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Chapter 10Chemical
Bonding II
Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro
Tro, Chemistry: A Molecular Approach 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
Tro, Chemistry: A Molecular Approach 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
O N O ••
••
••
••
•••• there are 3 e- groups on N1 lone pair1 single bond1 double bond (counted as 1 group)
Tro, Chemistry: A Molecular Approach 7
5 Basic Molecular Geometries
• 5 arrangements of e- groups
• for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same
•••• •• ••••••••
•• ••O S O O S O•••••• ••••
••••
••••
Tro, Chemistry: A Molecular Approach 8
2 e- Groups: Linear Geometry
• occupy positions opposite, around the central atomlinear geometry - bond angle is 180°
e.g. CO2
ClBeCl
O C O
Tro, Chemistry: A Molecular Approach 9
3 e- Groups: Trigonal Geometry
• occupy triangular positionstrigonal planar geometry - bond angle is 120°
e.g. BF3
F
F B F
Tro, Chemistry: A Molecular Approach 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.
Tro, Chemistry: A Molecular Approach 11
4 e- Groups: Tetrahedral Geometry
• occupy tetrahedron positions around the central atomtetrahedral geometry - bond angle is 109.5°
e.g. CH4
Tro, Chemistry: A Molecular Approach 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
P
ClCl
ClCl
Cl••
•• •
•••
•••
•
••••
••
••
••••
••
••
••
Tro, Chemistry: A Molecular Approach 13
6 e- Groups: Octahedral Geometry
• occupy positions in the shape of two square-base pyramids that are base-to-baseoctahedral geometry
e.g. SF6
S
F F
FF
F
F
••
•• •
•••
••
••
••
••
••••
••
••
••••
••
••
••••
Tro, Chemistry: A Molecular Approach 16
Effect of Lone Pairs: Derivative Shapes
• the molecule’s 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
Tro, Chemistry: A Molecular Approach 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 < 120°e.g. SO2
O S O
O S O
O S O
Tro, Chemistry: A Molecular Approach 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 pair
trigonal pyramidal shape – bond angle is 107 °e.g. NH3
Tro, Chemistry: A Molecular Approach 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°
Tro, Chemistry: A Molecular Approach 22
Tetrahedral-Bent Shape
Tro, Chemistry: A Molecular Approach 24
Tro, Chemistry: A Molecular Approach 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 < 120°
• the bond angles between axial and equatorial positions is < 90° linear = 180° axial-to-axial
Tro, Chemistry: A Molecular Approach 26
Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System
Tro, Chemistry: A Molecular Approach 27
See-Saw Shape
F S F
F
F
•••
•
••
••
••
••
••
••
••
••
••
••
••
Tro, Chemistry: A Molecular Approach 28
T-Shape
Tro, Chemistry: A Molecular Approach 29
Linear Shape
Tro, Chemistry: A Molecular Approach 30
Br
FF
FF
F••
•• •
•••
•••
•
••••
••
••
••••
••
••
••
••
• 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°
6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry
Tro, Chemistry: A Molecular Approach 31
F Xe F
F
F
•••
•
••
••
••
••
••
••
••
••
••
••
••
••
• 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°
6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry
Tro, Chemistry: A Molecular Approach 32
Tro, Chemistry: A Molecular Approach 33
Predicting the Shapes Around Central Atoms
1) Draw the Lewis Structure2) Determine the Number of Electron Groups around the Central
Atom3) 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
Tro, Chemistry: A Molecular Approach 36
Practice – Predict the Molecular Geometry and Bond Angles in ClO2F (Chloryl Fluoride)
Tro, Chemistry: A Molecular Approach 37
Practice – Predict the Molecular Geometry and Bond Angles in ClO2F
Cl = 7e─
O2 = 2(6e─) = 12e─
F = 7e─
Total = 26e─
4 Electron Groups on Cl
3 Bonding Groups1 Lone Pair
Shape = Trigonal Pyramidal
Bond AnglesO-Cl-O < 109.5°O-Cl-F < 109.5°
Cl Least Electronegative
Cl Is Central Atom
O Cl
O
F
••
••
•• •
•••
••
••
••
••
••
Tro, Chemistry: A Molecular Approach 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
Tro, Chemistry: A Molecular Approach 39
Tro, Chemistry: A Molecular Approach 40
SF6
S
F
F
F
F F
F
S
F F
FF
F
F
••
•• •
•••
••
••
••
••
••••
••
••
••••
••
••
••••
Tro, Chemistry: A Molecular Approach 41
Tro, Chemistry: A Molecular Approach 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 sequence
e.g. acetic acid
H|
HOCCH|||
OH
shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
Tro, Chemistry: A Molecular Approach 43
Describing the Geometryof Methanol
Tro, Chemistry: A Molecular Approach 44
Describing the Geometryof Glycine
Tro, Chemistry: A Molecular Approach 45
Practice – Predict the Molecular Geometries in H3BO3
46
Practice – Predict the Molecular Geometries in H3BO3
B = 3e─
O3 = 3(6e─) = 18e─
H3 = 3(1e─) = 3e─
Total = 24e─
3 Electron Groups on B
B has3 Bonding Groups0 Lone Pairs
Shape on B = Trigonal Planar
B Least Electronegative
B Is Central Atom
oxyacid, so H attached to O
O B
O
OH H
H••
••
••
••
••
•• 4 Electron Groups on O
O has2 Bonding Groups2 Lone Pairs
Shape on O = Bent
Tro, Chemistry: A Molecular Approach 47
Practice – Predict the Molecular Geometries in C2H4
48
Practice – Predict the Molecular Geometries in C2H4
C = 2(4e─) = 8e ─
H = 4(1e─) = 4e─
Total = 12e─
3 Electron Groups on C
Shape on each C = Trigonal Planar
0 Lone Pairs
Tro, Chemistry: A Molecular Approach 49
Practice – Predict the Molecular Geometries in CH3OCH3
50
Practice – Predict the Molecular Geometries in Dimethyl Ether (CH3OCH3)
C = 2(4e─) = 8e ─
H = 6(1e─) = 6e─
O = 6(1e─) = 6e─
Total = 20e─
4 Electron Groups on C
Shape on each C = Tetrahedral
2 Lone Pairs on O
Shape on O = Bent
Tro, Chemistry: A Molecular Approach 51
Reminder about Eletronegativity!
• Electronegativity, is a chemical property that describes the tendency of an atom to e- towards itself
Tro, Chemistry: A Molecular Approach 52
Polarity of Molecules
• 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
Tro, Chemistry: A Molecular Approach 53
Molecule Polarity
The H-Cl bond is polarBonding e- are pulled toward the Cl end of the molecule
Net result is a polar molecule.
Tro, Chemistry: A Molecular Approach 54
Vector Addition
Tro, Chemistry: A Molecular Approach 55
Tro, Chemistry: A Molecular Approach 56
Molecule Polarity
The O-C bond is polarThe bonding e- are pulled equally toward both O’sSymmetrical molecule
Net result is a nonpolar molecule
Tro, Chemistry: A Molecular Approach 57
Molecule Polarity
The H-O bond is polarBoth sets of bonding e- are pulled toward the O
Net result is a polar molecule
Tro, Chemistry: A Molecular Approach 58
Molecule Polarity
Tro, Chemistry: A Molecular Approach 59
Molecule Polarity
The H-N bond is polarAll the sets of bonding electrons are pulled toward the NNot symmetrical
Net result is a polar molecule
Tro, Chemistry: A Molecular Approach 60
Molecule Polarity
The C-H bond is polarFour equal dipoles cancel each other out due to symmetry
Net result is a non-polar molecule
Tro, Chemistry: A Molecular Approach 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 waterand ionic compounds as well
Tro, Chemistry: A Molecular Approach 62
Molecular Polarity Affects Solubility in Water
• Oil and water do not mix!
Mutual attraction causes polar molecules to clump together
• 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…
Unique Properties
Tro, Chemistry: A Molecular Approach 64
Molecular Polarity Affects Solubility in Water
• some molecules have both polar and nonpolar partse.g. soap
Tro, Chemistry: A Molecular Approach 65
Practice - Decide Whether the Following Are Polar
O N Cl ••
••
••
••
••••
O S
O
O
••
••
•• •
•••••
••
••ENO = 3.5N = 3.0Cl = 3.0S = 2.5
Tro, Chemistry: A Molecular Approach 66
Practice - Decide Whether the Following Are Polar
polarnonpolar
1) polar bonds, N-O2) asymmetrical shape 1) polar bonds, all S-O
2) symmetrical shape
O N Cl ••
••
••
••
••••
O S
O
O
••
••
•• •
•••••
••
••
TrigonalBent Trigonal
PlanarCl
N
O
3.0
3.0
3.5
O
O
OS
3.5
3.5 3.52.5