Post on 22-Jan-2023
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Chemical Bonding II:Molecular Geometry and
Hybridization of Atomic OrbitalsChapter 10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 10; Molecular Structure
I. Molecular Structure (3-D Shape) of Molecules Following Octet Rulea) VSEPR Theory
i. Regions of High Electron Densityii. 3-D Arrangement
II. Polarity of MoleculesIII. Solubility
Chapter 10; Molecular Structure
IV. Valence Bond Theorya) Hybrid Orbitalsβ) σ vs. Π bonds
V. Expanded Octets and Molecular Structure
Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs.
10.1
2
Predicting Molecular Structure
A. Diatomic Molecules (N2, Cl2, ICl)i. All all linear since 2 points define a line
B. Polyatomic Moleculesi. Draw Lewis Dot Structureii. Determine Regions of High Electron Density
(HED) around central atom in Lewis Structure.
Regions of High Electron Density
• Chemical Bonds• Single, double, triple bonds all count as 1 region
• Lone Pair• Free Radical
Predicting Molecular Structure(cont)
iii. Choose 3-D arrangement that places regions of High Electron Density (HED) as far apart as possible
AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonePairs/fr on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonalplanar
trigonalplanar
10.1
AB4 4 0 tetrahedral tetrahedral
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Cl ClBe
2 atoms bonded to central atom0 lone pairs on central atom 10.1 10.1
10.1
NO2 vs CO2
Class Molecular Structure
Bond Angle
NO2 AB2E Bent, Angular
< 120°
CO2 AB2 Linear 180°
C OO
N
O O
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BH3 vs NH3
Class Molecular Structure
Bond Angle
BH3 AB3 TrigonalPlanar
120°
NH3 AB3E TrigonalPyramidal
<109.5°
B
HH
H
NH
HH
Effect of Lone Pair on Central AtomTotal – 3 HED
Class
# of atomsbonded to
central atom
# lonePairs/fr
on central atom
Arrangement ofelectron pairs
MolecularGeometry
AB3 3 0 trigonalplanar
trigonalplanar
AB2E 2 1 trigonalplanar bent
Effect of Lone Pair on Central AtomTotal – 4 HED
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
AB4 4 0 tetrahedral tetrahedral
AB3E 3 1 tetrahedral trigonalpyramidal
AB2E2 2 2 tetrahedral bent
HO
H
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Molecular Structure Depends on # Atoms and # HED
2 3 4
2 Linear, 180°
Angular (Bent), <120°
Angular (Bent), <109.5°
3 Trigonal Planar, 120°
Trigonal Pyramidal, 109.5°
4 Tetrahedral, 109.5°
Regions of HED around Central Atom
# A
tom
s S
urro
undi
ng C
entra
l Ato
m
Dipole Moments and Polar Molecules
10.2
H F
electron richregionelectron poor
region
δ+ δ−
10.2
Determining Polarity of Molecules
1) Draw Lewis Dot Structure2) Determine Molecular Structure (3-D
Shape)3) Look for Dipoles Within Molecule4) Determine if Dipoles Reinforce or Cancel
• Reinforce => Polar Molecule• Cancel => Nonpolar Molecule
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10.2
Which of the following molecules have a dipole moment?H2O, CO2, SO2, and CH4
O HHdipole momentpolar molecule
SO O
CO O
no dipole momentnonpolar molecule
dipole momentpolar molecule
C
H
H
HH
no dipole momentnonpolar molecule
Solubility
• “Like” dissolves “Like– Polar substance dissolves in Polar Substance– NonPolar substance dissolves in NonPolar
Substance– Polar substance will not dissolve in NonPolar
Substance
I. Determine what type of orbital the unpaired electron is in for each atom the bond is between
II. Draw a picture of each orbital before overlap
III. Overlap orbitals to show how bond forms
Valence bond theory – bonds are formed by sharing of e- from overlapping atomic orbitals.
Change in electron density as two hydrogen atoms approach each other.
10.3
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Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals.
1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals.
2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.
3. Covalent bonds are formed by:
a. Overlap of hybrid orbitals with atomic orbitals
b. Overlap of hybrid orbitals with other hybrid orbitals
10.4
Formation of sp Hybrid Orbitals
10.4
Formation of sp2 Hybrid Orbitals
10.4 10.4
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10.4
Determination of Atom Hybridization from Lewis Structures
Hybrid Orbital
# of Hybrid Orbitals
Total # HED
3-D Orbital Orientation
sp 2 2 Linear
sp2 3 3 TrigonalPlanar
sp3 4 4 Tetrahedral
10.4
Predict correctbond angle
10.5
sp2 Hybridized C
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Sigma bond (σ) – electron density between the 2 atomsPi bond (π) – electron density above and below plane of nuclei of the bonding atoms 10.5 10.5
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10.5
Sigma (σ) and Pi Bonds (π)
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 σ and π bonds are in the acetic acid(vinegar) molecule CH3COOH?
C
H
H
CH
O
O Hσ bonds = 6 + 1 = 7π bonds = 1
10.5
AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR
AB3 3 0 trigonalplanar
trigonalplanar
10.1
AB4 4 0 tetrahedral tetrahedral
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB6 6 0 octahedraloctahedral
10.1
12
10.1
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedron
TA p298
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedron
AB3E2 3 2 trigonalbipyramidal T-shaped
ClF
F
F
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR-5 HED
10.1
AB5 5 0 trigonalbipyramidal
trigonalbipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedron
AB3E2 3 2 trigonalbipyramidal T-shaped
AB2E3 2 3 trigonalbipyramidal linear
I
I
I
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR
10.1
AB6 6 0 octahedraloctahedral
AB5E 5 1 octahedral square pyramidal
Br
F F
FF
F
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
VSEPR – 6 HED
10.1
AB6 6 0 octahedraloctahedral
AB5E 5 1 octahedral square pyramidal
AB4E2 4 2 octahedral square planar
Xe
F F
FF
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Molecular Orbital Theory (MO)• Combine bonding atomic orbitals into
Molecular Orbitals
• 2 Atomic Orbitals 2 Molecular Orbitals
– ‘Wave-Function Properties’ of Orbitals
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Delocalized Electrons: Resonance
When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.
Delocalized Electrons: Resonance
• In reality, each of the four atoms in the nitrate ion has a p orbital.
• The p orbitals on all three oxygens overlap with the porbital on the central nitrogen.
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Delocalized Electrons: Resonance
This means the π electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.
Resonance
The organic molecule benzene has six σ bonds and a p orbital on each carbon atom.
Resonance
• In reality the π electrons in benzene are not localized, but delocalized.
• The even distribution of the π electrons in benzene makes the molecule unusually stable.
Molecular Orbital (MO) Theory
Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals.
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Molecular Orbital (MO) Theory
• In MO theory, we invoke the wave nature of electrons.
• If waves interact constructively, the resulting orbital is lower in energy: a bonding molecular orbital.
Molecular Orbital (MO) Theory
If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.
MO Theory
• In H2 the two electrons go into the bonding molecular orbital.
• The bond order is one half the difference between the number of bonding and antibonding electrons.
MO Theory
For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is
12 (2 - 0) = 1
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MO Theory
• In the case of He2, the bond order would be
12 (2 - 2) = 0
• Therefore, He2does not exist.
MO Theory
• For atoms with both s and p orbitals, there are two types of interactions:– The s and the p orbitals that
face each other overlap in σfashion.
– The other two sets of porbitals overlap in πfashion.
MO Theory
• The resulting MO diagram looks like this.
• There are both σ and πbonding molecular orbitals and σ* and π* antibonding molecular orbitals.
MO Theory
• The smaller p-block elements in the second period have a sizeable interaction between the s and porbitals.
• This flips the order of the s and p molecular orbitals in these elements.