molecular structure. copyright © houghton mifflin company. all rights reserved. 13a–2 a computer...
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MOLECULAR STRUCTURE
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A computer representation of K3C60, a superconducting substance formed by reacting
potassium with buckminster fullerine (C60)
Source: Photo Researchers, Inc.
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Two forms of carbon; graphite and diamond.
Source: Grant Hellman
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Quartz grows in beautiful, regular crystals
SiO2
Vs
CO2
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Figure 13.1: (a) The interaction of two hydrogen atoms (b) Energy profile as a function of the distance
between the nuclei of the hydrogen atoms.
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Figure 13.1: (a) The interaction of two hydrogen atoms (b) Energy profile as a function of the distance
between the nuclei of the hydrogen atoms.
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WHERE DO THE ELECTRONS GO?
• Are they shared equally?
• Are they more on one atom than the other?
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WHERE DO THE ELECTRONS GO?
• Are they shared equally?
• Are they more on one atom than the other?
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WHERE DO THE ELECTRONS GO?
• Are they shared equally?
• Are they more on one atom than the other?
ANSWER: It depends who is pulling harder
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WHERE DO THE ELECTRONS GO?
• Are they shared equally?
• Are they more on one atom than the other?
ANSWER: It depends who is pulling harder(“Electro negativity”)
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The HCL molecule has a dipole moment
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Pauling and his electronegativity
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Pauling and his electronegativity
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Figure 13.3: The Pauling electronegativity values as updated by A.L. Allred in 1961. (cont’d)
Arbitrarily set F as 4
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The HCL molecule has a dipole moment
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X
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X
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models
Now lets consider more than two atoms
in a molecule
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Linear molecules
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Planar molecules
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Tetrahedral molecules
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MODELS
• These are only models
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MODELS
• These are only models
• But…. Models are very useful for describing properties.
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MODELS
• These are only models
• But…. Models are very useful for describing properties.
Newton:particles
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MODELS
• These are only models
• But…. Models are very useful for describing properties.
Newton:particles
Huygens:waves
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models
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Ball-and-stick model of a protein segment illustrating the alpha helix.
Source: Photo Researchers, Inc.
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The concept of individual bonds makes it much easier to deal with complex molecules such as DNA.
Source: Photo Researchers, Inc.
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Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic MoleculesExample: 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.
O C O
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Skeletal Structure
• Hydrogen atoms are always terminal atoms.• Central atoms are generally those with the lowest
electronegativity.• Carbon atoms are always central atoms.• Generally structures are compact and
symmetrical.
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Skeletal Structure
• Identify central and terminal atoms in the molecule C2H6O (ethyl alcohol or ethanol).
C
H
H
H
H
H
H
OC
NOTE: Terminal atoms are all bonded to only one other atoms. Central atoms are bonded to two or more other atoms
Now where do the electrons go?
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Writing Lewis Structures
• All the valence e- of atoms must appear.
• Usually, the e- are paired.
• Usually, each atom requires an octet.– H only requires 2 e-.
• Multiple bonds may be needed.– Readily formed by C, N, O, S, and P.
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Lewis Structures
Draw Lewis structures for:
HF:
H2O:
NH3:
CH4:
H F
or H F
H O H
or H O H
H N HH
or H N H
H
H C HH
H
or H C H
H
H
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Benzene
Fredrich August von Kekule (German chemist) said that he discovered the ring-shaped chemical structure of benzene because of a strange, reptilian dream he had in 1865: "I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. ... My mental eye... could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... " ( From "Creativity, Beyond the Myth of Genius" by Robert Weisberg published by W. H. Freeman 1992 .) Although some scholars now believe that Kekule's dream was a hoax to avoid sharing credit for the discovery of the hexagonal shape of benzene, it still makes a wonderful story.
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Resonance Forms
C6H6 30 ve’s
C C
C C
C
C
H
H
H
H
H
H
C C
C C
C
C
H
H
H
H
H
H
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Electron-Rich Atoms
PF5
P
F
F F
FF
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••
Exceptions to the Octet Rule
• Expanded octets.
P
Cl
Cl
••••
••
••
••••
••
P
Cl
Cl
••••
Cl
••••
••••
•••• •• ••
••
Cl
••••
••Cl••
S
F
F
••
••
F
••••
••
••
•••• •• ••
••F
••••
••F••
F
••
••
•••• ••
••
Cl
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Molecular ShapesMolecular Shapes
•Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which.
•The shape of a molecule is determined by its bond angles.
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Molecular ShapesMolecular Shapes
In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion.
We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.
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The VSEPR ModelThe VSEPR Model
Predicting Molecular GeometriesPredicting Molecular Geometries
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The VSEPR ModelThe VSEPR Model
Predicting Molecular GeometriesPredicting Molecular Geometries
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Molecular ShapesMolecular Shapes
experimentally we find all Cl-C-Cl bond angles are 109.5. Therefore, the molecule cannot be planar.All Cl atoms are located at the vertices of a tetrahedron with the C at its center.
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The VSEPR ModelThe VSEPR Model
Predicting Molecular GeometriesPredicting Molecular GeometriesTo determine the electron pair geometry:
•draw the Lewis structure•count the total number of electron pairs around the central atom•arrange the electron pairs in one of the above geometries to minimize e-e repulsion•multiple bounds count as one bonding pair
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The VSEPR ModelThe VSEPR Model
Predicting Molecular GeometriesPredicting Molecular Geometries
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The VSEPR ModelThe VSEPR Model
Predicting Molecular GeometriesPredicting Molecular Geometries
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Molecular ShapesAB2
Linear
AB3
Trigonal planar AB4
Tetrahedral
AB5
Trigonal bipyramidal
AB6
Octahedral
AB2EAngular or Bent AB3E
Trigonalpyramidal
AB2E2
Angular or Bent
AB4EIrregular tetrahedral(see saw)
AB3E2
T-shaped
AB2E3
Linear
AB5ESquare pyramidal
AB4E2
Square planar
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The VSEPR ModelThe VSEPR Model
The valence electrons in a molecule are the bonding pairs of electrons as well as the lone pairs.
There are 11 shapes that are important to us:Number of atoms, formula Shapes
(3 atoms, AB2) linear or bent(4 atoms, AB3) trigonal planar, trigonal bipyramidal, or T-shaped(5 atoms, AB4) tetrahedral, square planar, or see-saw(6 atoms, AB5) trigonal bipyramidal or square pyramidal(7 atoms, AB6) octahedral
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The VSEPR ModelThe VSEPR Model
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The VSEPR ModelThe VSEPR ModelMolecules with More than One Central AtomMolecules with More than One Central AtomIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.
Number of electron domains
Electron-domain geometry
Predicted bond angles
TetrahedralTrigonalplanar Tetrahedral
109.5o 120o 109.5o
C OH H
H
H O
4 3 4
C
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The VSEPR ModelThe VSEPR ModelMolecules with More than One Central AtomMolecules with More than One Central AtomIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.
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Figure 13.6: The carbon dioxide molecule
• Nonpolar Molecules– Dipole moments are symmetrical and cancel out.
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Determining Molecular Polarity
• Nonpolar Molecules– Dipole moments are symmetrical and cancel out.
BF3
F
F F
B
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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Determining Molecular Polarity
• Polar Molecules– Dipole moments are asymmetrical and don’t
cancel .
netdipolemoment
H2OH H
O
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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CHCl3
H
Cl ClCl
Determining Molecular Polarity
polar molecules have...– asymmetrical shape (lone pairs) or – asymmetrical atoms
netdipolemoment
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
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Figure 13.5: (a) The structure and charge distribution of the ammonia molecule. (b) The dipole moment of the
ammonia molecule oriented in an electric field.
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Sulfur has a partial positive charge
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Hydrogen atoms have a partial positive charge
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Hydrogen atoms and a small partial negative charge on the carbon
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......
Polar Bonds
H Cl
Polar
A molecule has a zero dipole moment when bond dipoles cancel one another.
H HO
PolarF F
B
F
Nonpolar
HH
H
N
Polar
Polar Nonpolar
F FCl
F
F F
Xe
F FCl
ClC
Cl
Nonpolar Polar
Cl
HC
Cl
H
H
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Comparing fuels
• Natural gas: CH4 + 2O2 → CO2 + 2 H2O ΔH=-808 kJ/mol
• Coal: C + O2 → CO2 ΔH=-393.5 kJ/mol
• Oil: C20H42 + 30½O2 → 20CO2 + 21 H2O ΔH=-13315 kJ/molΔH=-666 kJ/mol.CO2
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Comparing fuels
Production of 1 GigaJoule of energy releases:
• Natural gas: 109 J x 0.044 kg/mol ÷ 808,000 J/mol = 54.5 kg CO2
• Coal: 112 kg CO2
• Oil: 66 kg CO2
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N3 high energy density
3 2
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ATP energy
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ATP energy
Repulsion weakens these bonds
Resonance!