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Chapter 10
Chemical Bonding II:Molecular Geometry
VValence alence SShell hell EElectron lectron PPair air RRepulsion theory.epulsion theory.
Central atom without lone pair
Central atom with lone pair
Molecular geometry • General shape of a molecule• Three-dimensional arrangement of atoms around the central atom
Predict the geometry of the molecules and ions based on the electrostatic repulsions between the electron (bonding and nonbonding/lone) pairs.
VSEPR model
In Lewis structures there are two types of valence electron pairs: •bonding pairs (shared by atoms in bonds) •nonbonding pairs (also called lone pairs)
Molecule adopts the shape that minimizes the electron pair repulsions.
Valence shell electron pair repulsion (VSEPR) model:
1. Molecule in which central atom has no lone pairs
2. Molecule in which central atom has one/more lone pairs
1. Molecule in which central atom has no lone pairs:
ABxA = central atomB = surrounding atomx = number of surrounding atoms = 2,3,……
AB2 2 0
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
linear linear
B B
Cl ClBe
2 atoms bonded to central atom
0 lone pairs on central atomlinear
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
AB3 3 0 trigonal planar
trigonal planar
trigonal planar
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
AB4 4 0 tetrahedral tetrahedral
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
AB5 5 0 trigonalbipyramidal
trigonalbipyramid
al
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
AB6 6 0 octahedraloctahedral
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AB3
AB2
AB6
AB4
AB5
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2. Molecule in which central atom has one/more lone pairs:
ABxEy A = central atomB = surrounding atomE= lone pairs on the central atom x = number of surrounding atoms = 2,3,……y = number of lone pairs on the central atom = 1,2,……..
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
AB3 3 0 trigonal planar
trigonal planar
AB2E 2 1 trigonal planar bent
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
AB3E 3 1
AB4 4 0 tetrahedral tetrahedral
tetrahedral trigonal pyramidal
AB2E2 2 2 tetrahedral bent
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tetrahedral trigonal pyramidal
bent
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Predicting Molecular Geometry1. Count the number of e- pairs around the central atom
(bonding pairs and lone pairs).
2. Multiple bounds count as one bonding pair
3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and CH4?
SO O
AB2E
bentC
H
H
H H
AB4
tetrahedral
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Intermolecular Forces & Liquids and SolidsChapter 11
Dipole Moments and Polar Molecules
H F
electron rich
region
electron poor
region
Dipole moment () measure the polarity of a bond
Diatomic molecules containing atoms of different elements have dipole moments polar molecules (HCl, CO, NO)
Diatomic molecules containing atoms of the same elements do not have dipole moments nonpolar molecules (H2, O2, F2)
Intermolecular ForcesIntermolecular forces are attractive forces between molecules.Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular•41 kJ to vaporize 1 mole of water (inter)•930 kJ to break all O-H bonds in 1 mole of water (intra)
Generally, intermolecular forces are much weaker than intramolecular forces.
“Measure” of intermolecular force boiling point melting point
Intermolecular Forces1) Dipole-Dipole Forces
3) Hydrogen bonding
2) Dispersion (London) force Van der Waals forces
Relative strength Hydrogen bonding > dipole-dipole forces > dispersion force
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Intermolecular Forces1) Dipole-Dipole ForcesAttractive forces between polar molecules with dipole moments
Orientation of Polar Molecules in a Solid
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Intermolecular Forces2) Hydrogen BondThe hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.
A H…B A H…Aor
A & B are N, O, or F
Intermolecular Forces3) Dispersion/London Forces
Attractive forces that arise as a result of temporary dipoles in atoms/molecules
The weakest intermolecular force.
Every atom and molecule experience dispersion forces
27GreatestOrder
LeastOrder
Phase Changes
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A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.
Typical phase diagram
triple point is the condition which all three phases exist in equilibrium
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Phase Diagram of WaterSlope -
Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water
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At 1 atm,CO2 (s) CO2 (g)
Phase Diagram of Carbon Dioxide
Slope +