Post on 27-Jan-2021
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Chapter 11 Theories of Covalent
Bonding
Atomic Orbitals Molecules
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Bonding and Molecular Structure
Questions —
• How are molecules held together?
• Why is O2 paramagnetic? And how is this property connected with bonding in the molecule?
• How can noble gases form molecules such as XeF2, XeF4, XeOF4, XeO3, etc…?
N2 with
triple
bond
Phosphorus
is a
tetrahedron
of P4 atoms.
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© 2009 Brooks/Cole - Cengage Hydrogen fluoride, HF.
11.1 Valence Bond (VB) Theory: The Orbital Overlap Model of Bonding
by Linus Pauling (1901-1994)
A covalent bond forms when the orbitals of two atoms overlap and a pair of electrons with opposite spins occupy the overlap region.
The Central Themes of VB Theory
The greater the orbital overlap, the stronger the
bond. Extent of orbital overlap depends on orbital shape & direction
Fluorine, F2.
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VB Theory and Orbital Hybridization The orbitals that form when bonding occurs are different
from the atomic orbitals in the isolated atoms. If no change occurred, we could not account for the molecular shapes
that are observed.
Atomic orbitals “mix” or hybridize when bonding occurs
to form hybrid orbitals. The spatial orientation of these hybrid orbitals correspond with
observed molecular shapes.
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Features of Hybrid Orbitals The number of hybrid orbitals formed equals the number
of atomic orbitals mixed.
The type of hybrid orbitals formed varies with the types of
atomic orbitals mixed.
The shape and orientation of a hybrid orbital maximizes
overlap with the other atom in the bond.
atomic
orbitals
hybrid
orbitals
One 2s and one 2p atomic orbital mix to form two sp hybrid orbitals.
Formation and orientation of sp hybrid orbitals and the bonding in BeCl2.
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Types of Hybridization
# bond shape hybrid # p left
2 linear sp 2
3 trigonal planar sp2 1
4 tetrahedral sp3 0
5 trigonal bipyramidal sp3d
6 octahedral sp3d2
The 5 types of orbital hybridization correspond to the 5
electron group arrangements in VSEPR theory. Available atomic orbitals mix to form a new set of HYBRID ORBITALS that will give the
maximum overlap in the correct geometry
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sp Hybridization Formation and orientation of sp hybrid orbitals and the
bonding in BeCl2.
One 2s and one 2p
atomic orbital mix
to form two sp
hybrid orbitals
The two sp orbitals are in
linear arrangement , 180°
apart.
Each half-filled sp orbital
overlaps with the half-
filled 2p orbital of a Cl
atom.
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sp2 Hybridization
Bonding in BF3
Bond
angle
120o
One 2s and two 2p
atomic orbitals mix to
form three sp2 hybrid
orbitals
Overlap of B and F orbitals to form BF3 - The three sp2 orbitals point to
the corners of an equilateral
triangle, their axes 120° apart.
- Each half-filled sp2 orbital
overlaps with the half-filled 2p
orbital of a F atom.
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The sp3 hybrid orbitals in CH4
sp3 Hybridization
One 2s and three 2p atomic
orbitals mix to form four sp3
hybrid orbitals
Overlap of C and H orbitals to form CH4 - The four sp3 orbitals point to the
corners of a tetrahedral shape, their
axes 109° apart.
- Each half-filled sp3 orbital overlaps
with the half-filled 1s orbital of a H
atom.
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The sp3 hybrid orbitals in NH3
The N lone pair
occupies an sp3
hybrid orbital,
giving a trigonal
pyramidal
shape.
The sp3 hybrid orbitals in H2O
The O lone pairs occupy two sp3
hybrid orbitals, giving a bent shape
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The sp3d hybrid orbitals in PCl5.
The formation of more than
four bonding orbitals requires
d orbital involvement in
hybridization. One 3s, three
3p, and one 3d atomic orbitals
mix to form five sp3d hybrid
orbitals
sp3d Hybridization
Overlap of P and Cl orbitals to form PCl5 - The five sp3d orbitals point to the
corners of a trigonal bipyramidal
shape, their axes 120° and 90° apart.
- Each half-filled sp3d orbital overlaps
with the half-filled 3p orbital of a Cl
atom.
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The sp3d2 hybrid orbitals in SF6
sp3d2 Hybridization
The formation of more than
four bonding orbitals requires
d orbital involvement in
hybridization. One 3s, three
3p, and two 3d atomic orbitals
mix to form six sp3d2 hybrid
orbitals
Overlap of S and F orbitals to form SF6 - The six sp3d2 orbitals point to the
corners of a square pyramidal
shape, their axes 90° apart.
- Each half-filled sp3d2 orbital
overlaps with the half-filled 2p
orbital of an F atom.
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Table 11.1 Composition and Orientation of Hybrid Orbitals.
From molecular formula
to hybrid orbitals
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Bonding in Glycine
O
C
O H
H
H
NH
Hsp
3
sp3
sp3
sp2
••
••
••C
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Problems
Use Valence Bond (VB) Theory to describe
the bonding in H3O+, CH3NH2, and SF4.
Strategy: Write the Lewis dot structure for
the molecule. The electron group geometry
around each central atom determines the
hybrid orbital set for that atom.
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11.2 Modes of Orbital Overlap &
the Types of Covalent Bonds
A sigma (σ) bond is formed by end-to-end overlap of orbitals.
A pi (p) bond is formed by sideways overlap of orbitals.
All single bonds are σ bonds.
A double bond consists of one σ bond and
one p bond. A triple bond consists of one σ
bond and two p bonds
A p bond is weaker than a σ bond because
sideways overlap is less effective than end-to-
end overlap.
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The s bonds in ethane (C2H6)
both C are sp3
hybridized
s bond formed by s-sp3
overlap
End-to-end sp3-sp3 overlap to
form a s bond
A σ bond is cylindrically
symmetrical, with its
highest electron density
along the bond axis.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
There is relatively even
distribution of electron
density over all s bonds.
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The s and p bonds in ethylene (C2H4)
A p bond has two
regions of electron
density.
unhybridized 2p orbitals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Each C is sp hybridized and has
two unhybridized p orbitals.
The s and p bonds in acetylene (C2H2) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Electron density and bond order in
ethane, ethylene, and acetylene
A double bond is less than twice as strong as a single bond,
because a p bond is weaker than a σ bond.
However, in terms of bond order, a single bond has BO = 1, a
double bond has BO = 2, and a triple bond has BO = 3.
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Problem
Use VB Theory and Orbital Hybridization
to describe the bonding in acetone,
CH3COCH3, and acetonitrile, CH3CN.
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Orbital Overlap & Molecular Rotation
There is restricted rotation around C=C bond.
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Consequence of Multiple Bonds
cis-1,2-Dichloroethylene
polar molecule, bp 60 0C
trans-1,2-Dichloroethylene
non-polar molecule, bp 47.5 0C
The two isomers of 1,2-dichloroethylene (C2H4 Cl2) are
different molecules
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Limitation of VB Theory: The Paramagnetic Properties of O2
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11.3 Molecular Orbital (MO) Theory
& Electron Delocalization The combination of orbitals to form bonds is viewed as the
combination of wave functions.
Atomic wave functions (AOs) combine to form molecular
wave functions (MOs).
# of AOs combined always equals # of MOs formed.
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Key Distinctions between VB & MO Theories
Valence Bond (VB) Theory and Orbital Hybridization
- closely related to Lewis’s idea of bonding and lone pairs of localized electrons on a particular atom.
- used for qualitative, visual picture of molecular structure,
particularly for polyatomic molecules and all molecules in
ground (lowest energy) states.
Molecular Orbital (MO) Theory
- ‘spread out’ or delocalized electrons in MOs. - used for quantitative picture of bonding, especially for
molecules in excited states.
- used in explaining magnetic and spectral properties
(e.g., the colors of compounds).
- the only theory that can describe accurately the bonding
in molecules such as O2 and NO.
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Central Themes of MO Theory
Addition of AOs forms a bonding MO, which has a region of high electron density between the nuclei.
Subtraction of AOs forms an antibonding MO, which has a node, or region of zero electron density, between the
nuclei.
Bonding MO and Anti-bonding MO have different
shapes and energies
The bonding MO is
lower in energy and
the antibonding MO
is higher in energy
than the AOs that
combined to form
them.
Contours and energies of H2 bonding s and antibonding s* MOs
© 2009 Brooks/Cole - Cengage
Molecular Orbital Diagrams An MO diagram, just like an atomic orbital diagram, shows
the relative energy and number of electrons in each MO.
The MO diagram also shows the AOs from which each MO
is formed.
1. # of MO’s = # of atomic orbitals used.
2. Bonding MO is lower in energy than its parent atomic
orbitals. Antibonding MO is higher.
3. Atomic orbitals combine to form MOs most effectively
when the atomic orbitals are of similar energy.
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Filling MOs with Electrons 1. Electrons assigned to MO’s of increasing energy
2. An MO can hold a maximum of 2 electrons with
opposite spins
3. Orbitals of equal energy are half-filled, with spins
parallel, before any of them are filled
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Dihelium (He2) Molecule (s1s)
2 (s*1s)2
BO = ½(2-2) = 0
Bond order = 1/2 [# e- in bonding MOs
- # e- in antibonding MOs]
Bond Order
Hydrogen (H2) Molecule: (s1s)
2 (s*1s)0
BO = ½(2-0) = 1
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Learning Check
What is the electron configuration of the H2+
ion in MO terms. Compare the bond order of
this ion with He2+ and H2
-. Do you expect H2+
to exist?
Strategy: Fill the number of valence
electrons in the MO diagram or write
electron configuration in MO terms. Then
calculate Bond Order.
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Bonding in s-block homonuclear diatomic molecules
Li2
Li2 bond order = 1
Be2
Be2 bond order = 0
Homomuclear Diatomic Molecules
of Period 2 Elements
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Shapes and energies of s and p MOs
from combinations of 2p atomic orbitals
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Relative MO energy levels for Period 2
homonuclear diatomic molecules
MO energy levels
for O2, F2, and Ne2
without 2s-2p
mixing
MO energy levels
for B2, C2, and N2
with 2s-2p
mixing
Note the
change in
energy of s2p
and p2p MOs
with s-p mixing
s-p mixing
occurs
when s &
p-orbitals
are close in
energy (< ½
filled 2p, or
< 12
valence
electrons)
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MO occupancy
and molecular
properties for
B2 through Ne2.
The MO
theory fully
explains the
paramagnetic
properties of
O2.
Note the change in order of
energy of s2p and p2p MOs on
going from N2 to O2.
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Learning Check
The cation O2+ and N2
+ are important
components of Earth’s upper atmosphere.
Write electron configurations in MO terms
for these ions. Predict their bond orders and
magnetic behavoirs.
Strategy: Count the number of valence
electrons and place them in the MO
diagrams.
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The MO diagram
for HF.
- No p MOs. The s MO
between 1s of H and 2p
of F. As F is much higher
in EN than H, it holds
electrons tighter
resulting the bonding MO
is closer F 2p orbitals.
- The 2 lone pairs
electrons belong to F and
do not form bonds. They
are in nonbonding
MOs.
Electron Configurations for
Heteronuclear Diatomic Molecules
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The MO diagram for NO.
- Total 11 valence electrons:
s-p mixing and electron
configuration:
(s2s)2(s2s*)
2(p2p)4(s2p)
2(p2p*)1
- B.O. = 2.5, consistent with
2 resonance structures
- the single electron is
closer to N than to O
Electron Configurations for
Heteronuclear Diatomic Molecules
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Problems
Write electron configurations in MO terms and
calculate bond order for CO, and ClF.
© 2009 Brooks/Cole - Cengage
Fig. 9-22, p. 431
Molecular Orbitals in Ozone, O3, molecule
Polyatomic Molecules:
Resonance and MO Theory
The p
electrons
delocalize
through-
out the
molecule
© 2009 Brooks/Cole - Cengage
Fig. 9-24, p. 432
Molecular Orbitals in Benzene, C6H6, molecule