Transcript Chapter 10 Chemical Bonding II

Chemistry II
Chapter 10
Chemical Bonding II
Chemical Bonding II
Molecular Shapes
Chemical Bonding II
VSEPR Theory
• e- groups (lone pairs and bonds) are most stable when they are as
far apart as possible –
v________ s____ e_______ p_____ r_________ theory
• Maximum separation
• 3-D representation allows us to predict the shapes and bond angles
in the molecule
Chemical Bonding II
VSEPR Theory
e.g. draw the 2 possible Lewis dot structures for NO2- and
discuss the behavior of the associated e- groups
there are _____ e- groups on N
____ lone pair
____ single bond
____ double bond (counted as 1 group)
Chemical Bonding II
2 e- Groups: Linear Geometry
• 5 basic shapes of molecules:
linear,
trigonal planar,
tetrahedral,
trigonal bipyramidal,
octahedral
Chemical Bonding II
2 e- Groups: Linear Geometry
• Draw both 2-dimensional and 3-dimensional pictures of the
molecules in the following slides
Chemical Bonding II
2 e- Groups: Linear Geometry
• occupy positions opposite, around the central atom
linear geometry - bond angle is ________
e.g. CO2
Chemical Bonding II
3 e- Groups: Trigonal Geometry
• occupy triangular positions
trigonal planar geometry - bond angle is __________
e.g. BF3
Chemical Bonding II
3 e- Groups: Trigonal Geometry
e.g. Formaldehyde, CH2O
3 e– groups around central atom
– why not 120° ?
Chemical Bonding II
4 e- Groups: Tetrahedral Geometry
• occupy tetrahedron positions around the central atom
tetrahedral geometry - bond angle is ________
e.g. CH4
Chemical Bonding II
5 e- Groups: Trigonal Bipyramidal Geometry
• occupy positions in the shape of a two tetrahedra that are base-to-base
trigonal bipyramidal geometry
e.g. PCl5
Chemical Bonding II
6 e- Groups: Octahedral Geometry
• occupy positions in the shape of two square-base pyramids that are
base-to-base
octahedral geometry
e.g. SF6
Chemical Bonding II
3 e- Groups with Lone Pairs: Derivative of Trigonal Geometry
• when there are 3 e- groups around central atom, and 1 of them is a
lone pair
trigonal planar - bent shape - bond angle < 120°
e.g. SO2
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O
S
O
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O
S
O
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O
S
Chemical Bonding II
4 e- Groups with Lone Pairs : Derivatives of Tetrahedral Geometry
• when there are 4 e- groups around the central atom, and 1 is a
lone pair
trigonal pyramidal shape – bond angle is 107 °
e.g. NH3
Chemical Bonding II
4 e- Groups with Lone Pairs: Derivatives of Tetrahedral Geometry
• when there are 4 e- groups around the central atom, and 2 are lone
pairs
tetrahedral-bent shape – bond angle is 104.5 °
e.g. H2O
Chemical Bonding II
Tetrahedral-Bent Shape
Chemical Bonding II
5 e- Groups with Lone Pairs
Derivatives of Trigonal Bipyramidal Geometry
• when there are 5 e- groups around the central atom, and some are lone
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pairs, they will occupy the equatorial positions because there is more room
when there are 5 e- groups around the central atom, and 1 is a lone pair, the
result is called see-saw shape
 aka distorted tetrahedron
when there are 5 e- groups around the central atom, and 2 are lone pairs,
the result is called T-shaped
when there are 5 e- groups around the central atom, and 3 are lone pairs,
the result is called a linear shape
the bond angles between equatorial positions is < 120°
the bond angles between axial and equatorial positions is < 90°
 linear = 180° axial-to-axial
Chemical Bonding II
Replacing Atoms with Lone Pairs
in the Trigonal Bipyramid System
Chemical Bonding II
See-Saw Shape
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•F
S
F
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Chemical Bonding II
T-Shape
Chemical Bonding II
Linear Shape
Tro, Chemistry: A Molecular Approach
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Chemical Bonding II
6 e- Groups with Lone Pairs: Derivatives of Octahedral Geometry
• when there are 6 e- groups around the central atom, and 1 is a lone pair,
the result is called a square pyramid shape
 the bond angles between axial and equatorial positions is < 90°
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•• •• F •• ••
•F
F•
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Br ••
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F•
•F
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6 e- Groups with Lone Pairs
Derivatives of Octahedral Geometry
• when there are 6 e- groups around the central atom, and 2 are lone
pairs, the result is called a square planar shape
 the bond angles between equatorial positions is 90°
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•F•
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•F
Xe
F
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Chemical Bonding II
Predicting the Shapes
Around Central Atoms
1. Draw the Lewis Structure
2. Determine the Number of Electron Groups around the Central Atom
3. Classify Each Electron Group as Bonding or Lone pair, and Count
each type
remember, multiple bonds count as 1 group
4. Use Table 10.1 to Determine the Shape and Bond Angles
Practice – Predict the Molecular Geometry
and Bond Angles in ClO2F (Chloryl Fluoride)
Practice – Predict the Molecular Geometry
and Bond Angles in ClO2F
Cl Least Electronegative
Cl Is Central Atom
Cl = 7e─
O2 = 2(6e─) = 12e─
F = 7e─
Total = 26e─
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•O
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•O•
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Cl
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4 Electron Groups on Cl
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F ••
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3 Bonding Groups
1 Lone Pair
Shape = Trigonal Pyramidal
Bond Angles
O-Cl-O < 109.5°
O-Cl-F < 109.5°
Chemical Bonding II
Representing 3-Dimensional Shapes on a 2-Dimensional Surface
• one of the problems with drawing molecules is trying to show their
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dimensionality
by convention, the central atom is put in the plane of the paper
put as many other atoms as possible in the same plane and indicate
with a straight line
for atoms in front of the plane, use a solid wedge
for atoms behind the plane, use a hashed wedge
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•F
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•F
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S
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F ••
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SF6
F
F
F
S
F
F
F
Multiple Central Atoms
• many molecules have larger structures with many interior atoms
• we can think of them as having multiple central atoms
• when this occurs, we describe the shape around each central atom in
sequence
e.g. acetic acid
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H O 
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shape around center C is trigonal planarH  C  C  O  H
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shape around right O is tetrahedral-bent
H
shape around left C is tetrahedral
Describing the Geometry
of Methanol
Describing the Geometry
of Glycine
Practice – Predict the Molecular Geometries
in H3BO3
Tro, Chemistry: A Molecular Approach
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Practice – Predict the Molecular Geometries
in H3BO3
oxyacid, so H attached to O
B Least Electronegative
B Is Central Atom
B = 3e─
O3 = 3(6e─) = 18e─
H3 = 3(1e─) = 3e─
Total = 24e─
H
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O
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B
H
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O
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34 Electron
Electron Groups
Groups on
on B
O
H
O has
B
has
3 Bonding Groups
2
0 Lone Pairs
2
Shape on B = Trigonal Plana
Shape on O = Bent
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Practice – Predict the Molecular Geometries
in C2H4
Tro, Chemistry: A Molecular Approach
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Practice – Predict the Molecular Geometries
in C2H4
C = 2(4e─) = 8e ─
H = 4(1e─) = 4e─
Total = 12e─
3 Electron Groups on C
0 Lone Pairs
Shape on each C =
Trigonal Planar
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Practice – Predict the Molecular Geometries
in CH3OCH3
Practice – Predict the Molecular Geometries
in Dimethyl Ether (CH3OCH3)
4 Electron Groups on C
C = 2(4e─) = 8e ─
H = 6(1e─) = 6e─
O = 6(1e─) = 6e─
Total = 20e─
2 Lone Pairs on O
Shape on each C = Tetrahedral
Shape on O = Bent
Reminder about Eletronegativity!
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Electronegativity, is a chemical property that describes the
tendency of an atom to e- towards itself
Polarity of Molecules
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in order for a molecule to be polar it must
1) have polar bonds
 electronegativity difference
 dipole moments (charge x distance)
2) have an unsymmetrical shape
 vector addition
polarity affects the intermolecular forces of attraction
 therefore boiling points and solubilities
 like dissolves like
nonbonding pairs strongly affect molecular polarity
Molecule Polarity
The H-Cl bond is polar
Bonding e- are pulled toward the Cl end of the molecule
Net result is a polar molecule.
Vector Addition
Molecule Polarity
The O-C bond is polar
The bonding e- are pulled equally toward both O’s
Symmetrical molecule
Net result is a nonpolar molecule
Molecule Polarity
The H-O bond is polar
Both sets of bonding e- are pulled toward the O
Net result is a polar molecule
Molecule Polarity
Molecule Polarity
The H-N bond is polar
All the sets of bonding electrons are pulled toward the N
Not symmetrical
Net result is a polar molecule
Molecule Polarity
The C-H bond is polar
Four equal dipoles cancel each other out due to symmetry
Net result is a non-polar molecule
Molecular Polarity Affects
Solubility in Water
• polar molecules are attracted to other
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polar molecules
since water is a polar molecule, other
polar molecules dissolve well in water
 and ionic compounds as well
Molecular Polarity Affects
Solubility in Water
• Oil and water do not mix!
Mutual attraction
causes polar
molecules to clump
together
Unique Properties
• Water shrinks on melting (ice floats
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on water)
Unusually high melting point
Unusually high boiling point
Unusually high surface tension
Unusually high viscosity
Unusually high heat of vaporization
Unusually high specific heat
capacity
And more…
Molecular Polarity Affects
Solubility in Water
• some molecules have both polar and nonpolar parts
e.g. soap
Practice - Decide Whether the Following Are
Polar
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EN
O = 3.5
N = 3.0
Cl = 3.0
S = 2.5
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•O
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N
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Cl ••
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•O
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S
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O ••
Practice - Decide Whether the Following Are
Polar
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N
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•O
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Cl ••
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Trigonal
Bent
3.0
3.0
Cl
N
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3.5
polar
S
S
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O ••
Trigonal
Planar
3.5O
O
1) polar bonds, N-O
2) asymmetrical shape
•O•
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2.5
O3.5
1) polar bonds, all S-O
2) symmetrical shape
nonpolar
3.5 O