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Chapter 10 Chemical Bonding II

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Title: Chapter 10 Chemical Bonding II


1
Chapter 10Chemical Bonding II
Chemistry A Molecular Approach, 1st Ed.Nivaldo
Tro
2
Structure Determines Properties!
  • properties of molecular substances depend on
    structures
  • the structure includes many factors, including
  • skeletal structure
  • Bonding - ionic, polar covalent, or covalent
  • Shape
  • bonding theory should allow you to predict the
    shapes of molecules

3
Molecular Geometry
  • Molecules are 3-D
  • Describe molecular shape using geometric terms
  • Geometry has characteristic angles that we call
    bond angles

4
Using Lewis Theory to PredictMolecular Shapes
  • Lewis theory - regions of e- in an atom based on
    placing shared pairs and unshared pairs of
    valence e-
  • predicts the shapes of molecules based on
    negatively charged regions which repel

5
VSEPR Theory
  • e- groups (lone pairs and bonds) are most stable
    when they are as far apart as possible valence
    shell electron pair repulsion theory
  • Maximum separation
  • the resulting geometric arrangement will allow us
    to predict the shapes and bond angles in the
    molecule
  • 3-D representation

6
Electron Groups
  • the Lewis structure predicts the arrangement of
    valence e- around the central atom(s)
  • each lone pair of e- constitutes one e- group on
    a central atom
  • each type of bond constitutes one electron group
    on a central atom
  • e.g. NO2

there are 3 e- groups on N 1 lone pair 1 single
bond 1 double bond (counted as 1 group)
7
5 Basic Molecular Geometries
  • 5 arrangements of e- groups
  • for molecules that exhibit resonance, it doesnt
    matter which resonance form you use the
    molecular geometry will be the same

8
2 e- Groups Linear Geometry
  • occupy positions opposite, around the central
    atom
  • linear geometry - bond angle is 180
  • e.g. CO2

9
3 e- Groups Trigonal Geometry
  • occupy triangular positions
  • trigonal planar geometry - bond angle is 120
  • e.g. BF3

10
Not Quite Perfect Geometry
3 e groups around central atom why not 120 ?
Because the bonds are not identical, the observed
angles are slightly different from ideal.
11
4 e- Groups Tetrahedral Geometry
  • occupy tetrahedron positions around the central
    atom
  • tetrahedral geometry - bond angle is 109.5
  • e.g. CH4

12
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

13
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

14
The Effect of Lone Pairs
  • lone pair groups occupy more space on the
    central atom
  • because their e- density is exclusively on the
    central atom rather than shared like bonding
    electron groups
  • relative sizes of repulsive force interactions
    is
  • Lone Pair Lone Pair gt Lone Pair Bonding Pair
    gt Bonding Pair Bonding Pair
  • this effects the bond angles, making them smaller
    than expected

15
Effect of Lone Pairs
The bonding electrons are shared by two atoms, so
some of the negative charge is removed from the
central atom.
The nonbonding electrons are localized on the
central atom, so area of negative charge takes
more space.
16
Effect of Lone Pairs Derivative Shapes
  • the molecules shape will be one of basic
    molecular geometries if all the e- groups are
    bonds and all the bonds are equivalent
  • molecules with lone pairs or different kinds of
    surrounding atoms will have distorted bond angles
    and different bond lengths, but the shape will be
    a derivative of one of the basic shapes

17
3 e- Groups with Lone PairsDerivative of
Trigonal Geometry
  • when there are 3 e- groups around central atom,
    and 1 of them is a lone pairtrigonal planar -
    bent shape - bond angle lt 120e.g. SO2

18
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 pairtrigonal pyramidal
    shape bond angle is 107 e.g. NH3

19
Bond Angle Distortion from Lone Pairs
20
HW Which species has the smaller bond angle,
Perchlorate (ClO4-) or Chlorate (ClO3-)?
21
4 e- Groups with Lone Pairs Derivatives of
Tetrahedral Geometry
  • when there are 4 e- groups around the central
    atom, and 2 are lone pairstetrahedral-bent
    shapee.g. H2O
  • it looks similar to the trigonal planar-bent
    shape, except the angles are smaller

104.5
22
Tetrahedral-Bent Shape
23
Bond Angle Distortion from Lone Pairs
24
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25
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 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
    lt 120
  • the bond angles between axial and equatorial
    positions is lt 90
  • linear 180 axial-to-axial

26
Replacing Atoms with Lone Pairsin the Trigonal
Bipyramid System
27
See-Saw Shape
28
T-Shape
29
Linear Shape
30
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 lt 90

31
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

32
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33
Predicting the Shapes Around Central Atoms
  • Draw the Lewis Structure
  • Determine the Number of Electron Groups around
    the Central Atom
  • Classify Each Electron Group as Bonding or Lone
    pair, and Count each type
  • remember, multiple bonds count as 1 group
  • Use Table 10.1 to Determine the Shape and Bond
    Angles

34
Practice Predict the Molecular Geometry and
Bond Angles in SiF5-
35
Practice Predict the Molecular Geometry and
Bond Angles in SiF5-
Si Least Electronegative
5 Electron Groups on Si
Si Is Central Atom
5 Bonding Groups 0 Lone Pairs
Si 4e- F5 5(7e-) 35e- (-) 1e- total 40e-
Shape Trigonal Bipyramid
Bond Angles Feq-Si-Feq 120 Feq-Si-Fax 90
36
Practice Predict the Molecular Geometry and
Bond Angles in ClO2F (Chloryl Fluoride)
37
Practice Predict the Molecular Geometry and
Bond Angles in ClO2F
Cl Least Electronegative
4 Electron Groups on Cl
Cl Is Central Atom
3 Bonding Groups 1 Lone Pair
Cl 7e- O2 2(6e-) 12e- F 7e- Total 26e-
Shape Trigonal Pyramidal
Bond Angles O-Cl-O lt 109.5 O-Cl-F lt 109.5
38
Representing 3-Dimensional Shapes on a
2-Dimensional Surface
  • one of the problems with drawing molecules is
    trying to show their 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

39
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40
SF6
41
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42
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 sequencee.g. acetic acid

shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
43
Describing the Geometryof Methanol
44
Describing the Geometryof Glycine
45
Practice Predict the Molecular Geometries in
H3BO3
46
Practice Predict the Molecular Geometries in
H3BO3
oxyacid, so H attached to O
3 Electron Groups on B
4 Electron Groups on O
B Least Electronegative
B has 3 Bonding Groups 0 Lone Pairs
O has 2 Bonding Groups 2 Lone Pairs
B Is Central Atom
B 3e- O3 3(6e-) 18e- H3 3(1e-)
3e- Total 24e-
Shape on B Trigonal Planar
Shape on O Bent
47
Practice Predict the Molecular Geometries in
C2H4
48
Practice Predict the Molecular Geometries in
C2H4
3 Electron Groups on C
C 2(4e-) 8e - H 4(1e-) 4e- Total 12e-
0 Lone Pairs
Shape on each C Trigonal Planar
49
Practice Predict the Molecular Geometries in
CH3OCH3
50
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
51
Reminder about Eletronegativity!
  • Electronegativity, is a chemical property that
    describes the tendency of an atom to e- towards
    itself

52
Polarity of Molecules
  • in order for a molecule to be polar it must
  • have polar bonds
  • electronegativity difference
  • dipole moments (charge x distance)
  • 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

53
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.
54
Vector Addition
55
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56
Molecule Polarity
The O-C bond is polar The bonding e- are pulled
equally toward both Os Symmetrical molecule Net
result is a nonpolar molecule
57
Molecule Polarity
The H-O bond is polarBoth sets of bonding e- are
pulled toward the O Net result is a polar
molecule
58
Molecule Polarity
59
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
60
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
61
Molecular Polarity Affects Solubility in Water
  • polar molecules are attracted to other polar
    molecules
  • since water is a polar molecule, other polar
    molecules dissolve well in water
  • and ionic compounds as well

62
Molecular Polarity Affects Solubility in Water
  • Oil and water do not mix!

Mutual attraction causes polar molecules to clump
together
63
Unique Properties
  • Water shrinks on melting (ice floats 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

64
Molecular Polarity Affects Solubility in Water
  • some molecules have both polar and nonpolar
    partse.g. soap

65
Practice - Decide Whether the Following Are Polar
EN O 3.5 N 3.0 Cl 3.0 S 2.5
66
Practice - Decide Whether the Following Are Polar
Trigonal Bent
Trigonal Planar
2.5
1) polar bonds, N-O 2) asymmetrical shape
1) polar bonds, all S-O 2) symmetrical shape
polar
nonpolar
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