Transcript General Chemistry

Chapter 9
Molecular Structures
Linus Pauling 1627-1691.** Defined electronegativity.
Wrote “The Nature of the Chemical Bond” 1939.
Pioneered crystal and protein structures.
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Molecular Shapes
•Lewis structures give atomic connectivity: they tell us
which atoms are physically connected to which.
•Lewis structures do not give us information about
geometry or shape of molecules.
H
For example, methane, CH4, is drawn as: H C H
Where all the angles appear to be 90o.
H
Actually, the bond angles are
(tetrahedral angle)
H
C
109.5o,
H
H
H
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Molecular Shapes
Methane: Tetrahedral geometry
Tetrahedron
(sp3-hybridization)
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Some Molecular Geometries
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Molecular Shapes
In order to predict molecular shape, we assume the
valence electrons repel each other.
Therefore, the molecule adopts whichever 3D geometry
minimized this repulsion.
This is the idea behind Valence Shell Electron Pair
Repulsion (VSEPR) theory.
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Consider water, H2O
Dot structure is:
O
..
H : .O. : H
BD: 2
NBD: 2
TED: 4
H
H
TED=4 tells us that
electron domain geometry
(EDG) is tetrahedral and,
BD=2 tells us that:
molecular geometry (MG)
is bent
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PCl5
SF6
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Some examples:
PBr3
SO3
SO32-
H2O
BeF2
BCl3
CO32-
CH2=CH2
HCN
Work out the molecular geometries.
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Polarity of Molecules
Polar molecules interact with electric fields.
If the centers of negative and positive charge do not
coincide, then the molecule is polar.
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Polarity of Molecules
Dipole Moments of Polyatomic Molecules
Example: in CO2, each C-O dipole is canceled because
the molecule is linear. In H2O, the H-O dipoles do not
cancel because the molecule is bent.
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Polarity of Molecules
Dipole Moments of Polyatomic Molecules
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Covalent Bonding and Orbital Overlap
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Hybrid Orbitals
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Hybrid Orbitals
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Multiple Bonds
-Bonds: electron density lies on the axis between the
nuclei.
All single bonds are -bonds.
-Bonds: electron density lies above and below the
plane of the nuclei.
A double bond consists of one -bond and one -bond.
A triple bond has one -bond and two -bonds.
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Multiple Bonds
Two p orbitals
overlap to form
a π-bond
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Multiple Bonds
Ethylene, C2H4, H2C=CH2, has:
•one - and one -bond between the carbon atoms;
•both C atoms sp2 hybridized;
•both C atoms with trigonal planar molecular geometries.
2 p-orbitals
1 π-bond
5 σ-bonds
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Multiple Bonds
Consider acetylene, C2H2, H-CC-H, which has
•one - and two -bonds between the carbon atoms;
•both C atoms sp-hybridized;
•both C atoms with linear molecular geometries.
•When triple bonds form (e.g. HCCH, N2) one -bond
is always above and below and the other is in front
and behind the axis of the nuclei.
4 p-orbitals
2 π-bonds
3 σ-bonds H C
C H
:NN :
4 p-orbitals
2 π-bonds
1 σ-bond
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Delocalized  Bonding
In the case of benzene (C6H6) there are
 6 C-C  bonds, all equal;
 6 C-H  bonds, all equal.
 Each C atom is sp2 hybridized.
• There are 6 p orbitals on each C atom.
σ-bonds
p orbitals
molecular
orbital
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