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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

11

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 +