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Copyright 1999, PRENTICE HALL Chapter 9 1
Molecular Shapes
Lewis structures give atomic connectivity: they tell uswhich atoms are physically connected to which.
The shape of a molecule is determined by its bond
angles.
Consider CCl4: 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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Copyright 1999, PRENTICE HALL Chapter 9 2
Molecular Shapes
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Copyright 1999, PRENTICE HALL Chapter 9 3
Molecular ShapesIn order to predict molecular shape, we assume the
valence electrons repel each other. Therefore, themolecule adopts whichever 3D geometry minimizes this
repulsion.
We call this process Valence Shell Electron Pair
Repulsion (VSEPR) theory.
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Copyright 1999, PRENTICE HALL Chapter 9 4
The VSEPR ModelThe 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) linearor bent
(4 atoms, AB3) tr igonal planar, tr igonal bipyramidal,
or T-shaped
(5 atoms, AB4) tetrahedral, square planar, or see-saw
(6 atoms, AB5) tr igonal bipyramidalor square pyramidal(7 atoms, AB6) octahedral
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Copyright 1999, PRENTICE HALL Chapter 9 6
The VSEPR ModelPredicting Molecular Geometries
To determine the shape of a molecule, we distinguishbetween lone pairs (or non-bonding pairs, those not in a
bond) of electrons and bonding pairs (those found
between two atoms).
We define the electron pair geometry by the positions in
3D space ofALL electron pairs (bonding or non-
bonding).
The electrons adopt an arrangement in space to
minimize e--e- repulsion.
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7/25Copyright 1999, PRENTICE HALL Chapter 9 7
The VSEPR ModelPredicting Molecular Geometries
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8/25Copyright 1999, PRENTICE HALL Chapter 9 8
The VSEPR ModelPredicting Molecular Geometries
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9/25Copyright 1999, PRENTICE HALL Chapter 9 9
The VSEPR ModelPredicting Molecular Geometries
To 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- repulsionmultiple bounds count as one bonding pair
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Copyright 1999, PRENTICE HALL Chapter 9 10
The VSEPR ModelPredicting Molecular Geometries
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Copyright 1999, PRENTICE HALL Chapter 9 12
The VSEPR ModelThe Effect of Nonbonding Electrons and
Multiple Bonds on Bond AnglesWe determine the electron pair geometry only looking
at electrons.
We name the molecular geometry by the positions of
atoms.
We ignore lone pairs in the molecular geometry.
All the atoms that obey the octet rule have tetrahedral
electron pair geometries.
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Copyright 1999, PRENTICE HALL Chapter 9 13
The VSEPR ModelThe Effect of Nonbonding Electrons and
Multiple Bonds on Bond AnglesBy experiment, the H-X-H bond angle decreases on
moving from C to N to O:
Since electrons in a bond are attracted by two nuclei, they do
not repel as much as lone pairs.Therefore, the bond angle decreases as the number of lone
pairs increase.
OH
H104.5
O107
O
NH
HH
C
H
HHH
109.5O
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Copyright 1999, PRENTICE HALL Chapter 9 14
The VSEPR ModelThe Effect of Nonbonding Electrons and
Multiple Bonds on Bond AnglesSimilarly, electrons in multiple bonds repel more than
electrons in single bonds.
C O
Cl
Cl
111.4o
124.3
o
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Copyright 1999, PRENTICE HALL Chapter 9 15
The VSEPR ModelMolecules with Expanded Valence Shells
Atoms that have expanded octets have AB5 (trigonal
bipyramidal) or AB6 (octahedral) electron pairgeometries.
For trigonal bipyramidal structures there is a plane
containing three electrons pairs. The fourth and fifth
electron pairs are located above and below this plane.
Axial pairs are not equivalent to equatorial pairs
For octahedral structures, there is a plane containing
four electron pairs. Similarly, the fifth and sixth
electron pairs are located above and below this plane.
All positions are equivalent in an octahedron.
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Copyright 1999, PRENTICE HALL Chapter 9 16
The VSEPR ModelMolecules with Expanded Valence Shells
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Copyright 1999, PRENTICE HALL Chapter 9 17
The VSEPR Model
Molecules with Expanded
Valence ShellsTo minimize e--e- repulsion,lone pairs are always placed in
equatorial positions.
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Copyright 1999, PRENTICE HALL Chapter 9 18
The VSEPR ModelMolecules with Expanded Valence Shells
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Copyright 1999, PRENTICE HALL Chapter 9 19
The VSEPR ModelMolecules with More than One Central Atom
In acetic acid, CH3COOH, there are three centralatoms.
We assign the geometry about each central atom
separately.
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Copyright 1999, PRENTICE HALL Chapter 9 20
Polar i ty of MoleculesPolar molecules interact with electric fields.
If the centers of negative and positive charge do notcoincide, then the molecule is polar.
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Copyright 1999, PRENTICE HALL Chapter 9 21
Polar i ty of MoleculesIf two charges, equal in magnitude and opposite in sign,
are separated by a distance r, then a dipole isestablished.
The dipole moment, m, is given bym = Qr
where Qis the magnitude of charge.
Dipole Moments of Polyatomic MoleculesIn a polyatomic molecule, each bond can be a dipole.
The orientation of these individual dipole moments
determines whether the molecule has an overall dipole
moment.
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Copyright 1999, PRENTICE HALL Chapter 9 22
Polar i ty of MoleculesDipole Moments of Polyatomic Molecules
Example: in CO2, each C-O dipole is canceled becausethe molecule is linear. In H2O, the H-O dipoles do not
cancel because the molecule is bent.
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Copyright 1999, PRENTICE HALL Chapter 9 23
Polar i ty of MoleculesDipole Moments of Polyatomic Molecules
It is possible for a molecule with polar bonds to beeither polar or non-polar.
For diatomic molecules:polar bonds always result in an overall dipole moment.
For triatomic molecules:if the molecular geometry is trigonal pyramidal, there is an
overall dipole moment;
if the molecular geometry is trigonal planar and all three
bonds are identical, there is no overall dipole moment;if the molecular geometry is trigonal planar and one or two
bonds are different, there is an overall dipole moment.
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Copyright 1999, PRENTICE HALL Chapter 9 24
Polar i ty of MoleculesDipole Moments of Polyatomic Molecules
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Copyright 1999 PRENTICE HALL Chapter 9 25
Polar i ty of MoleculesDipole Moments of Polyatomic Molecules
For tetraatomic molecules with identical bonds:if the molecular geometry is tetrahedral or square planar,
then the molecules are nonpolar;
if the molecular geometry is see-saw, the molecule is polar.
For tetraatomic molecules in which one, two, or threebonds are different:
the molecule is polar.
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