Chem Chapter10 LEC
description
Transcript of Chem Chapter10 LEC
Chapter 10Chemical
Bonding II
2008, Prentice Hall
Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro
Roy KennedyMassachusetts Bay Community College
Wellesley Hills, MA
Tro, Chemistry: A Molecular Approach 2
Structure Determines Properties!• properties of molecular substances depend on
the structure of the molecule• the structure includes many factors, including:
the skeletal arrangement of the atomsthe kind of bonding between the atoms
ionic, polar covalent, or covalentthe shape of the molecule
• bonding theory should allow you to predict the shapes of molecules
Tro, Chemistry: A Molecular Approach 3
Molecular Geometry• Molecules are 3-dimensional objects
• We often describe the shape of a molecule with terms that relate to geometric figures
• These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure
• The geometric figures also have characteristic angles that we call bond angles
Tro, Chemistry: A Molecular Approach 4
Using Lewis Theory to PredictMolecular Shapes
• Lewis theory predicts there are regions of electrons in an atom based on placing shared pairs of valence electrons between bonding nuclei and unshared valence electrons located on single nuclei
• this idea can then be extended to predict the shapes of molecules by realizing these regions are all negatively charged and should repel
Tro, Chemistry: A Molecular Approach 5
VSEPR Theory• electron groups around the central atom will be
most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theorysince electrons are negatively charged, they should
be most stable when they are separated as much as possible
• the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule
Tro, Chemistry: A Molecular Approach 6
Tro, Chemistry: A Molecular Approach 7
Electron Groups• the Lewis structure predicts the arrangement of valence
electrons around the central atom(s)
• each lone pair of electrons constitutes one electron group on a central atom
• each bond constitutes one electron group on a central atom regardless of whether it is single, double, or triple
O N O ••
••
••
••
•••• there are 3 electron groups on N1 lone pair1 single bond1 double bond
Tro, Chemistry: A Molecular Approach 8
Molecular Geometries• there are 5 basic arrangements of electron groups
around a central atombased on a maximum of 6 bonding electron groups
though there may be more than 6 on very large atoms, it is very rare
• each of these 5 basic arrangements results in 5 different basic molecular shapes in order for the molecular shape and bond angles to be a
“perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent
• for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same
Tro, Chemistry: A Molecular Approach 9
Linear Geometry• when there are 2 electron groups around the central
atom, they will occupy positions opposite each other around the central atom
• this results in the molecule taking a linear geometry• the bond angle is 180°
ClBeCl
O C O
Tro, Chemistry: A Molecular Approach 10
Linear Geometry
Tro, Chemistry: A Molecular Approach 11
Trigonal Geometry• when there are 3 electron groups around the central
atom, they will occupy positions in the shape of a triangle around the central atom
• this results in the molecule taking a trigonal planar geometry
• the bond angle is 120°
F
F B F
Tro, Chemistry: A Molecular Approach 12
Trigonal Geometry
Tro, Chemistry: A Molecular Approach 13
Not Quite Perfect Geometry
Because the bonds are not identical, the observed angles are slightly different from ideal.
Tro, Chemistry: A Molecular Approach 14
Tro, Chemistry: A Molecular Approach 15
Tetrahedral Geometry• when there are 4 electron groups around the central
atom, they will occupy positions in the shape of a tetrahedron around the central atom
• this results in the molecule taking a tetrahedral geometry
• the bond angle is 109.5°
F
F C F
F
Tro, Chemistry: A Molecular Approach 16
Tetrahedral Geometry
Tro, Chemistry: A Molecular Approach 17
Methane
Tro, Chemistry: A Molecular Approach 18
Trigonal Bipyramidal Geometry• when there are 5 electron groups around the central atom, they
will occupy positions in the shape of a two tetrahedra that are base-to-base with the central atom in the center of the shared bases
• this results in the molecule taking a trigonal bipyramidal geometry
• the positions above and below the central atom are called the axial positions
• the positions in the same base plane as the central atom are called the equatorial positions
• the bond angle between equatorial positions is 120°
• the bond angle between axial and equatorial positions is 90°
Tro, Chemistry: A Molecular Approach 19
Trigonal Bipyramid
Tro, Chemistry: A Molecular Approach 20
Trigonal Bipyramidal Geometry
P
ClCl
ClCl
Cl••
•• •
•••
•••
•
••••
••
••
••••
••
••
••
Tro, Chemistry: A Molecular Approach 21
Tro, Chemistry: A Molecular Approach 22
Octahedral Geometry• when there are 6 electron groups around the central
atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases
• this results in the molecule taking an octahedral geometry it is called octahedral because the geometric figure has 8
sides
• all positions are equivalent• the bond angle is 90°
Tro, Chemistry: A Molecular Approach 23
Octahedral Geometry
Tro, Chemistry: A Molecular Approach 24
Octahedral Geometry
S
F F
FF
F
F
••
•• •
•••
••
••
••
••
••••
••
••
••••
••
••
••••
Tro, Chemistry: A Molecular Approach 25
Tro, Chemistry: A Molecular Approach 26
The Effect of Lone Pairs• lone pair groups “occupy more space” on the central
atombecause their electron 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 > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair • this effects the bond angles, making them smaller
than expected
Tro, Chemistry: A Molecular Approach 27
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.
Tro, Chemistry: A Molecular Approach 28
Derivative Shapes
• the molecule’s shape will be one of basic molecular geometries if all the electron 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 29
Derivative of Trigonal Geometry• when there are 3 electron groups around the central
atom, and 1 of them is a lone pair, the resulting shape of the molecule is called a trigonal planar - bent shape
• the bond angle is < 120°
O S O
O S O
O S O
Tro, Chemistry: A Molecular Approach 30
Derivatives of Tetrahedral Geometry
• when there are 4 electron groups around the central atom, and 1 is a lone pair, the result is called a pyramidal shapebecause it is a triangular-base pyramid with the central
atom at the apex
• when there are 4 electron groups around the central atom, and 2 are lone pairs, the result is called a tetrahedral-bent shape it is planar it looks similar to the trigonal planar-bent shape, except the
angles are smaller
• for both shapes, the bond angle is < 109.5°
Tro, Chemistry: A Molecular Approach 31
Pyramidal Shape
Tro, Chemistry: A Molecular Approach 32
Bond Angle Distortion from Lone Pairs
Tro, Chemistry: A Molecular Approach 33
Tetrahedral-Bent Shape
Tro, Chemistry: A Molecular Approach 34
Pyramidal Shape
F
FAsF
Tro, Chemistry: A Molecular Approach 35
Bond Angle Distortion from Lone Pairs
Tro, Chemistry: A Molecular Approach 36
Tetrahedral-Bent Shape
1
OClO
Tro, Chemistry: A Molecular Approach 37
Derivatives of theTrigonal Bipyramidal Geometry
• when there are 5 electron 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 electron 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 electron groups around the central atom, and 2 are lone pairs, the result is called T-shaped
• when there are 5 electron 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 38
Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System
Tro, Chemistry: A Molecular Approach 39
See-Saw Shape
F S F
F
F
•••
•
••
••
••
••
••
••
••
••
••
••
••
Tro, Chemistry: A Molecular Approach 40
T-Shape
Tro, Chemistry: A Molecular Approach 41
T-Shaped
F Cl F
F
•••
•
••
••
•• ••
••
••
••
••
••
Tro, Chemistry: A Molecular Approach 42
Linear Shape
Tro, Chemistry: A Molecular Approach 43
Derivatives of theOctahedral Geometry
• when there are 6 electron groups around the central atom, and some are lone pairs, each even number lone pair will take a position opposite the previous lone pair
• when there are 6 electron 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°
• when there are 6 electron 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°
Tro, Chemistry: A Molecular Approach 44
Square Pyramidal Shape
Br
FF
FF
F••
•• •
•••
•••
•
••••
••
••
••••
••
••
••
••
Tro, Chemistry: A Molecular Approach 45
Square Planar Shape
F Xe F
F
F
•••
•
••
••
••
••
••
••
••
••
••
••
••
••
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Tro, Chemistry: A Molecular Approach 47
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 48
Practice – Predict the Molecular Geometry and Bond Angles in SiF5
-1
Tro, Chemistry: A Molecular Approach 49
Practice – Predict the Molecular Geometry and Bond Angles in SiF5
─
Si = 4e─
F5 = 5(7e─) = 35e─
(─) = 1e─
total = 40e─
5 Electron Groups on Si
5 Bonding Groups0 Lone Pairs
Shape = Trigonal Bipyramid
Si
FF
FF
F••
•• •
•••
•••
•
••••
••
••
••••
••
••
••
-1
Bond AnglesFeq-Si-Feq = 120°Feq-Si-Fax = 90°
Si Least Electronegative
Si Is Central Atom
Tro, Chemistry: A Molecular Approach 50
Practice – Predict the Molecular Geometry and Bond Angles in ClO2F
Tro, Chemistry: A Molecular Approach 51
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 52
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 53
Tro, Chemistry: A Molecular Approach 54
SF6
S
F
F
F
F F
F
S
F F
FF
F
F
••
•• •
•••
••
••
••
••
••••
••
••
••••
••
••
••••
Tro, Chemistry: A Molecular Approach 55
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
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 56
Describing the Geometryof Methanol
Tro, Chemistry: A Molecular Approach 57
Describing the Geometryof Glycine
Tro, Chemistry: A Molecular Approach 58
Practice – Predict the Molecular Geometries in H3BO3
59
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 = Tetrahedral Bent
Tro, Chemistry: A Molecular Approach 60
Polarity of Molecules
• in order for a molecule to be polar it must1) have polar bonds
electronegativity difference - theory bond dipole moments - measured
2) have an unsymmetrical shape vector addition
• polarity affects the intermolecular forces of attraction therefore boiling points and solubilities
like dissolves like
• nonbonding pairs affect molecular polarity, strong pull in its direction
Tro, Chemistry: A Molecular Approach 61
Molecule Polarity
The H-Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule.
Tro, Chemistry: A Molecular Approach 62
Vector Addition
Tro, Chemistry: A Molecular Approach 63
Tro, Chemistry: A Molecular Approach 64
Molecule Polarity
The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.
Tro, Chemistry: A Molecular Approach 65
Molecule Polarity
The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.
Tro, Chemistry: A Molecular Approach 66
Molecule Polarity
The H-N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule.
Tro, Chemistry: A Molecular Approach 67
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
• some molecules have both polar and nonpolar parts
Tro, Chemistry: A Molecular Approach 68
A Soap MoleculeSodium Stearate
Tro, Chemistry: A Molecular Approach 69
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 70
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
Tro, Chemistry: A Molecular Approach 71
Problems with Lewis Theory• Lewis theory gives good first approximations of
the bond angles in molecules, but usually cannot be used to get the actual angle
• Lewis theory cannot write one correct structure for many molecules where resonance is important
• Lewis theory often does not predict the correct magnetic behavior of moleculese.g., O2 is paramagnetic, though the Lewis structure
predicts it is diamagnetic
Tro, Chemistry: A Molecular Approach 72
Valence Bond Theory
• Linus Pauling and others applied the principles of quantum mechanics to molecules
• they reasoned that bonds between atoms would arise when the orbitals on those atoms interacted to make a bond
• the kind of interaction depends on whether the orbitals align along the axis between the nuclei, or outside the axis
Tro, Chemistry: A Molecular Approach 73
Orbital Interaction• as two atoms approached, the partially filled or
empty valence atomic orbitals on the atoms would interact to form molecular orbitals
• the molecular orbitals would be more stable than the separate atomic orbitals because they would contain paired electrons shared by both atomsthe interaction energy between atomic orbitals is
negative when the interacting atomic orbitals contain a total of 2 electrons
Tro, Chemistry: A Molecular Approach 74
Orbital Diagram for the Formation of H2S
+
↑
H
1s ↑↓ H-S bond
↑
H
1sS↑ ↑ ↑↓↑↓
3s 3p
↑↓ H-S bond
Predicts Bond Angle = 90°Actual Bond Angle = 92°
Tro, Chemistry: A Molecular Approach 75
Valence Bond Theory - Hybridization• one of the issues that arose was that the number of
partially filled or empty atomic orbital did not predict the number of bonds or orientation of bonds C = 2s22px
12py12pz
0 would predict 2 or 3 bonds that are 90° apart, rather than 4 bonds that are 109.5° apart
• to adjust for these inconsistencies, it was postulated that the valence atomic orbitals could hybridize before bonding took placeone hybridization of C is to mix all the 2s and 2p
orbitals to get 4 orbitals that point at the corners of a tetrahedron
Tro, Chemistry: A Molecular Approach 76
Unhybridized C Orbitals Predict the Wrong Bonding & Geometry
Tro, Chemistry: A Molecular Approach 77
Valence Bond TheoryMain Concepts
1. the valence electrons in an atom reside in the quantum mechanical atomic orbitals or hybrid orbitals
2. a chemical bond results when these atomic orbitals overlap and there is a total of 2 electrons in the new molecular orbital
a) the electrons must be spin paired
3. the shape of the molecule is determined by the geometry of the overlapping orbitals
Tro, Chemistry: A Molecular Approach 78
Hybridization• some atoms hybridize their orbitals to maximize
bondinghybridizing is mixing different types of orbitals to
make a new set of degenerate orbitalssp, sp2, sp3, sp3d, sp3d2
more bonds = more full orbitals = more stability
• better explain observed shapes of molecules
• same type of atom can have different hybridization depending on the compoundC = sp, sp2, sp3
Tro, Chemistry: A Molecular Approach 79
Hybrid Orbitals• H cannot hybridize!!
• the number of standard atomic orbitals combined = the number of hybrid orbitals formed
• the number and type of standard atomic orbitals combined determines the shape of the hybrid orbitals
• the particular kind of hybridization that occurs is the one that yields the lowest overall energy for the molecule in other words, you have to know the structure of the
molecule beforehand in order to predict the hybridization
Tro, Chemistry: A Molecular Approach 80
Orbital Diagrams with Hybridization
• place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy
• when bonding, bonds form between hybrid orbitals and bonds form between unhybridized orbitals that are parallel
Tro, Chemistry: A Molecular Approach 81
Carbon Hybridizations
Unhybridized
2s 2p
sp hybridized
2sp
sp2 hybridized
2p
sp3 hybridized
2p
2sp2
2sp3
Tro, Chemistry: A Molecular Approach 82
sp3 Hybridization• atom with 4 areas of electrons
tetrahedral geometry109.5° angles between hybrid orbitals
• atom uses hybrid orbitals for all bonds and lone pairs
H C N H
H
H H
s
•• sp3
s
sp3
Tro, Chemistry: A Molecular Approach 83
sp3 Hybridization of C
Tro, Chemistry: A Molecular Approach 84
Tro, Chemistry: A Molecular Approach 85
sp3 Hybridized AtomsOrbital Diagrams
Unhybridized atom
2s 2p
sp3 hybridized atom
2sp3
C
2s 2p
2sp3
N
Tro, Chemistry: A Molecular Approach 86
Methane Formation with sp3 C
Tro, Chemistry: A Molecular Approach 87
Ammonia Formation with sp3 N
Tro, Chemistry: A Molecular Approach 88
CH3NH2 Orbital Diagram
sp3 C sp3 N
1s H
1s H 1s H 1s H 1s H
Tro, Chemistry: A Molecular Approach 89
Practice - Draw the Orbital Diagram for the sp3 Hybridization of Each Atom
Unhybridized atom
3s 3p
Cl
2s 2p
O
Tro, Chemistry: A Molecular Approach 90
Practice - Draw the Orbital Diagram for the sp3 Hybridization of Each Atom
Unhybridized atom
3s 3p
Cl
2s 2p
O
sp3 hybridized atom
3sp3
2sp3
Tro, Chemistry: A Molecular Approach 91
Types of Bonds• a sigma () bond results when the bonding atomic
orbitals point along the axis connecting the two bonding nucleieither standard atomic orbitals or hybrids
s-to-s, p-to-p, hybrid-to-hybrid, s-to-hybrid, etc.
• a pi () bond results when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nucleibetween unhybridized parallel p orbitals
• the interaction between parallel orbitals is not as strong as between orbitals that point at each other; therefore bonds are stronger than bonds
Tro, Chemistry: A Molecular Approach 92
Tro, Chemistry: A Molecular Approach 93
Tro, Chemistry: A Molecular Approach 94
Bond Rotation• because orbitals that form the bond point
along the internuclear axis, rotation around that bond does not require breaking the interaction between the orbitals
• but the orbitals that form the bond interact above and below the internuclear axis, so rotation around the axis requires the breaking of the interaction between the orbitals
Tro, Chemistry: A Molecular Approach 95
Tro, Chemistry: A Molecular Approach 96
Tro, Chemistry: A Molecular Approach 97
sp2
• atom with 3 areas of electrons trigonal planar system
C = trigonal planar N = trigonal bent O = “linear”
120° bond anglesflat
• atom uses hybrid orbitals for bonds and lone pairs, uses nonhybridized p orbital for bond
H C O H
O ••
sp2s ••
••
••sp2
sp3 s
Tro, Chemistry: A Molecular Approach 98
Tro, Chemistry: A Molecular Approach 99
+
sp2 sp2
Hybrid orbitals overlap to form bondUnhybridized p orbitals overlap to form bond
Tro, Chemistry: A Molecular Approach 100
Tro, Chemistry: A Molecular Approach 101
sp2 Hybridized AtomsOrbital Diagrams
Unhybridized atom
2s 2p
sp2 hybridized atom
2sp2
2p
C3 1
2s 2p
2sp2
2p
N2 1
Tro, Chemistry: A Molecular Approach 102
CH2NH Orbital Diagram
sp2 C
sp2 N
1s H
1s H 1s H
p C p N
Tro, Chemistry: A Molecular Approach 103
Practice - Draw the Orbital Diagram for the sp2 Hybridization of Each Atom. How many and bonds would you expect each to form?
Unhybridized atom
2s 2p
B
2s 2p
O
Tro, Chemistry: A Molecular Approach 104
Practice - Draw the Orbital Diagram for the sp2 Hybridization of Each Atom. How many and bonds would you expect each to form?
Unhybridized atom
2s 2p
B
3 0
2s 2p
O1 1
sp2 hybridized atom
2sp2
2p
2sp2
2p
Tro, Chemistry: A Molecular Approach 105
sp• atom with 2 areas of electrons
linear shape180° bond angle
• atom uses hybrid orbitals for bonds or lone pairs, uses nonhybridized p orbitals for bonds
H C N
sp sps
Tro, Chemistry: A Molecular Approach 106
Tro, Chemistry: A Molecular Approach 107
Tro, Chemistry: A Molecular Approach 108
sp Hybridized AtomsOrbital Diagrams
Unhybridized atom
2s 2p
sp hybridized atom
2sp
2p
C22
2s 2p
2sp
2p
N12
Tro, Chemistry: A Molecular Approach 109
HCN Orbital Diagram
sp C
sp N
1s H
p C p N
Tro, Chemistry: A Molecular Approach 110
sp3d• atom with 5 areas of electrons
around ittrigonal bipyramid shapeSee-Saw, T-Shape, Linear120° & 90° bond angles
• use empty d orbitals from valence shell
• d orbitals can be used to make bonds
••
I
F
FO
OO
••
••••
••••
••
••
••
••
••
••
••
-1
Tro, Chemistry: A Molecular Approach 111
Tro, Chemistry: A Molecular Approach 112
sp3d Hybridized AtomsOrbital Diagrams
Unhybridized atom
3s 3p
sp3d hybridized atom
3sp3d
S
3s 3p
3sp3d
P
3d
3d
(non-hybridizing d orbitals not shown)
Tro, Chemistry: A Molecular Approach 113
sp3d2
• atom with 6 areas of electrons around itoctahedral shapeSquare Pyramid, Square Planar90° bond angles
• use empty d orbitals from valence shell
• d orbitals can be used to make bonds
•• ••
••••
••Br
F
F
F
F
F••••
••
•• ••••
•• ••
••
••
••
Tro, Chemistry: A Molecular Approach 114
Tro, Chemistry: A Molecular Approach 115
sp3d2 Hybridized AtomsOrbital Diagrams
Unhybridized atom sp3d2 hybridized atom
S
3s 3p 3sp3d2
↑↓
3d
↑↓ ↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
I
5s 5p
↑↓
5d
↑↓ ↑↓ ↑
5sp3d2
↑↓ ↑ ↑ ↑ ↑ ↑
(non-hybridizing d orbitals not shown)
Tro, Chemistry: A Molecular Approach 116
Tro, Chemistry: A Molecular Approach 117
Example - Predict the Hybridization of All the Atoms in H3BO3
O B
O
OH H
H••
••
••
••
••
••
H = can’t hybridizeB = 3 electron groups = sp2
O = 4 electron groups = sp3
Tro, Chemistry: A Molecular Approach 118
Practice - Predict the Hybridization and Bonding Scheme of All the Atoms in NClO
O N Cl ••
••
••
••
••••
Tro, Chemistry: A Molecular Approach 119
Practice - Predict the Hybridization and Bonding Scheme of All the Atoms in NClO
O N Cl ••
••
••
••
••••
N = 3 electron groups = sp2
O = 3 electron groups = sp2
Cl = 4 electron groups = sp3
Tro, Chemistry: A Molecular Approach 120
Predicting Hybridization and Bonding Scheme
1) Start by drawing the Lewis Structure2) Use VSEPR Theory to predict the electron group
geometry around each central atom3) Use Table 10.3 to select the hybridization scheme
that matches the electron group geometry4) Sketch the atomic and hybrid orbitals on the atoms in
the molecule, showing overlap of the appropriate orbitals
5) Label the bonds as or
Tro, Chemistry: A Molecular Approach 121
Ex 10.8 – Predict the hybridization and bonding scheme for CH3CHO
Draw the Lewis Structure
Predict the electron group geometry around inside atoms
C1 = 4 electron areas
C1= tetrahedral
C2 = 3 electron areas
C2 = trigonal planar
C 1H
H
H
C 2 H
O
Tro, Chemistry: A Molecular Approach 122
Ex 10.8 – Predict the hybridization and bonding scheme for CH3CHO
Determine the hybridization of the interior atoms
C1 = tetrahedral
C1 = sp3
C2 = trigonal planar
C2 = sp2
Sketch the molecule and orbitals
C 1H
H
H
C 2 H
O
Tro, Chemistry: A Molecular Approach 123
Ex 10.8 – Predict the hybridization and bonding scheme for CH3CHO
Label the bonds
C 1H
H
H
C 2 H
O
Tro, Chemistry: A Molecular Approach 124
Problems with Valence Bond Theory
• VB theory predicts many properties better than Lewis Theorybonding schemes, bond strengths, bond lengths,
bond rigidity
• however, there are still many properties of molecules it doesn’t predict perfectlymagnetic behavior of O2
Tro, Chemistry: A Molecular Approach 125
Molecular Orbital Theory• in MO theory, we apply Schrödinger’s wave equation
to the molecule to calculate a set of molecular orbitals in practice, the equation solution is estimated we start with good guesses from our experience as to what
the orbital should look like then test and tweak the estimate until the energy of the
orbital is minimized
• in this treatment, the electrons belong to the whole molecule – so the orbitals belong to the whole moleculeunlike VB Theory where the atomic orbitals still exist in the
molecule
Tro, Chemistry: A Molecular Approach 126
LCAO• the simplest guess starts with the atomic orbitals
of the atoms adding together to make molecular orbitals – this is called the Linear Combination of Atomic Orbitals methodweighted sum
• because the orbitals are wave functions, the waves can combine either constructively or destructively
Tro, Chemistry: A Molecular Approach 127
Molecular Orbitals• when the wave functions combine constructively, the
resulting molecular orbital has less energy than the original atomic orbitals – it is called a Bonding Molecular Orbital, most of the electron density between the nuclei
• when the wave functions combine destructively, the resulting molecular orbital has more energy than the original atomic orbitals – it is called a Antibonding Molecular Orbital*, *most of the electron density outside the nuclei nodes between nuclei
Tro, Chemistry: A Molecular Approach 128
Interaction of 1s Orbitals
Tro, Chemistry: A Molecular Approach 129
Molecular Orbital Theory• Electrons in bonding MOs are stabilizing
Lower energy than the atomic orbitals
• Electrons in anti-bonding MOs are destabilizingHigher in energy than atomic orbitalsElectron density located outside the
internuclear axisElectrons in anti-bonding orbitals cancel
stability gained by electrons in bonding orbitals
Tro, Chemistry: A Molecular Approach 130
MO and Properties• Bond Order = difference between number of
electrons in bonding and antibonding orbitalsonly need to consider valence electronsmay be a fractionhigher bond order = stronger and shorter bonds if bond order = 0, then bond is unstable compared
to individual atoms - no bond will form.
• A substance will be paramagnetic if its MO diagram has unpaired electrons if all electrons paired it is diamagnetic
2
Elec. Antibond# - Elec. Bond # Order Bond
Tro, Chemistry: A Molecular Approach 131
1s 1s
Hydrogen AtomicOrbital
Hydrogen AtomicOrbital
Dihydrogen, H2 MolecularOrbitals
Since more electrons are in bonding orbitals than are in antibonding orbitals,
net bonding interaction
Tro, Chemistry: A Molecular Approach 132
H2
* Antibonding MOLUMO
bonding MOHOMO
Tro, Chemistry: A Molecular Approach 133
1s 1s
Helium AtomicOrbital
Helium AtomicOrbital
Dihelium, He2 MolecularOrbitals
Since there are as many electrons in antibonding orbitals as in bonding orbitals,
there is no net bonding interaction
BO = ½(2-2) = 0
134
1s 1s
Lithium AtomicOrbitals
Lithium AtomicOrbitals
Dilithium, Li2 MolecularOrbitals
Since more electrons are in bonding orbitals than are in
antibonding orbitals, net bonding interaction
2s 2s
Any fill energy level will generate filled bonding and
antibonding MO’s;therefore only need to consider valence shell
BO = ½(4-2) = 1
Tro, Chemistry: A Molecular Approach 135
bonding MOHOMO
Antibonding MOLUMO
Li2
Tro, Chemistry: A Molecular Approach 136
Interaction of p Orbitals
Tro, Chemistry: A Molecular Approach 137
Interaction of p Orbitals
Tro, Chemistry: A Molecular Approach 139
O2
• dioxygen is paramagnetic• paramagnetic material have unpaired electrons• neither Lewis Theory nor Valence Bond Theory
predict this result
Tro, Chemistry: A Molecular Approach 140
O2 as described by Lewis and VB theory
Tro, Chemistry: A Molecular Approach 141
OxygenAtomicOrbitals
OxygenAtomicOrbitals
2s 2s
2p 2p
Since more electrons are in bonding orbitals than are in
antibonding orbitals, net bonding interaction
Since there are unpaired electrons in the
antibonding orbitals, O2 is paramagnetic
O2 MO’s
BO = ½( 8 be – 4 abe)BO = 2
Tro, Chemistry: A Molecular Approach 142
O2
Antibonding MOHOMO
Bonding MOHOMO-1
Antibonding MOLUMO+1
Antibonding MOLUMO
Tro, Chemistry: A Molecular Approach 143
Draw a molecular orbital diagram of N2 and predict its bond order and magnetic properties
Tro, Chemistry: A Molecular Approach 144
NitrogenAtomicOrbitals
NitrogenAtomicOrbitals
2s 2s
2p 2p
Since there are no unpaired electrons, N2 is diamagnetic
N2 MO’s
BO = ½( 8 be – 2 abe)BO = 3
Tro, Chemistry: A Molecular Approach 145
Heteronuclear Diatomic Molecules• the more electronegative atom has lower energy orbitals• when the combining atomic orbitals are identical and
equal energy, the weight of each atomic orbital in the molecular orbital are equal
• when the combining atomic orbitals are different kinds and energies, the atomic orbital closest in energy to the molecular orbital contributes more to the molecular orbital lower energy atomic orbitals contribute more to the bonding MOhigher energy atomic orbitals contribute more to the
antibonding MO
• nonbonding MOs remain localized on the atom donating its atomic orbitals
Tro, Chemistry: A Molecular Approach 146
NO
Free-Radical
2s Bonding MOmainly O’s 2s atomic orbital
Tro, Chemistry: A Molecular Approach 147
HF
Tro, Chemistry: A Molecular Approach 148
Polyatomic Molecules
• when many atoms are combined together, the atomic orbitals of all the atoms are combined to make a set of molecular orbitals which are delocalized over the entire molecule
• gives results that better match real molecule properties than either Lewis or Valence Bond theories
Tro, Chemistry: A Molecular Approach 149
Ozone, O3
Delocalized bonding orbital of O3