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Chemistry-140 Lecture 26. Chapter 10: Bonding & Molecular Structure: Orbital Hybridization, Molecular Orbitals. Chapter Highlights intro to VB & MO theory orbital overlap orbital hybridization multiple bonding ( p bonds) bond order MO theory. Chemistry-140 Lecture 26. - PowerPoint PPT Presentation
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Chapter 10:
Bonding & Molecular Structure:
Orbital Hybridization, Molecular Orbitals
Chapter Highlights
intro to VB & MO theory
orbital overlap
orbital hybridization
multiple bonding ( bonds)
bond order
MO theory
Chemistry-140 Lecture 26
Valence Bond (VB) Theory: (Linus Pauling, 1954) assumes
covalent bonding is due to overlap of atomic orbitals which
create a region of shared electron density between the
nuclei
Molecular Orbital (MO) Theory: (Robert Mulliken, 1966)
assumes valence electrons are in molecular orbitals which
extend over several atoms
Two Approaches to Chemical Bonding
Chemistry-140 Lecture 26
Valence Bond Theory: Orbital Overlap
Orbital overlap: If two H-atoms approach each other
closely enough their 1s orbitals can partially occupy the
same region of space….
Chemistry-140 Lecture 26
VB Theory: Orbital Overlap
HA:1sA
Overlap Region
HB:1sB
1sA 1sB
H-atoms
H2 molecule
Chemistry-140 Lecture 26
Energy Profile of a Covalent Bond
Chemistry-140 Lecture 23
Orbital Overlap
The valence bond orbital between two atoms is a region of
high probability of finding the electron.
There is an optimum distance between the two nuclei,
called the bond length: the distance of separation at which
the total energy is minimized.
The imaginary line that passes through both nuclei is called
the internuclear axis
Chemistry-140 Lecture 26
Sigma () Bonds
Sigma () bond:
A bond in which
the electron density
is circularly
symmetrical about
the internuclear
axis. The orbital
overlap is along the
internuclear axis.
Chemistry-140 Lecture 26
C: [He]2s22p2
What Orbitals Do We Use to Make the Tetrahedral Molecule CH4 ?
H: 1s1
Chemistry-140 Lecture 26
Hybridization: The process of mathematically mixing two
or more atomic orbitals, on a single atom.
Hybrid orbital: The result of this blending of orbitals. The
number of hybrid orbitals formed is always the same as the
number of atomic orbitals used
Hybrid Orbitals
Chemistry-140 Lecture 26
CH4 has four equivalent C-H bonds
sp3 Hybridization
1s 2s 2p
ground state
promoted state
CH
HH
H
Chemistry-140 Lecture 26
sp3 hybrid orbitals: are formed from the mixing of one
s-orbital and three p-orbitals. The arrangement of
the four sp3 hybrid orbitals is tetrahedral, with a
109.5° angle between the hybrid orbitals
sp3 Hybridization
1s 2sp3
Chemistry-140 Lecture 26
sp3 Hybridization
Chemistry-140 Lecture 26
BF3 has three equivalent B-F bonds
sp2 Hybridization
1s 2s 2p
ground state
promoted state
B
F
F
F
Chemistry-140 Lecture 26
sp2 hybrid orbitals: are formed from the mixing of one
s-orbital and two p-orbitals. The arrangement of the
three sp2 hybrid orbitals is trigonal planar, with a
120° angle between the hybrid orbitals
sp2 Hybridization
1s 2sp2 2p
Chemistry-140 Lecture 26
Chemistry-140 Lecture 26 November 8th, 1996
sp2 HybridizationChemistry-140 Lecture 26
BeF2 has two equivalent Be-F bonds
sp Hybridization
1s 2s 2p
ground state
promoted state
F Be F
Chemistry-140 Lecture 26
sp hybrid orbitals: are formed from the mixing of one
s-orbital and one p-orbital. The arrangement of the
two sp hybrid orbitals is linear, with a 180° angle
between the hybrid orbitals
sp Hybridization
1s 2sp 2p
Chemistry-140 Lecture 26
sp Hybridization
Chemistry-140 Lecture 26
Chemistry-140 Lecture 26
Chapter 10:Bonding & Molecular Structure:
Orbital Hybridization, Molecular Orbitals
Chapter Highlights
intro to VB & MO theory
orbital overlap
orbital hybridization
multiple bonding ( bonds)
bond order
MO theory
Chemistry-140 Lecture 28
BF3 has three equivalent B-F bonds
sp2 Hybridization
1s 2s 2p
ground state
promoted state
B
F
F
F
Chemistry-140 Lecture 28
sp2 hybrid orbitals: are formed from the mixing of one
s-orbital and two p-orbitals. The arrangement of the
three sp2 hybrid orbitals is trigonal planar, with a
120° angle between the hybrid orbitals
sp2 Hybridization
1s 2sp2 2p
Chemistry-140 Lecture 28
Chemistry-140 Lecture 26 November 8th, 1996
sp2 Hybridization
Chemistry-140 Lecture 28
BeF2 has two equivalent Be-F bonds
sp Hybridization
1s 2s 2p
ground state
promoted state
F Be F
Chemistry-140 Lecture 28
sp hybrid orbitals: are formed from the mixing of one
s-orbital and one p-orbital. The arrangement of the
two sp hybrid orbitals is linear, with a 180° angle
between the hybrid orbitals
sp Hybridization
1s 2sp 2p
Chemistry-140 Lecture 28
sp Hybridization
Chemistry-140 Lecture 28
Chemistry-140 Lecture 28
Chemistry-140 Lecture 28
Sigma () Bonds
Sigma () bond:
A bond in which
the electron density
is circularly
symmetrical about
the internuclear
axis. The orbital
overlap is along the
internuclear axis.
Chemistry-140 Lecture 28
Multiple Bonds
In almost all cases, single bonds are -bonds
BUT: To explain double and triple bonds we need another
kind of bond.
CC
H H
HH
C2H4 ethylene
C2H2 acetylene
CC HH
Chemistry-140 Lecture 28
-Orbital Overlap
-bonds: those in which the electron density is above and
below the internuclear axis. The internuclear axis is a
region of zero electron density.
Chemistry-140 Lecture 28
10 of 12 valence electrons are used to form the C-H (four) and
C-C (one) -bonds. The extra p-orbitals are perpendicular to
the plane of the molecule and contain a single electron
Ethylene (sp2 hybridization)
ground state
promoted state
sp2 hybridization
1s 2sp2 2p
1s 2s 2p
Chemistry-140 Lecture 28
Ethylene (sp2 hybridization)
Chemistry-140 Lecture 28
Ethylene (sp2 hybridization)
Chemistry-140 Lecture 28
Ethylene (sp2 hybridization)
H(1s)C(sp2)
C(p)
-bonds
-bond
Chemistry-140 Lecture 28
6 of 10 valence electrons are used to form the C-H (two) and C-
C (one) -bonds. The TWO extra p-orbitals are perpendicular
to the axis of the molecule and contain a single electron each
Acetylene (sp hybridization)
ground state
promoted state
sp hybridization
1s 2s 2p
1s 2sp 2p
Chemistry-140 Lecture 28
Acetylene (sp hybridization)
Chemistry-140 Lecture 28
Acetylene (sp hybridization)
Chemistry-140 Lecture 28
Consequences of Multiple Bonding
Cl
H
C
Free rotation occurs aroundthe axis of a single -bond
This cannot occur for a multiple -bond
system and isomers may result
cis trans
Chemistry-140 Lecture 28
Bond Order and Hybridization in Resonance Structures
TWO -electrons over THREE atoms. O-O bond order is 1.5!!O-O distance & energy an average of a single & a double bond
O
O O
O
O O
Chemistry-140 Lecture 28
Identifying Orbital Hybridization Schemes
H C C O
H
H
O
H
Question
Complete this Lewis structure and assign hybridization
schemes to all the non-hydrogen atoms. How many electrons
are there in -orbitals in this compound?
Chemistry-140 Lecture 28
Identifying Orbital Hybridization Schemes
Answer
H C C O
H
H
O
H
sp3
sp3sp2
sp2
Since there is only ONE -bond, the
number of electrons in -bonds is TWO!
Chemistry-140 Lecture 28
Chapter 10:Bonding & Molecular Structure:
Orbital Hybridization, Molecular Orbitals
Chapter Highlights
intro to VB & MO theory
orbital overlap
orbital hybridization
multiple bonding ( bonds)
bond order
MO theory
Chemistry-140 Lecture 29
An Introduction to Molecular Orbitals
Molecular Orbitals: Valence electrons are in molecular
orbitals, MO’s extending over the whole molecule.
Emphasizes the uniqueness of each molecule rather
than being the sum of its atoms (VB theory)
Why Bother!!!
O2 is paramagnetic!! That's a good reason!!
O O =
Chemistry-140 Lecture 29
TWO atomic orbitals HA(1s) and HB(1s) combine
mathematically (a linear combination) to produce TWO
molecular orbitals H2 (1s) and H2(1s*).
1s* = Antibonding MO 1s* = Antibonding MO
1s = Bonding MO1s = Bonding MO
Molecular Orbitals From Atomic Orbitals
Chemistry-140 Lecture 29
Bonding MO (1s): From addition of the two atomic
orbitals. Leads to an increased probability that the
electrons are found in this region. Electrons and orbital
are concentrated between the nuclei.
Antibonding MO (1s*): From subtraction of the two
atomic orbitals. Leads to a reduced probability that the
electrons are found in this region. Without significant
electron density between the nuclei, they are repelled.
Molecular Orbitals From Atomic Orbitals
Chemistry-140 Lecture 29
HA(1s) HB(1s)
HA(1s) HB(1s)
sigma*antibonding MO
with node
sigmabonding MO
Molecular Orbital Description of H2
Chemistry-140 Lecture 29
A Molecular Orbital Diagram for H2
Chemistry-140 Lecture 29
A first principle: The number of molecular orbitals (MO)
produced is always equal to the number of atomic orbitals
(AO) used in the combination.
A second principle: Bonding MO’s are always lower in
energy and antibonding MO’s higher in energy than their
parent AO’s.
A third principle: Electrons are assigned to MO’s with
successively higher energies; obeying the Pauli exclusion
principle and Hund’s rule.
Some Basic Principles of MO Theory
Chemistry-140 Lecture 29
Bond Order in MO Theory
Recall: Bond order was defined as the number of bonding
electron pairs linking two atoms.
In MO Theory:
Bond order = 1/2 [(number of electrons in bonding MO’s)- (number of electrons in antibonding MO’s)]
Chemistry-140 Lecture 29
Bond Order From an MO Diagram
H2: (1s)2 (1s*)
Bond order for H2 = 1He2: (1s)2
(1s*)2
Bond order for He2 = 0
Chemistry-140 Lecture 29
A Molecular Orbital Diagram for Li2
Chemistry-140 Lecture 29
Formation of (2p) and (2p)* MO’s
Chemistry-140 Lecture 29
Formation of (2p) and (2p)* MO’s
Chemistry-140 Lecture 29
MO’s Derived From the 2p Orbitals
Chemistry-140 Lecture 29
MO Diagram for First Row Diatomics X2
Chemistry-140 Lecture 29
MO Diagram for N2 (Highest Occupied MO)
HOMO(Lowest Unoccupied MO)
LUMO
Chemistry-140 Lecture 29
N2:(1s)2(1s*)2(2s)2(2s*)2(2p)4(2p)2(2p*)(2p*)
Bond order for N2 = 1/2 (8 - 2) = 3
Electron Configuration and
Bond Order for the N2 Molecule
N2:[core](2s)2(2s*)2(2p)4(2p)2
Chemistry-140 Lecture 29
MO Diagram for O2
Chemistry-140 Lecture 29
O2:(1s)2(1s*)2(2s)2(2s*)2(2p)4(2p)2(2p*)2(2p*)
Bond order for O2 = 1/2 (8 - 4) = 2
Electron Configuration and
Bond Order for the O2 Molecule
O2:[core](2s)2(2s*)2(2p)4(2p)2(2p*)2
MO Theory predicts that OMO Theory predicts that O22
has TWO unpaired electronshas TWO unpaired electronsand is therefore PARAMAGNETIC!!and is therefore PARAMAGNETIC!!
Chemistry-140 Lecture 29
Textbook Questions From Chapter # 10Review concepts: 1, 2, 3, 4, 5
Hybrid orbitals: 16, 18, 20, 24
Molecular orbital theory: 30, 34
General questions: 36, 40, 43, 45, 53
Conceptual questions: 61
Textbook Questions From Chapter # 10Review concepts: 1, 2, 3, 4, 5
Hybrid orbitals: 16, 18, 20, 24
Molecular orbital theory: 30, 34
General questions: 36, 40, 43, 45, 53
Conceptual questions: 61
Chemistry-140 Lecture 29