Chapter 9
Lewis Theory-VSEPR Valence Bond Theory
Molecular Orbital Theory
Problems with Lewis Theory
Lewis theory generally predicts trends in properties, but does not give good numerical predictions.
Lewis theory gives good first approximations of the
bond angles in molecules, but usually cannot be used to get actual bond angles.
Lewis theory cannot write one correct structure for many molecules where resonance is important.
Lewis theory often does not predict the correct magnetic behavior of molecules.
Valence Bond Theory
Linus Pauling and others applied the principles of quantum mechanics to molecules.
They reasoned that bonds between atoms would occur when the atomic orbitals interacted to
make new bonds.
The types of interactions depend on whether the orbitals align along the axis between the nuclei, or
outside the axis.
Orbital Interaction
As two atoms approached, the half-filled valence atomic orbitals on each atom would interact to
form molecular orbitals.
The molecular orbitals would be more stable than the separate atomic orbitals because they would
contain paired electrons shared by both atoms.
Orbital Diagram for the Formation of H2S
S ↑ ↑ ↑↓ ↑↓
3s 3p
↑
H
1s
↑
H
1s
↑↓ H─S bond
↑↓ H─S bond
H S H
Predicts bond angle = 90° Actual bond angle = 92°
“Unhybridized” C Orbitals Predict the Wrong Bonding & Geometry
H 1s
H 1s
C 2s 2p
Valence Bond Theory - Main Concepts
Valence electrons of the atoms in a molecule reside in quantum-mechanical atomic orbitals. The orbitals
can be the standard s, p, d, and f orbitals, or they may be hybrid combinations of these.
A chemical bond results when two of these atomic
orbitals interact and there is a total of two electrons in a new molecular orbital.
The shape of the molecule is determined by the geometry of the interacting orbitals.
Hybridization
Hybridization is mixing different types of orbitals in the valence shell to make a new set of degenerate orbitals.
# of new orbitals-----> 2, 3, 4, 5, 6 orbital designation---> sp, sp2, sp3, sp3d, sp3d2
The same type of atom can have different types of hybridization:
C,N = sp, sp2, sp3
The particular kind of hybridization that occurs is the one that yields the lowest overall energy for the molecule.
The sp Hybrid Orbitals in Gaseous BeCl2
Cl Be Cl
The sp2 Hybrid Orbitals in BF3
B
F
FF
The sp3 Hybrid Orbitals in CH4
The sp3 Hybrid Orbitals in NH3
The sp3 Hybrid Orbitals in H2O
Hybridization and VSEPR Theory
All three central atoms are sp3 hybridized !!
The sp3d Hybrid Orbitals in PCl5
The sp3d2 Hybrid Orbitals in SF6
sp3 hybridized ↑ ↑ ↑ ↑ Unhybridized
2s 2p
↑↓ ↑ ↑
Carbon Hybridizations
Unhybridized
2s 2p
↑↓ ↑ ↑ sp2 hybridized
2p 2sp2
↑ ↑ ↑ ↑
Unhybridized
2s 2p
↑↓ ↑ ↑ sp hybridized
2sp
↑ ↑ ↑ ↑
2p
2 sp3
Different Carbon Hybridizations Lead to Different Molecular Geometries
electron density
sp3
sp2
sp
sp3 Hybridization
Atom with four electron groups around it
tetrahedral electron group geometry ~109.5° angles between hybrid orbitals
tetrahedral molecular geometry for carbon trigonal pyramidal geometry for nitrogen
bent geometry for oxygen
Atom uses hybrid orbitals for all bonds & lone pairs
sp3 Hybridized Atoms
Place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy
CUnhybridized atom
2s 2p
↑↓ ↑
sp3 hybridized atom
2sp3
↑ ↑ ↑ ↑ ↑
N2s 2p
↑↓ ↑
2sp3
↑ ↑ ↑ ↑ ↑ ↑
↑
O↑
↑
2s 2p
↑↓ ↑ ↑ ↑↓
2sp3
↑ ↑ ↑↓
Bonding with Valence Bond Theory
Bonding takes place between atoms when their atomic or hybrid orbitals interact (“overlap”).
To interact, the orbitals must either be aligned along the axis between the atoms, or
The orbitals must be parallel to each other and perpendicular to the interatomic axis.
Bonding in Methane
Ammonia Formation with sp3 N
O
H
H
1s1ssp3
sp3
Water Formation with sp3 O
Types of Bonds
Sigma (σ) bond - when the interacting atomic orbitals point along the axis connecting the two bonding nuclei
Pi (π) bond - when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the
two bonding nuclei. (Usually from overlap of two unhybridized p orbitals)
The interaction between parallel orbitals is not as strong as
between orbitals that point at each other;
Therefore, σ bonds are stronger than π bonds.
Types of Bonds
sp2 Hybridization
Atom with three electron groups around it
trigonal electron group planar system ~120° bond angles - flat
C = trigonal planar molecular geometry N = bent molecular geometry
O = linear geometry
Atom uses hybrid orbitals for σ bonds and lone pairs
Atom uses a nonhybridized p orbital for a π bond
sp2 Hybridized Atoms Orbital Diagrams
Unhybridized atom
2s 2p
↑↓ ↑
sp2 hybridized atom
2sp2
↑ ↑ ↑ ↑ ↑
2p
2s 2p
↑↓ ↑
2sp2
↑ ↑ ↑ ↑
2p
↑ ↑
↑
2s 2p
↑↓ ↑ ↑ ↑↓
2sp2
↑ ↑
2p
↑
↑
↑↓
C
N
O
Formaldehyde, CH2O
↑ sp2 C ↑ ↑
↑ ↑
sp2 O ↑ ↑↓
↑ ↑
1s H
σ
σ σ
1s H
p C p O
π
↑↓
C OH
H
C OH
H
Bonding in Formaldehyde
Hybrid orbitals overlap to form σ bonds. Unhybridized p orbitals overlap to form a π bond.
Ethene, CH2CH2C C
H
H H
H
↑ sp2 C ↑ ↑
↑
↑ ↑
1s H
σ σ
1s H
p C
σ
π
↑ ↑ ↑
↑
↑ ↑
1s H
σ σ
1s H
sp2 C
p C
Bonding in Ethene, C2H4
π
π
Bonding in Ethene
CH2NH Orbital Diagram
↑ sp2 C ↑ ↑
↑ ↑
sp2 N ↑ ↑ ↑↓ ↑ ↑ ↑
1s H
σ
σ σ σ
1s H 1s H
p C p N
C N
H
H H
・ ・
π
C NH
H H
Bond RotationRotation around a σ bond does not require breaking the
interaction between atomic orbitals.
Rotation around a π bond requires the breaking of the interaction between atomic orbitals.
Restricted Rotation Around π-bonded Atoms in C2H2Cl2
Restricted Rotation Around π-bonded Atoms in C2H2Cl2
“cis” “trans”
Restricted Rotation Around π-bonded Atoms in C2H2Cl2
nonet
dipole
sp hybridization
Atom with two electron groups
linear shape 180° bond angle
Atoms use hybrid orbitals for σ bonds or lone pairs
Atom use nonhybridized p orbitals for π bonds
sp Hybridized Atoms Orbital Diagrams
Unhybridized atom
2s 2p
↑↓ ↑
sp hybridized atom
2sp
↑ ↑ ↑ ↑ ↑
2p
2s 2p
↑↓ ↑
2sp
↑ ↑ ↑ ↑
2p
↑ ↑
↑
C
N
C
HCN Orbital Diagram
↑ sp C ↑
↑ ↑ ↑
sp N
↑
↑ ↑↓ ↑
1s H
σ
s
p C p N
2π
C NH
σ
Bonding in HCNH C N
⇵H
⇵
⇵
C
N
HCCH (C2H2) Orbitals C CH H
↑ sp C ↑
↑ ↑
↑
1s H
s
p C σ
2π
↑ ↑
↑ ↑
↑
1s H
s
p C
sp C
σ σ
Bonding in C2H2
Bonding in HCCHH C C H
H
C
C
H
Bonding in C2H2
sp3d hybridization
sp3d hybridization
sp3d hybridization
P
S
Unhybridized atom
3s 3p
↑↓ ↑
sp3d hybridized atom
3sp3d
↑ ↑ ↑ ↑ ↑ ↑
3d
↑
3s 3p
↑↓
3sp3d
↑ ↑ ↑ ↑ ↑ ↑↓
3d
↑↓ ↑
(non-hybridizing d orbitals not shown)
4s 4p
↑↓ ↑↓ ↑↓ ↑
4d 4sp3d
↑↓ ↑ ↑ ↑ ↑↓ Br
sp3d hybridization
sp3d hybridization
sp3d hybridization
sp3d2 hybridization
sp3d2 Hybridized AtomsOrbital Diagrams
S
Br
3s 3p 3sp3d2
↑↓
3d
↑↓ ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
Unhybridized atom sp3d2 hybridized atom
4s 4p
↑↓
4d
↑↓ ↑↓ ↑
4sp3d2
↑↓ ↑ ↑ ↑ ↑ ↑
sp3d2 hybridization
sp3d2 hybridization
Atom with six electron groups around it octahedral electron geometry
Square Pyramid, Square Planar 90° bond angles
Use empty d orbitals from valence shell. d orbitals can be used to make π bonds.
Predicting Hybridization and Bonding Scheme
1. Start by drawing the Lewis structure 2. Use VSEPR Theory to predict the electron group
geometry around each central atom. 3. Select the hybridization scheme that matches the
electron group geometry. 4. Sketch the atomic and hybrid orbitals on the atoms
in the molecule, showing overlap of the appropriate orbitals
5. Label the bonds as σ or π
Predict the hybridization and bonding scheme for CH2CH2
1. Start by drawing the Lewis structure
2. Use VSEPR Theory to predict the electron group geometry around each central atom
The molecule has two interior atoms. Since each atom has three electron groups (one double bond and two single bonds), the electron geometry about each atom is trigonal planar.
Predict the hybridization and bonding scheme for CH2CH2
3. Select the hybridization scheme that matches the electron group geometry
4. Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals
C1 = trigonal planar C1 = sp2
C2 = trigonal planar C2 = sp2
continued…
Predict the hybridization and bonding scheme for CH3CHO
C1 = 4 electron areas C1= tetrahedral C2 = 3 electron areas C2 = trigonal planar
1. Start by drawing the Lewis structure
2. Use VSEPR Theory to predict the electron group geometry around each central atom
Predict the hybridization and bonding scheme for CH3CHO
3. Select the hybridization scheme that matches the electron group geometry
4. Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals
C1 = tetrahedral C1 = sp3
C2 = trigonal planar C2 = sp2
Label the bonds as σ or π
Predict the hybridization and bonding scheme for CH3CHO
σ
HC C
OH
HH
π
Additional Examples Follow
Predict the hybridization of all the atoms in H3BO3
H = can’t hybridizeB = 3 electron groups = sp2
O = 4 electron groups = sp3
Predict the hybridization and bonding scheme of all the atoms in NClO
• •
O N C l •
•
•
•
• • • • • •
N = 3 electron groups = sp2
O = 3 electron groups = sp2
Cl = 4 electron groups = sp3
Cl
O N
Cl
O N
SOF4 Orbital Diagram
↑ sp3d S ↑ ↑
↑ ↑
sp2 O ↑ ↑↓ ↑
2p F
σ
σ
d S p O
π
↑ ↑ ↑↓ ↑
2p F
σ
↑
2p F
σ
↑
2p F
σ
sp3d hybridization
Atom with five electron groups around ittrigonal bipyramid electron geometry Seesaw, T–Shape, Linear 120° & 90° bond angles
Use empty d orbitals from valence shelld orbitals can be used to make π bonds