Chapter 11: Theories of Covalent Bonding

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Chapter 11 Theories of Covalent Bonding
11.1 Valence Bond (VB) Theory and Orbital
Hybridization 11.2 The Mode of Orbital Overlap
and the Types of Covalent Bonds 11.3
Molecular Orbital (MO) Theory and Electron
Delocalization
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Central Themes of Valence Bond Theory
Basic Principle of Valence Bond Theory a
covalent bond forms when the orbitals from two
atoms overlap and a pair of electrons occupies
the region between the nuclei.
1) Opposing spins of the electron pair. The
region of space formed by the overlapping
orbitals has a maximum capacity of two electrons
that must have opposite spins.
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Central Themes of Valence Bond Theory
Basic Principle of Valence Bond Theory a
covalent bond forms when the orbitals from two
atoms overlap and a pair of electrons occupies
the region between the nuclei.
1) Opposing spins of the electron pair. The
region of space formed by the overlapping
orbitals has a maximum capacity of two electrons
that must have opposite spins. 2) Maximum
overlap of bonding orbitals. The bond strength
depends on the attraction of nuclei for the
shared electrons, so the greater the orbital
overlap, the stronger the bond.
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Central Themes of Valence Bond Theory
3) Hybridization of atomic orbitals. To explain
the bonding in simple diatomic molecules such
as HF it is sufficient to propose the direct
overlap of the s and p orbitals of isolated
ground state atoms. In cases such as methane
CH4 where 4 hydrogen atoms are bonded to a
central carbon atom it is impossible to obtain
the correct bond angles. Pauling proposed that
the valence atomic orbitals in the molecule are
different from those in the isolated atoms.We
call this Hybridization!
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Fig. 11.1
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Hybrid Orbital Types - Periodic Groups
Hybrid Orbital Types Groups in the
Periodic Table Associated
SP Group IIA
Alkaline Earth Elements SP2
Group IIIA Boron
Family SP3
Group IVA Carbon Family SP3d
Group VA
Nitrogen Family SP3d2
Group VIA Oxygen Family
The exception is carbon which can have SP, SP
2, SP 3 hybrid orbitals
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The sp Hybrid Orbitals in Gaseous BeCl2
Fig. 11.2 AB
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Fig. 11.2 CD
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Fig. 11.3
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Fig. 11.4
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The sp3 Hybrid Orbitals in NH3 and H2O
Fig. 11.5
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The sp3d Hybrid Orbitals in PCl5
Fig. 11.6
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The sp3d2 Hybrid Orbitals in SF6
Sulfur Hexafluoride -- SF6
Fig. 11.7
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Fig. 11.8
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Postulating the Hybrid Orbitals in a Molecule
Problem Describe how mixing of atomic orbitals
on the central atoms leads to the hybrid
orbitals in the following a) Methyl amine,
CH3NH2 b) Xenon tetrafluoride, XeF4 Plan
From the Lewis structure and molecular shape, we
know the number and arrangement of electron
groups around the central atoms, from which we
postulate the type of hybrid orbitals involved.
Then we write the partial orbital diagram for
each central atom before and after the orbitals
are hybridized.
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Postulating the Hybrid Orbitals in a Molecule
Problem Describe how mixing of atomic orbitals
on the central atoms leads to the hybrid
orbitals in the following a) Methyl amine,
CH3NH2 b) Xenon tetrafluoride, XeF4 Plan
From the Lewis structure and molecular shape, we
know the number and arrangement of electron
groups around the central atoms, from which we
postulate the type of hybrid orbitals involved.
Then we write the partial orbital diagram for
each central atom before and after the orbitals
are hybridized. Solution a) For CH3NH2 The
shape is tetrahedral around the C and N
atoms. Therefore, each central atom is sp3
hybridized. The carbon atom has four half-filled
sp3 orbitals
2s
2p
sp3
Isolated Carbon Atom
Hybridized Carbon Atom
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Postulating the Hybrid Orbitals in a Molecule
Problem Describe how mixing of atomic orbitals
on the central atoms leads to the hybrid
orbitals in the following a) Methyl amine,
CH3NH2 b) Xenon tetrafluoride, XeF4 Plan
From the Lewis structure and molecular shape, we
know the number and arrangement of electron
groups around the central atoms, from which we
postulate the type of hybrid orbitals involved.
Then we write the partial orbital diagram for
each central atom before and after the orbitals
are hybridized. Solution a) For CH3NH2 The
shape is tetrahedral around the C and N
atoms. Therefore, each central atom is sp3
hybridized. The carbon atom has four half-filled
sp3 orbitals
2s
2p
sp3
Isolated Carbon Atom
Hybridized Carbon Atom
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The N atom has three half-filled sp3 orbitals and
one filled with a lone pair.
2s
sp3
2p
..
H
C
H
N
H
H
H
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b) The Xenon atom has filled 5 s and 5 p
orbitals with the 5 d orbitals empty.
Isolated Xe atom
5 d
5 s
5 p
Hybridized Xe atom
5 d
sp3d2
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b) continuedFor XeF4. for Xenon, normally it has
a full octet of electrons,which would mean an
octahedral geometry, so to make the compound,
two pairs must be broken up, and bonds made to
the four fluorine atoms. If the two lone pairs
are on the equatorial positions, they will be at
900 to each other, whereas if the two polar
positions are chosen, the two electron groups
will be 1800 from each other. Thereby
minimizing the repulsion between the two electron
groups.
F
F
F
F
Xe
Xe
1800
F
F
F
F
Square planar
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Fig. 11.9
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Fig. 11.10
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Fig. 11.11
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Restricted Rotation of ?-Bonded Molecules
A) Cis - 1,2 dichloroethylene B)
trans - 1,2 dichloroethylene
Fig. 11.12
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An Analogy between Light Waves and
Atomic Wave Functions
Fig. 11.13
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Fig. 11.14
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Filling Molecular Orbitals with Electrons
1) Orbitals are filled in order of increasing
Energy ( Aufbau principle ) 2) An
orbital has a maximum capacity of two electrons
with opposite spins ( Pauli exclusion
principle ) 3) Orbitals of equal energy are
half filled, with spins parallel, before any
is filled ( Hunds rule )
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Fig. 11.15
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Fig. 11.16
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Fig. 11.17
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Fig. 11.18
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Fig. 11.19
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Fig. 11.20
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Fig. 11.22
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Fig. 11.23
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Fig. 11.24