AP Chemistry 108

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AP Chemistry 10/8
CHAPTER 9 Notes Notebooks, homework due tomorrow
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Lewis structures are used to account for the
formulas of covalent compounds, but do not
indicate the shape of molecules The shape and
size of a molecule, together with the strength
and polarity of its bonds, largely determine its
properties
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(a)A tetrahedron is an object with four faces and
four vertices. (b) The geometry of the CCl4
molecule. All C-Cl bonds are the same length and
have the same bond angles (c) A space-filling
model of the molecule, showing relative sizes of
the atoms
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The overall shape of a molecule is determined by
its bond angles and bond lengths Molecules with a
single central atom bonded to 2 or more atoms of
the same type ABn
AB2
BeH2
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The shapes of some simple AB2 and AB3 molecules
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Valence Shell Electron Pair Repulsion
Theory VSEPR Theory States that The best
arrangement of a given number of electron domains
(region where electrons will most likely be
found) is the one that minimizes the repulsions
among them
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VSEPR Theory

• Types of e- Pairs
• Bonding pairs - form bonds
• Lone pairs - nonbonding electrons

Courtesy Christy Johannesson www.nisd.net/communic
ationsarts/pages/chem
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Molecular Shapes
It is very important to consider nonbonded
electron pairs (domains), as these act just like
bonded electron pairs Molecules with nonbonded
domains are sometimes written showing the domains
as E AB2E (nonlinear)
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Five fundamental geometries on which the shapes
of ABn molecules are based
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Molecular Shapes
AB2E2 (Bent)
H2O
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Molecular Shapes
AB3E2 (trigonal planar)
BF3
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Molecular Shapes
AB3E (pyramidal)
NH3
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Molecular Shapes
AB2E3 (linear)
I3
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Molecular Shapes
AB4 (tetrahedral)
CH4
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Molecular Shapes
AB4E2 (square planar)
XeF4
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VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory
• Electron pairs orient themselves in order to
  minimize repulsive forces.

Courtesy Christy Johannesson www.nisd.net/communic
ationsarts/pages/chem
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Valence shell electron pair repulsion (VSEPR)
model
Predict the geometry of the molecule from the
electrostatic repulsions between the electron
(bonding and nonbonding) pairs.
AB2
2
0
10.1
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10.1
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VSEPR
AB2
2
0
linear
linear
AB3
3
0
10.1
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10.1
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VSEPR
AB2
2
0
linear
linear
AB4
4
0
10.1
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10.1
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VSEPR
AB2
2
0
linear
linear
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
10.1
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10.1
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VSEPR
AB2
2
0
linear
linear
AB4
4
0
tetrahedral
tetrahedral
AB6
6
0
10.1
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10.1
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VSEPR
trigonal planar
trigonal planar
AB3
3
0
AB2E
2
1
10.1
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VSEPR
AB4
4
0
tetrahedral
tetrahedral
AB3E
3
1
10.1
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VSEPR
AB4
4
0
tetrahedral
tetrahedral
AB2E2
2
2
10.1
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VSEPR
trigonal bipyramidal
trigonal bipyramidal
AB5
5
0
AB4E
4
1
10.1
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VSEPR
trigonal bipyramidal
trigonal bipyramidal
AB5
5
0
AB3E2
3
2
10.1
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VSEPR
trigonal bipyramidal
trigonal bipyramidal
AB5
5
0
AB2E3
2
3
10.1
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VSEPR
AB5E
5
1
10.1
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VSEPR
AB4E2
4
2
10.1
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Predicting Molecular Geometry

• Draw Lewis structure for molecule.
• Count number of lone pairs on the central atom
  and number of atoms bonded to the central atom.
• Use VSEPR to predict the geometry of the molecule.

AB4E
AB2E
distorted tetrahedron
bent
10.1
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Dipole Moments and Polar Molecules
electron rich region
electron poor region
m Q x r
Q is the charge
r is the distance between charges
1 D 3.36 x 10-30 C m
10.2
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10.2
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10.2
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dipole moment polar molecule
dipole moment polar molecule
no dipole moment nonpolar molecule
no dipole moment nonpolar molecule
10.2
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10.2
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Dipoles (polar molecules) and Microwaves
10.2
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Sharing of two electrons between the two atoms.
Valence bond theory bonds are formed by sharing
of e- from overlapping atomic orbitals.
10.3
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10.4
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Change in electron density as two hydrogen atoms
approach each other.
10.3
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Valence Bond Theory and NH3
N 1s22s22p3
3 H 1s1
If use the 3 2p orbitals predict 900
Actual H-N-H bond angle is 107.30
10.4
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Hybridization mixing of two or more atomic
orbitals to form a new set of hybrid orbitals.

• Mix at least 2 nonequivalent atomic orbitals
  (e.g. s and p). Hybrid orbitals have very
  different shape from original atomic orbitals.
• Number of hybrid orbitals is equal to number of
  pure atomic orbitals used in the hybridization
  process.
• Covalent bonds are formed by
• Overlap of hybrid orbitals with atomic orbitals
• Overlap of hybrid orbitals with other hybrid
  orbitals

10.4
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10.4
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10.4
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10.4
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Formation of sp Hybrid Orbitals
10.4
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Formation of sp2 Hybrid Orbitals
10.4
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Count the number of lone pairs AND the number of
atoms bonded to the central atom
of Lone Pairs of Bonded Atoms
Hybridization
Examples
2
sp
BeCl2
3
sp2
BF3
4
sp3
CH4, NH3, H2O
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sp3d
PCl5
6
sp3d2
SF6
10.4
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10.5
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10.5
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10.5
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10.5
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10.5
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10.5
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Sigma (s) and Pi Bonds (p)
1 sigma bond
Single bond
1 sigma bond and 1 pi bond
Double bond
Triple bond
1 sigma bond and 2 pi bonds
s bonds 6
1 7
p bonds 1
10.5
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No unpaired e-
Should be diamagnetic
Molecular orbital theory bonds are formed from
interaction of atomic orbitals to form molecular
orbitals.
10.6
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Energy levels of bonding and antibonding
molecular orbitals in hydrogen (H2).
A bonding molecular orbital has lower energy and
greater stability than the atomic orbitals from
which it was formed.
An antibonding molecular orbital has higher
energy and lower stability than the atomic
orbitals from which it was formed.
10.6
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10.6
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10.6
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10.6
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• Molecular Orbital (MO) Configurations
• The number of molecular orbitals (MOs) formed is
  always equal to the number of atomic orbitals
  combined.
• The more stable the bonding MO, the less stable
  the corresponding antibonding MO.
• The filling of MOs proceeds from low to high
  energies.
• Each MO can accommodate up to two electrons.
• Use Hunds rule when adding electrons to MOs of
  the same energy.
• The number of electrons in the MOs is equal to
  the sum of all the electrons on the bonding atoms.

10.7
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bond order
½
1
0
½
10.7
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10.7
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Delocalized molecular orbitals are not confined
between two adjacent bonding atoms, but actually
extend over three or more atoms.
10.8
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Electron density above and below the plane of the
benzene molecule.
10.8