the number of lone pairs AND the number of atomsbonded to the central atom. Predict the overall arrangementof electron pairs using VSEPR model ( Table 10.1)
3.Deduce hybridization of central atom by matching thearrangement of the electron pairs with those of hybrid orbitalsshown in Table 10.4.
10.4
Hybrid orbitals # of hybrid shape
orbitals
• sp3 4 tetrahedral
• sp2 3 trigonal planar
• sp 2linear
• sp3d 5 trigonal bipyramidal
• sp3d2 6 octahedral
10.4
Hybridization in molecules containing double and triple bonds
Two Kinds of covalent Bonds
• Sigma bond: bonding density is along the internuclear axis
– head to head overlap of two hybrid orbitals
– head to head overlap of p + hybrid
• any s character to the bond = sigma bond
• Pi bond: bonding density is above and below the
internuclear axis
– Sideway overlap p + p
10.5
Sigma bond
Pi bond
Multiple bonds
• Double bond: One sigma and one pi
bond between the same atoms
–Example: ethylene
10.5
• Triple bond: One sigma and two pi
bonds between the same atoms
–Example: acetylene
Review of 2 Types of covalent bonds:
1. Sigma bond
e-density (overlap region) is along the internuclear axis
2. Pi bond
e-density (overlap region) is above and below the plane of
the internuclear axis.
Relative reactivity of bonds:
• Pi bonds are more reactive than sigma bonds
• Less energy is needed to break a pi bond than a sigma bond
Sigma () and Pi Bonds ()
Single bond
1 sigma bond
Double bond
1 sigma bond and 1 pi bond
Triple bond
1 sigma bond and 2 pi bonds
How many and bonds are in the acetic acid
(vinegar) molecule CH3COOH?
C
H
H
C
H
O
O
H
bonds = 6
+ 1 = 7
bonds = 1
10.5
Summary: VB Theory
framework of the molecule is determined by the arrangement ofthe sigma-bonds; Hybrid orbitals are used to form the sigmabonds and the lone pairs of electrons.
Molecular orbital theory – bonds are formed frominteraction of atomic orbitals to form molecularorbitals.
O
O
No unpaired e-
Should be diamagnetic
Experiments show O2 is paramagnetic
10.6
But experiment showthat there are twounpaired electrons—paramagnetic.
Valence bond theory
• Bonds are formed from interaction of atomic orbitals to formmolecular orbitals.
MO theory takes the view that a molecule is similar to an atom
• The molecule has molecular orbitals that can be populated byelectrons just like the atomic orbitals in atoms
• # of MOs formed = # of atomic orbitals combined
Molecular Orbital (MO) Theory
Energy levels of bonding and antibonding molecularorbitals in hydrogen (H2).
A bonding molecular orbital has lower energy and greaterstability than the atomic orbitals from which it was formed.
An antibonding molecular orbital has higher energy andlower stability than the atomic orbitals from which it wasformed.
10.6
1.The number of molecular orbitals (MOs) formed is alwaysequal to the number of atomic orbitals combined.
2.The more stable the bonding MO, the less stable thecorresponding antibonding MO.
3.The filling of MOs proceeds from low to high energies.
4.Each MO can accommodate up to two electrons.
5.Use Hund’s rule when adding electrons to MOs of thesame energy.(Electrons spread out as much as possible,with spins unpaired, over orbitals that have the sameenergy)
6.The number of electrons in the MOs is equal to the sum ofall the electrons on the bonding atoms.
10.7
Molecular Orbital (MO) Configurations
-MOs follow the same filling rules as atomic orbitals
bond order =
1
2
Number ofelectrons inbondingMOs
Number ofelectrons inantibondingMOs
(
-
)
10.7
bondorder
½
1
0
½