Transcript Honors Chemistry ch 10

Honors Chemistry
Chapter 10: Chemical Bonding II
10.1 Molecular Geometry
• Study of the shapes of molecules
• Molecule’s geometry affects properties
• Valence shell
• outermost occupied shell
• Holds electrons involved in bonding
• VSEPR Model
• Valence Shell Electron Pair Repulsion
• Accounts for arrangement of electron pairs around
a central atom in terms of electrostatic repulsion
10.1 Molecular Geometry
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AXnEm
Draw Lewis Structure
Identify central atom (A)
Count electron pairs
X = bonding pairs
E = lone pairs
Electron-group repulsions and the five basic molecular shapes.
linear
trigonal bipyramidal
tetrahedral
trigonal planar
octahedral
10.1 Molecular Geometry
10.1 Molecular Geometry
10.1 Molecular Geometry
10.1 Molecular Geometry
10.1 Molecular Geometry
10.1 Molecular Geometry
10.1 Molecular Geometry
• True bond angles deviate from ideal
• Lone pairs are more repulsive than
bonding pairs
• Triple bonds > double bonds > single
bonds in terms of repulsion
• Molecules with multiple centers
• Apply VSEPR to each central atom
10.1 Molecular Geometry
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Find the shapes and bond angles:
AsH3
CS2
OF2
NO3AlCl4I3C2H4
10.2 Dipole Moments
• Vector describing the polarity of the entire
molecule
• Symbol for dipole moment is m
• Measured in Debyes
• Depends on bond polarity and geometry
• Examples: O2, CO2, H2O, CH4, CCl4, NH3
• Larger molecules become more complex
• cis-dichloroethylene,trans-dichloroethylene
10.3 Valence Bond Theory
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More complete theory of bonding
Based on Quantum Mechanics
Explains bond energies and bond lengths
Consider overlap of 1s orbitals as two H
atoms approach each other
• Nucleus-electron attraction forces
• Nucleus-nucleus repulsion forces
10.3 Valence Bond Theory
10.4 Hybridization
• Consider compound of C and H
• Carbon has 2s2 sp2 configuration
• Basic QM would predict
• CH2
• 90o bond angles
• Actual compound is CH4, 109.5o angles
• Need to merge s and p orbitals into a new
set of atomic orbitals (hybrids)
The sp3 hybrid orbitals in CH4.
The sp3 hybrid orbitals in NH3.
The sp3 hybrid orbitals in H2O.
The sp2 hybrid orbitals in BF3.
The sp hybrid orbitals in gaseous BeCl2.
10.4 Hybridization
10.4 Hybridization
• Procedure for hybridizing atomic orbitals:
• 1. Draw Lewis structure
• 2. Use VSEPR to predict the overall
geometry of the electron pairs
• 3. Deduce the hybridization of the central
atom from the geometry
10.5 Double and Triple Bonds
• sp2 and sp hybrids don’t use all p orbitals
• “Leftover” p’s can overlap to form bonds
• Sigma bond (s)
• Overlap of s or head-on p orbitals
• e- density between nuclei
• Pi bond (p)
• Sideways overlap of p orbitals
• e- density above and below
nuclei
The s bonds in ethane(C2H6).
both C are sp3 hybridized
s-sp3 overlaps to s bonds
sp3-sp3 overlap to form a s bond
relatively even
distribution of electron
density over all s
bonds
The s and p bonds in ethylene (C2H4).
overlap in one position - s
p overlap - p
electron density
The s and p bonds in acetylene (C2H2).
overlap in one position - s
p overlap - p
10.5 Double and Triple Bonds
Electron density and bond order.
10.5 Double and Triple Bonds
Restricted rotation of p-bonded molecules in C2H2Cl2.
CIS
TRANS
10.6 Molecular Orbital Theory
• VB allows e- to stay in atomic orbitals
• This is only an approximation
• Fails to account for some properties of
molecules (eg, magnetism)
• Reality – orbitals are delocalized across
the entire molecule
• Molecular Orbital Theory – based on QM
• Rebuild y for the entire molecule
An analogy between light waves and atomic wave functions.
Amplitudes of wave
functions added
Amplitudes of
wave functions
subtracted.
Contours and energies of the bonding and antibonding
molecular orbitals (MOs) in H2.
The bonding MO is lower in energy and the antibonding MO is higher in
energy than the AOs that combined to form them.
The MO diagram for H2.
Filling molecular orbitals with electrons follows the
same concept as filling atomic orbitals.
Energy
s*1s
1s
1s
H2 bond order
= 1/2(2-0) = 1
s1s
AO
of H
MO
of H2
AO
of H
MO diagram for He2+ and He2.
s*1s
1s
1s
Energy
Energy
s*1s
1s
1s
s1s
AO of
He
MO of
He+
s1s
AO of
He+
He2+ bond order = 1/2
AO of
He
MO of
He2
AO of
He
He2 bond order = 0
Bonding in s-block homonuclear diatomic molecules.
s*2s
s*2s
2s
2s
s2s
2s
2s
s2s
Be2
Li2
Li2 bond order = 1
Be2 bond order = 0
Contours and energies of s and p MOs through
combinations of 2p atomic orbitals.
Relative MO energy levels for Period 2 homonuclear
diatomic molecules.
without 2s-2p
mixing
with 2s-2p
mixing
MO energy levels
for O2, F2, and Ne2
MO energy levels
for B2, C2, and N2
MO occupancy
and molecular
properties for B2
through Ne2
The paramagnetic
properties of O2
Energy
s*
2p
The MO diagram for NO
s
p*
p
s
p
p
s*
2p
possible Lewis
structures
p
s
2s
2s
AO of N
AO of O
s
s
MO of NO
0
0
N
O
-1
+1
N
O
10.8 Delocalized Molecular Orbitals
• In larger molecules, bonds are sometimes
spread over the entire molecule
benzene, C6H6
ozone,O3