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1
CHAPTER 9
• Structure and Molecular Bonding
2
Introduction
• Bonds - Attractive forces that hold atoms together in compounds
• Valence Electrons - The electrons involved in bonding are in the outermost (valence) shell.
Rules of the GameRules of the GameRules of the GameNo. of valence electrons of a No. of valence electrons of a main group atom = Group main group atom = Group numbernumber
••For Groups 1AFor Groups 1A--4A, no. of bond pairs = 4A, no. of bond pairs =
group number.group number.
•• For Groups 5A For Groups 5A --7A, BP7A, BP’’s = 8 s = 8 -- GrpGrp. No.. No.
5
Rules of the GameRules of the Game••No. of valence electrons of an atom = Group No. of valence electrons of an atom = Group
numbernumber
••For Groups 1AFor Groups 1A--4A, no. of bond pairs = group 4A, no. of bond pairs = group
numbernumber
•• For Groups 5A For Groups 5A --7A, BP7A, BP’’s = 8 s = 8 -- GrpGrp. No. . No.
••Except for H (and sometimes atoms of Except for H (and sometimes atoms of
3rd and higher periods), 3rd and higher periods),
BPBP’’s + LPs + LP’’s = 4s = 4
This observation is called theThis observation is called the
OCTET RULEOCTET RULE
6
Lewis Dot Formulas of Atoms
Li Be B C N O F Ne.... .. ..
..HeH
.
.. . .
.. ..
..
...
..
.. ..
.
...
.
. ...
7
Ionic Bonding
Formation of Ionic Compounds• An ion is an atom or a group of atoms
possessing a net electrical charge.
1. positive (+) ions or cations• These atoms have lost 1 or more electrons.
2. negative (-) ions or anions• These atoms have gained 1 or more electrons.
8
Formation of Ionic Compounds
• Monatomic ions consist of one atom.
• Examples:– Na+, Ca2+, Al3+ - cations
– Cl-, O2-, N3- -anions
• Polyatomic ions contain more than one atom.– NH4
+ - cation
– NO2-,CO3
2-, SO42- - anions
9
Formation of Ionic Compounds
• Reaction of Group IA Metals with Group VIIA Nonmetals
point melting
C842an with gas solid
solid whiteyellow silver
LiF 2 F Li 2
nometal 17 -G metal 1-G
o
(s)2(g)(s) →+
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Formation of Ionic Compounds
1s 2s 2p
Li ↑↓ ↑F ↑↓ ↑↓ ↑↓↑↓↑
These atoms form ions with these configurations.
Li+ ↑↓ same configuration as [He]
F- ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ same configuration as [Ne]
Li + F...
.... .
Li+F[ ]........
11
Formation of Ionic Compounds
• General trend
• Cations become isoelectronic with the preceding noble gas.
• Anions become isoelectronic with the following noble gas.
12
Formation of Ionic Compounds
• In general for the reaction of 1 metals and 17 nonmetals, the reaction equation is:
This leads to the following resonance This leads to the following resonance
structures.structures.
•
•O OS••
••
••
••••
•
•
bring inleft pair
OR bring inright pair
•
•O OS••
••
••
••
•
••
•O OS••
••
••
••
•
•
41
Writing Lewis Formulas:Limitations of the Octet Rule
• There are some molecules that violate the octet rule.– For these molecules the N - A = S rule does not apply:
1. - Be.
2. - Group - 13.
3. -Odd number of total electrons.
4. -Central element must have a share of more than 8 valence electrons to accommodate all of the substituents. (I.e. some S, P)
42
Writing Lewis Formulas:Limitations of the Octet Rule
• Example: Write Lewis formula for BBr3.
B··. Br·
···
··
.
BBr Br
Br
····
····
······
··
······
··
Br B
Br
Br····
··
·· ··
··
····
··
or
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Sulfur Tetrafluoride, SF4Sulfur Sulfur TetrafluorideTetrafluoride, SF, SF44•• Central atom = Central atom =
•• Valence electrons = ___ or ___ Valence electrons = ___ or ___
pairs.pairs.
•• Form sigma bonds and distribute Form sigma bonds and distribute
electron pairs.electron pairs.
F
••
•
•
••
F
F
S••
••
•
•
•
•
•
• F••
••
••
••
•• 5 pairs around the S atom. A common
occurrence outside the 2nd period.
5 pairs around the S 5 pairs around the S
atom. A common atom. A common
occurrence outside the occurrence outside the
2nd period. 2nd period.
44
Writing Lewis Formulas:Limitations of the Octet Rule
• Example: Write dot and dash formulas for AsF
5.
45
Formal Atom ChargesFormal Atom ChargesFormal Atom Charges• Atoms in molecules often bear a charge (+ or -).
• The predominant resonance structure of a molecule is the one with charges as close to 0 as possible.
•• Formal charge Formal charge = Group number = Group number –– 1/2 (no. of bonding electrons) 1/2 (no. of bonding electrons) -- (no. of LP electrons)(no. of LP electrons)
46
Calculated Partial Charges in CO2
CalculatedCalculated Partial Charges in Partial Charges in COCO22
Which is the most stable resonance form?Which is the most stable resonance form?
49
Calculated Partial Charges Calculated Partial Charges in SCNin SCN--
All atoms negative, but most on the S
All atoms negative, but All atoms negative, but
most on the Smost on the S••
•
•
• S NC•
•
•
50
Dipole Moments
• Asymmetric charge distribution
• The dipole moment has the symbol µ.
• µ is the product of the distance,d, separating charges of equal magnitude and opposite sign, and the magnitude of the charge, q.
51
Dipole Moments
• For example, HF and HI:
units Debye 0.38 units Debye 1.91
I-H F-H
-- δδδδ ++
aa
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Dipole Moments
• There are some nonpolar molecules that have polar bonds.
• There are two conditions that must be true for a molecule to be polar.
1. There must be at least one polar bond present or one lone pair of electrons.
2. The polar bonds, if there are more than one, and lone pairs must be arranged so that their dipole moments do not cancel one another.
3. Examples (water, CF4, CO2, NH3, NH4+)
53
Polar Molecules: The Influence of Molecular Geometry
• Molecular geometry affects molecular polarity.
– Due to the effect of the bond dipoles and how they either cancel or reinforce each other.
A B A
linear molecule
nonpolar
A B
A
angular molecule
polar
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Polar Molecules: The Influence of Molecular Geometry
• Polar Molecules must meet two requirements:
1. One polar bond or one lone pair of electrons on central atom.
2. Neither bonds nor lone pairs can be symmetrically arranged that their polarities cancel.
55
Polar and Nonpolar Covalent Bonds
• Covalent bonds in which the electrons are shared equally are designated as nonpolarcovalent bonds.
– Nonpolar covalent bonds have a symmetrical charge distribution.
N N······
··
·· N N·
···or H HorH H
.
.
56
Polar and Nonpolar Covalent Bonds
• Polar covalent bonds - electrons are not shared equally -different electronegativities.
bondpolar very 1.9 Difference
4.0 2.1 ativitiesElectroneg
F H
1.9
=
43421
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Polar and Nonpolar Covalent Bonds
• Compare HF to HI.
bondpolar slightly 0.4 Difference
2.5 2.1 ativitiesElectroneg
I H
0.4
=
43421
58
Two Simple Theories of Covalent Bonding
• Valence Shell Electron Pair Repulsion Theory
– Commonly designated as VSEPR
– Principal originator
• R. J. Gillespie in the 1950’s
• Valence Bond Theory (Chapter 10)
– Involves the use of hybridized atomic orbitals
– Principal originator
• L. Pauling in the 1930’s & 40’s
59
VSEPR Theory
• VSEPR - electron density around the central atom are arranged as far apart as possible to minimize repulsions.
• Five basic molecular shapes
60
VSEPR Theory
1 Two regions of high electron density around the central atom.
61
VSEPR Theory
2 Three regions of high electron density around the central atom.
62
VSEPR Theory
3 Four regions of high electron density around the central atom.
63
VSEPR Theory
4 Five regions of high electron density around the central atom.
64
VSEPR Theory
5 Six regions of high electron density around the central atom.
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VSEPR Theory
1.1. Electronic geometryElectronic geometry - locations of regions
of electron density around the central atom(s).
2.2. Molecular geometryMolecular geometry - arrangement of atoms around the central atom(s).
Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used.
66
VSEPR Theory• An example of a molecule that has the same electronic and molecular geometries is methane - CH4.
• Electronic and molecular geometries are tetrahedral.
H
C
HH
H
67
VSEPR Theory
• An example of a molecule that has different electronic and molecular geometries is water - H2O.
• Electronic geometry is tetrahedral.
• Molecular geometry is bent or angular.
H
C
HH
H
68
VSEPR Theory
• Lone pairs of electrons (unshared pairs) require more volume than shared pairs.
– Consequently, there is an ordering of repulsions of electrons around central atom.
• Criteria for the ordering of the repulsions:
69
VSEPR Theory
1 Lone pair to lone pair is the strongest repulsion.
2 Lone pair to bonding pair is intermediate repulsion.
3 Bonding pair to bonding pair is weakest repulsion.
• Mnemonic for repulsion strengths
lp/lp > lp/bp > bp/bp
• Lone pair to lone pair repulsion is why bond angles in water are less than 109.5o.
70
Molecular Shapes and BondingMolecular Shapes and Bonding
• In the next sections we will use the following terminology:
A = central atom
B = bonding pairs around central atom
U = lone pairs around central atom
• For example:
AB3U designates that there are 3 bonding pairs and 1 lone pair around the central atom.
71
Linear Electronic Geometry:AB2
Species (No Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are: BeCl
2, BeBr
2, BeI
2, HgCl
2, CdCl
2
• All of these examples are linear, nonpolar molecules.
• Important exceptions occur when the two substituents are not the same!BeClBr or BeIBr will be linear and polar!
72
Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of
Electrons on A)
• Some examples of molecules with this geometry are:
BF3, BCl3
• All of these examples are trigonal planar, nonpolar molecules.
• Important exceptions occur when the three substituents are not the same!
BF2Cl or BCI2Br will be trigonal planar and polar!
73
Tetrahedral Electronic Geometry: AB4
Species (No Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are:
CH4, CF
4, CCl
4, SiH
4, SiF
4
• All of these examples are tetrahedral, nonpolar molecules.
• Important exceptions occur when the four substituents are not the same!
CF3Cl or CH2CI2 will be tetrahedral and polar!
74
Tetrahedral Electronic Geometry: AB4Species (No Lone Pairs of Electrons on
A)
75
Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of
Electrons on A)
• Some examples of molecules with this geometry are: NH3, NF3, PH3, PCl3, AsH3
• These molecules are our first examples of central atoms with lone pairs of electrons.Thus, the electronic and molecular geometries are different.
All three substituents are the same but molecule is polarpolar.
• NH3 and NF3 are trigonal pyramidal, polar molecules.
76
Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of
Electrons on A)
• Some examples of molecules with this geometry are: H2O, OF2, H2S
• These molecules are our first examples of central atoms with two lone pairs of electrons.Thus, the electronic and molecular geometries are different.
Both substituents are the same but molecule is polarpolar.
• Molecules are angular, bent, or V-shaped and polar.
77
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3� Some examples of molecules with this geometry are: PF5, AsF5, PCl5, etc.
• These molecules are examples of central atoms with five bonding pairs of electrons.The electronic and molecular geometries are the same.
• Molecules are trigonal bipyramidal and nonpolar when all five substituents are the same.If the five substituents are not the same polarpolarmolecules can result, AsF4Cl is an example.
78
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
• If lone pairs are incorporated into the trigonalbipyramidal structure, there are three possible new shapes.
1. One lone pair - Seesaw shape
2. Two lone pairs - T-shape
3. Three lone pairs – linear
• The lone pairs occupy equatorial positions because they are 120o from two bonding pairs and 90o from the other two bonding pairs.
– Results in decreased repulsions compared to lone pair in axial position.
79
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3• AB4U molecules have:
1. trigonal bipyramid electronic geometry
2. seesaw shaped molecular geometry
3. and are polar
• One example of an AB4U molecule is
SF4
80
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Molecular Geometry
H
C
HH
H
81
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
• AB3U2 molecules have:
1. trigonal bipyramid electronic geometry
2. T-shaped molecular geometry
3. and are polar
• One example of an AB3U2 molecule is
IF3
82
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Molecular Geometry
H
C
HH
H
83
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3• AB2U3 molecules have:
1.trigonal bipyramid electronic geometry
2.linear molecular geometry
3.and are nonpolar
• One example of an AB3U2 molecule is
84
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Molecular Geometry
H
C
HH
H
85
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• Some examples of molecules with this geometry are:
SF6, SeF6, SCl6, etc.
• These molecules are examples of central atoms with six bonding pairs of electrons.
• Molecules are octahedraloctahedral and nonpolarnonpolarwhen all six substituents are the same.
If the six substituents are not the same polarpolarmolecules can result, SF5Cl is an example.
86
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• If lone pairs are incorporated into the octahedral
structure, there are two possible new shapes.
1. One lone pair - square pyramidal
2. Two lone pairs - square planar
• The lone pairs occupy axial positions because
they are 90o from four bonding pairs.
– Results in decreased repulsions compared to lone pairs
in equatorial positions.
87
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• AB5U molecules have:
1.octahedral electronic geometry
2.Square pyramidal molecular geometry
3.and are polar.
• One example of an AB4U molecule is
IF5
88
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry
H
C
HH
H
89
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• AB4U2 molecules have:
1.octahedral electronic geometry
2.square planar molecular geometry
3.and are nonpolar.
• One example of an AB4U2 molecule is
XeF4
90
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular GeometryPolarity
H
C
HH
H
91
Compounds Containing Double Bonds
• Ethene or ethylene, C2H4, is the simplest organic compound containing a double bond.
• Compound must have a double bond to obey octet rule.
92
Compounds Containing Double Bonds
Lewis Dot Formula
CC
H
HH
H
C CH
H
H
H··
··
···· ·
·
··
o r
93
Bond PropertiesBond Properties• What is the effect of bonding and
structure on molecular properties?
Free rotation Free rotation
around Caround C––C single C single
bondbond
No rotation No rotation
around C=C around C=C
double bonddouble bond
94
Bond Order# of bonds between a pair of atoms
Bond OrderBond Order# of bonds between a pair of atoms# of bonds between a pair of atoms
Double bondDouble bondDouble bondSingle bondSingle bondSingle bond
Triple bond
Triple Triple
bondbond
AcrylonitrileAcrylonitrileAcrylonitrile
95
Bond OrderBond Order
Fractional bond orders occur in molecules
with resonance structures.
Consider NO2-
Bond order = 3 e - pairs in N — O bonds
2 N — O bonds
Bond order = Total # of e - pairs used for a type of bond
Total # of bonds of that type
The N—O bond order = 1.5The NThe N——O bond order = 1.5O bond order = 1.5
O O O O
N••
••••
••
••••••••••
••
••••
••N
96
Bond OrderBond Order
Bond order is proportional to two important
bond properties:
(a) bond strength
(b) bond length
745 kJ745 kJ
414 kJ414 kJ
110 pm110 pm
123 pm123 pm
97
Bond LengthBond Length
• Bond length is the distance between the nuclei of two bonded atoms.
98
Bond LengthBond Length
Bond length depends on size of bonded atoms.
HH——FF
HH——ClCl
HH——II
Bond distances measured in Angstrom units where 1 A = 10-2 pm.
Bond distances measured Bond distances measured
in Angstrom units where 1 in Angstrom units where 1
A = 10A = 10--2 2 pm.pm.
99
Bond length depends on bond order.
Bond distances measured in Angstrom units where 1 A = 10-2 pm.
Bond distances measured Bond distances measured
in Angstrom units where 1 in Angstrom units where 1
A = 10A = 10--2 2 pm.pm.
Bond LengthBond Length
100
• —measured by the energy req’d to break a bond. See
Table 9.10.
• BOND STRENGTH (kJ/mol)
H—H 436
C—C 346
C=C 602
C≡C 835
N≡N 945
The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.
The GREATER the number of bonds (bond order) the The GREATER the number of bonds (bond order) the
HIGHER the bond strength and the SHORTER the HIGHER the bond strength and the SHORTER the
bond.bond.
Bond StrengthBond Strength
101
102
Bond StrengthBond Strength
• Measured as the energy req’d to break a
bond. See Table 9.10
103
• Measured as the energy req’d to break a
bond. See Table 9.10.
• BOND STRENGTH (kJ/mol)
H—H 436
C—C 346
C=C 602
C≡C 835
N≡N 945
The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.
The GREATER the number of bonds (bond order) the The GREATER the number of bonds (bond order) the
HIGHER the bond strength and the SHORTER the HIGHER the bond strength and the SHORTER the
bond.bond.
Bond StrengthBond Strength
104
Molecular PolarityMolecular Polarity
Why do ionic compounds dissolve in Why do ionic compounds dissolve in
water?water?
WaterWater
Boiling point Boiling point
= 100 = 100 ˚̊CC
MethaneMethane
Boiling point Boiling point
= = --161 161 ˚̊CC
Why do water and Why do water and
methane differ so methane differ so
much in their much in their
boiling points?boiling points?
105
Bond PolarityBond PolarityBond Polarity
•• Three molecules with Three molecules with
polar, covalent bonds.polar, covalent bonds.
•• Each bond has one Each bond has one
atom with a slight atom with a slight
negative charge (negative charge (--δδ) ) and and another with and and another with
a slight a slight positivepositivecharge (+ charge (+ δδ))
106
ElectronegativityElectronegativity, , χχχχ is a measure of the ability of an is a measure of the ability of an atom in a molecule to attract atom in a molecule to attract electrons to itself.electrons to itself.
Concept proposed by Linus Pauling 1901-1994Concept proposed by Concept proposed by LinusLinus Pauling 1901Pauling 1901--19941994