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CHAPTER 8
Bonding and Molecular Structure
2
Introduction
• Bonds: Attractive forces that hold atoms together in compounds
• Valence Electrons: the outermost electrons– -These e- are involved in bonding
3
Electrons are divided between core and valence electrons
B 1sB 1s22 2s 2s22 2p 2p11
Core = [He]Core = [He] , valence = 2s , valence = 2s22 2p 2p11
Br [Ar] 3dBr [Ar] 3d1010 4s 4s22 4p 4p55
Core = [Ar] 3dCore = [Ar] 3d1010 , valence = 4s , valence = 4s22 4p 4p55
Valence Electrons
4
-The number of valence electrons of a main group atom is the Group number
-For Groups IA-IVA, number of bonding -For Groups IA-IVA, number of bonding (unpaired) (unpaired) electrons is equal to the group numberelectrons is equal to the group number-For Groups VA -VIIA, number of bonding (unpaired) -For Groups VA -VIIA, number of bonding (unpaired)
electrons is equal to 8 - group numberelectrons is equal to 8 - group number
Valence Electrons
5
-Except for H (and sometimes atoms of -Except for H (and sometimes atoms of the 3the 3rdrd group and higher) group and higher)
-The total number of valence electrons -The total number of valence electrons around a given atom in a molecule will around a given atom in a molecule will be eight:be eight:
OCTET RULE- (with the exception of hydrogen) atoms in molecules prefer to be surrounded by 8 electrons (or have 4 bonds = 8 electrons)
Valence Electrons
6
Lewis Dot Formulas of Atoms
H He
Li Be B C N O F Ne
IA IIA IIIA IVA VA VIA VIIA VIIIA
7
Ionic Bonding
An ion is an atom or a group of atoms possessing a net electrical charge
• -cations: positive (+) ions• These atoms have lost 1 or more
electrons
1. -anions: negative (-) ions• 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
Examples:• NH4
+ - cation• NO2
-,CO32-, SO4
2- - anions
9
Formation of Ionic Compounds• General trend:
– metals become isoelectronic with the preceding noble gas electron configuration
– nonmetals become isoelectronic with the following noble gas electron configuration
10
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)
11
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[ ]...... ..
12
Formation of Ionic Compounds• In general:
the reaction of IA metals and VIIA nonmetals:
2 M(s) + X2 2 MX(s)
– where M is the metals Li to Cs– and X is the nonmetals F to I
Electronically it looks like: ns np ns npM M+ __ __ __ __X X-
13
Formation of Ionic Compounds
reaction of IIA metals with VIIA nonmetals:
Be(s) + F2(g) BeF2(g)
14
Formation of Ionic Compounds
The valence electrons in these two elements react like:
2s 2p 2s 2p Be [He] Be2+ __ __ __
__F [He] F-
Lewis dot structure representation:
15
Formation of Ionic Compounds
The remainder of the IIA metals and VIIA nonmetals react similarly:
M(s) + X2 MX
2
M can be any of the metals Be to BaX can be any of the nonmetals F to I
16
Formation of Ionic CompoundsFor the reaction of IA metals with
VIA nonmetals:
-2s22(g)(s) O Li2O Li4
Draw the valence electronic configurations for Li, O, and their appropriate ions
17
Formation of Ionic Compounds• Draw the electronic configurations
for Li, O, and their appropriate ionsYou do it!You do it!
2s 2p 2s 2p Li [He] Li1+
O [He] O2-
Draw the Lewis dot formula representation of this reaction
18
Formation of Ionic Compounds
Simple Binary Ionic Compounds Table• Reacting Groups General Formula Example
IA + VIIA MX NaF IIA + VIIA MX2 BaCl2IIIA + VIIA MX3 AlF3
IA + VIA M2X Na2O
IIA + VIA MX BaOIIIA + VIA M2X3 Al2S3
19
Formation of Ionic Compounds
• Reacting Groups General Formula Example
IA + VA M3X Na3N
IIA + VA M3X2 Mg3P2
IIIA + VA MX AlN
-H forms ionic compounds when bound to metals (IA and IIA metals
For example: LiH, KH, CaH2, and BaH2
-When H is bound to nonmetals, the compounds are covalent in nature
20
Formation of Covalent Bonds• -potential energy of an H2 molecule as
a function of the distance between the two H atoms
21
Covalent Bonding• Atoms share electrons
• If the atoms share: • 2 electrons a single covalent bond is
formed• 4 electrons - a double covalent bond• 6 electrons - a triple covalent bond
Atoms have a lower potential energy when bound…this is a more favorable situation (why?)
22
Writing Lewis Formulas:
• 1. Add the number of valence electrons for all the atoms that are present in the molecule
• 2. Add or subtract electrons based on the molecule’s (or ion’s) charge
• 3. Identify the central atom and draw a skeletal structure: – -the one that requires the most e- to complete octet – -the less electronegative
• 4. Place a bond between each atom (1 bond = 2 e-)
• 5. Fill in octet of outer atoms first• 6. Finish by completing the octet of central atom
– – if you run out of e- then multiple bonds must be created between the central atom and atoms bound to it
23
Writing Lewis Formulas
octet rule: representative elements usually attain stable noble gas electron configurations (8 valence e-) in most compounds
You must distinguish the difference between: – -bonding electrons and nonbonding
electrons -shared (paired) and unshared
(unpaired) electrons
24
Formation of Covalent Bonds
• Lewis dot structures:
• 1. H2 molecule formation:
2. HCl molecule formation:
25
Lewis Structures• Homonuclear diatomic molecules
– 1. Two atoms of the same element, H2:
H HorH H..
2. Fluorine, F2:
3. Nitrogen, N2:
26
Lewis Structures
heteronuclear diatomic molecules
1. hydrogen fluoride, HF
2. hydrogen chloride, HCl
3. hydrogen bromide, HBr
or ··H Cl··
··H Cl..
······
27
Lewis Structures
• Water, H2O
•Ammonia molecule , NH3
28
Lewis Structures
• Polyatomic ions:
• ammonium ion NH4+
Notice that the N-atom in this molecule has eight electrons around them (H does not)
29
Writing Lewis Formulas
• Sulfite ion, SO32-.
30
Double and even triple bonds are commonly observed for C, N, P, O, and S
••O OC
•• ••
••
••O OC
•• ••
••
HH22COCO
SOSO33
CC22FF44
31
Lewis Structures
• Example: Write Lewis dot and dash formulas for sulfur trioxide, SO3
32
Resonance
• There are three possible structures for SO3:
O S
O
O·· ······ ··
······
OS
O
O·· ···· ·· ··
··
······
O S
O
O·· ····
·· ··
····
-Two or more Lewis formulas are necessary to show the bonding in a molecule -use equivalent resonance structures to show the molecule’s structure
-Double-headed arrows are used to indicate resonance formulas
33
Resonance
Resonance is a flawed method of representing molecules– -There are no single or double bonds in
SO3
SO O
O
34
Sulfur Dioxide, SOSulfur Dioxide, SO22Sulfur Dioxide, SOSulfur Dioxide, SO22
1. Central atom =1. Central atom =
2. Valence electrons = ___2. Valence electrons = ___
or ___ pairsor ___ pairs
4. Form double bond so 4. Form double bond so that S has an octet — but that S has an octet — but note that there are two note that there are two ways of doing this.ways of doing this.
3. Write the Lewis 3. Write the Lewis structurestructure
35
Limitations of the Octet Rule
• There are some molecules that violate the octet rule:
1. - Be2. - Group IIIA3. -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. S and P)
36
Limitations of the Octet Rule
• Example: Write Lewis formula for BBr3.
37
Sulfur Tetrafluoride, SFSulfur Tetrafluoride, SF44Sulfur Tetrafluoride, SFSulfur Tetrafluoride, SF44
Central atom = Central atom =
Valence electrons = ___ or ___ Valence electrons = ___ or ___ pairs.pairs.
Form sigma bonds and Form sigma bonds and distribute electron pairs.distribute electron pairs.
F
••
••
••
F
F
S••
••
••
••
•• F
••
••
••
••
••
F
••
••
••
F
F
S••
••
••
••
•• F
••
••
••
••
•• 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.
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.
38
Limitations of the Octet Rule
• Example: Write dot structures for AsF
5.
39
• Atoms in molecules often bear a charge (+ or -)
• The predominant resonance structure of a molecule is the one with charges on atoms as close to 0 as possible
• Formal charge = Group number – 1/2 (# of bonding electrons) - (# of Lone electrons)
• • = Group number – (# of bonds) • – (# of Lone electrons)
Formal Atomic Charges
40
Formal Charge
CO CO22
Formal Charge
CO CO22
. .
. .
. .
. .
41
Thiocyanate Ion, SCN-
Thiocyanate Ion, SCN-
••
•
••S NC
•••
••
•
••S NC
•••
•••
••S NC
•••
Which is the most stable resonance form?Which is the most stable resonance form?
Formal Charge
42
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 9)– Involves the use of hybridized atomic
orbitals– Principal originator
• L. Pauling in the 1930’s & 40’s
43
VSEPR Theory
electron densities around the central atom are arranged as far apart as possible to minimize repulsions (why?)
• Five basic molecular shapes:• Linear, trigonal planar, tetrahedral,
trigonal bipyramidal, octahedral
44
VSEPR Theory
1. Two regions of high electron density around the central atom.
45
VSEPR Theory2. Three regions of high electron density around the
central atom.
46
VSEPR Theory3. Four regions of high electron density around
the central atom.
47
VSEPR Theory
4. Five regions of high electron density around the central atom.
48
VSEPR Theory
5. Six regions of high electron density around the central atom.
49
VSEPR Theory
1. Electronic geometry(family): locations of regions of electron density around the central atom(s)
2.2. Molecular geometry:Molecular geometry: arrangement of atoms around the central atom(s)
Electron pairs are not used in the molecular geometry determination
50
VSEPR Theory
Lone pairs (unshared pairs) of electrons require more volume than shared pairs– -there is an ordering of repulsions of lone
electrons around central atom
Criteria for the ordering of the repulsions: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.
51
Molecular Shapes and Molecular Shapes and BondingBonding
• Symbolism:A = central atomB = bonding pairs around central atomU = lone pairs around central atom
• For example:AB3U designates that there are 3 bonding
pairs and 1 lone pair around the central atom
52
Linear Electronic Geometry: AB2
Some examples of molecules with this geometry:BeCl
2, BeBr
2, BeI
2, HgCl
2, CdCl
2
53
Trigonal Planar Electronic Geometry: AB3
Some examples of molecules with this geometry are:
BF3, BCl3
54
Tetrahedral Electronic Geometry: AB
4
Some examples of molecules with this geometry are: CH
4, CF
4, CCl
4,
SiH
4,
SiF
4
55
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
HHH
56
Tetrahedral Electronic Geometry: AB4
57
Tetrahedral Electronic Geometry: AB3U
Some examples of molecules with this geometry are: NH3, NF3, PH3, PCl3, AsH3
– -trigonal pyramidal-electronic and molecular geometries are different.
. .
107.5°
. .
58
• Some examples of molecules with this geometry are: H2O, OF2, H2S– -bent
-electronic and molecular geometries are different
Tetrahedral Electronic Geometry: AB2U2
104.5°
59
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
HHH
60
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
Some examples of molecules with this geometry are: PF5, AsF5, PCl5
axial
axial
equatorial
61
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes:1. One lone pair - Seesaw shape2. Two lone pairs - T-shape3. Three lone pairs – linear
The lone pairs occupy equatorial positions first: -they are 120o from each other
-90o from the axial positions– Results in decreased repulsions compared to
lone pair in axial positionaxial
axial
equatorial
62
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
• AB4U molecules have:
1. trigonal bipyramid electronic geometry
2. seesaw shaped molecular geometry 3. polar
• One example of an AB4U molecule is
SF4
63
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
H
C
HHH
64
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
• AB3U2 molecules have: 1. 1. trigonal bipyramid electronic
geometry 2. T-shaped molecular geometry 3. polar
• One example of an AB3U2 molecule is
IF3
65
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
H
C
HHH
66
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
• AB2U3 molecules have:
1.trigonal bipyramid electronic geometry
2.linear molecular geometry 3.nonpolar
• One example of an AB3U2 molecule is
BrF2-
67
Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and
AB2U3
H
C
HHH
68
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• Some examples of molecules with this geometry are: SF6, SeF6, SCl6, etc.
69
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 pyramidal2. Two lone pairs - square planar
The lone pairs occupy any position because they are all 90o from all bonds positions:– -Additional lone pairs occupy the position
180º from the first set of lone pairs– -This results in decreased repulsions compared
to lone pairs in the other positions
70
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• AB5U molecules have:
1.octahedral electronic geometry 2.Square pyramidal molecular
geometry 3.polar.
• One example of an AB4U molecule is
IF5
71
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
72
Polarity and Polarity and ElectronegativityElectronegativity
Figure 8.11Figure 8.11
73
Dipole Moments
• For example, HF and HI:
units Debye 0.38 units Debye 1.91
I-H F-H
--
74
Dipole Moments
some “nonpolar molecules” that have polar bonds
Two conditions to be polar:1. 1. There must be at least one polar bond
present or one lone pair of electrons2. 2. the molecule must be nonsymmetric
Examples: water, CF4, CO2, NH3, NH4+
75
Polar Molecules
• Molecular geometry affects molecular polarity– -they either cancel or reinforce each
other
A B A
linear molecule nonpolar
A B A
angular molecule
polar
76
Polar and Nonpolar Bonds
• Covalent bonds in which the electrons are shared equally are designated as nonpolar covalent bonds– -Nonpolar covalent bonds have a
symmetrical charge distribution (electron distribution)
N N········ ·· N N·· ··or H HorH H..
77
Polar and Nonpolar Bonds
• Polar covalent bonds: electrons are not shared equally
• -they have different electronegativities
H FElectronegativities: 2.1 4.0
Difference = 1.9 very polar bond
78
Polar and Nonpolar Bonds
• Compare HF to HI:
H IElectronegativities: 2.1 2.5
Difference = 0.4 slightly polar bond
more complicated geometries exist…
79
• Three molecules with polar covalent bonds:
• -Each bond has one atom with a slight negative charge (-)
• -another with a slight positive charge (+)
Bond Polarity
80
Polar or Nonpolar?Polar or Nonpolar?
AB3 molecules: BF3, Cl2CO, and NH3
81
Polar or Nonpolar?Polar or Nonpolar?CO2 and H2O
Which one is polar?
82
CHCH44 … CCl … CCl44Polar or Not?Polar or Not?
• Only CH4 and CCl4 are NOT polar. These are the only two molecules that are “symmetrical.”
83
Compounds Containing Double Bonds
• Ethene or ethylene, C2H4, is the simplest organic compound containing a double bond.– -has a double bond to obey octet ruleLewis Dot Formula
CC
H
HH
H
C CH
H
H
H····
·· ·· ··
··or
84
• What is the effect of bonding and structure on molecular properties?
Free rotation Free rotation around C–C single around C–C single bondbond
No rotation No rotation around C=C around C=C double bonddouble bond
and
Double Bonds
85
Bond Order # of bonds between similar pairs of
atoms
Bond Order # of bonds between similar pairs of
atoms
Double bondDouble bondDouble bondDouble bondSingle bondSingle bondSingle bondSingle bond
Triple Triple bondbondTriple Triple bondbond
AcrylonitrileAcrylonitrileAcrylonitrileAcrylonitrile
86
Consider NO2-:
that typeof bound atoms of # Total
type-one of bonds of # Total = orderBond
that typeof bound atoms of # Total
type-one of bonds of # Total = orderBond
The N—O bond order = 1.5The N—O bond order = 1.5The N—O bond order = 1.5The N—O bond order = 1.5
O O O O
N••
••••
••••
••••••••••
••••
••N
Bond Order
87
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
Bond Order
88
the distance between the nuclei of two bonded atoms
Bond Length
89
Bond LengthBond LengthBond length depends on size of bonded atoms
H—FH—F
H—ClH—Cl
H—IH—I
Bond distances measured Bond distances measured in Angstrom units where 1 in Angstrom units where 1 ÅÅ = 10 = 10-2 -2 pm.pm.
Bond distances measured Bond distances measured in Angstrom units where 1 in Angstrom units where 1 ÅÅ = 10 = 10-2 -2 pm.pm.
90
Bond length depends on bond order
Bond distances measured Bond distances measured in Angstrom units where 1 in Angstrom units where 1 ÅÅ = 10 = 10-2 -2 pm.pm.
Bond distances measured Bond distances measured in Angstrom units where 1 in Angstrom units where 1 ÅÅ = 10 = 10-2 -2 pm.pm.
91
• Measure of the energy required to break a bond
• See Table 9.10• BOND STRENGTH (kJ/mol) H—H 436 KJ C—C 346 KJ C=C 602 KJ CC 835 KJ NN 945 KJ
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.
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 Strength