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Reading assignment: 8.1-8.4
As you read ask yourself:
What determines whether any two atoms react?
What are the forces that hold atoms together in molecules and
ions together in ionic compounds?
How do the characteristics of these forces and the bonds that
are formed determine the physical and chemical properties of
the compounds?
How does the unequal sharing of an electron pair affect the
q
sharing?
g
bond? What determines unequal
Chem 101
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Chapter 8
Basic concepts of chemical bonding
 Properties
p
of substances
 Ionic bonds
Electrostatic attractive force
between ions
 Covalent bonds
 Metallic bonds
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Chemical bonding involves
Use Lewis symbols for an atom to
represent the valence electrons
chemical symbol + valence
electrons as dots
example:
O
Octet rule
atoms gain, lose or share electrons
Why?
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Chem 101
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s and p block elements
transition metal elements
example:
Mn2+:
Mn [Ar] 4s2 3d5
Mn4+:
Chem 101
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Ionic bonds
ΔH = -617 kJ/mol exothermic
Li(s) + ½ F2(g) → LiF(s)

Lii

+


F



two atomic properties determine how easily electron transfer occurs:
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Energetics of Ionic bonds
Li(g) → Li+(g) + eF(g) + e- → F-(g)
I1=+520 kJ/mol
∆E = -328 kJ/mol
overall, change in energy = +192 kJ/mol
defined as:
energy required to completely
separate 1 mole of solid ionic
compound to its gaseous ions
LiF(s) → Li+(g) + F-(g)
Chem 101
breaking up the lattice requires energy,
forming the lattice is exothermic!
Li(g) → Li+(g) + e-
I1=+520 kJ/mol
F(g) + e- → F-(g)
∆E = -328 kJ/mol
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Li+(g) + F-(g) → LiF(s)
Li(g) + F(g) → LiF(s)
Chem 101
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Lattice energy
E el 
depends on
Q1Q2
d
Can you
lattice
energy
predict
increases
which has
as the
the highest
chargeenergy?
lattice
on the ions increases
Chem 101
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Covalent Bonds
formed by sharing a pair of electrons
useful for representing
Lewis structures covalent
molecules
1) show valence electrons
2) octet rule determines
3) each bond provides
4) represent shared electrons
so, for Cl2 Lewis structure is
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Lewis structures for covalent molecules
for nonmetals
predict that:
p
7A elements ((F,, Cl,, Br))
6A elements
4A elements (C, Si)
What is the structure of F2?
How can we explain O2?
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Multiple bonds
share a single pair of electrons:
Multiple bonds:
double bonds – two pair
triple bonds – three pair
Bond distance -
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Bond distances
Distance between atoms related
to forces
In multiple bonds electron density
bond strength:
bond distances:
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Bond polarity
For ionic compounds, there is no sharing of electrons
Identical atoms in covalent bonds
example: Cl2
This bond is called
Non-identical atoms may have
Example: HF
This bond is
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Electronegativity
Ability of an atom in a molecule to
defined quantity, related to ionization energy and electron affinity
large differences:
small differences:
intermediate
differences:
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Dipole and dipole moment
Unequal sharing creates a
magnitude of the dipole is the
μ  directly proportional to charge
and to distance
μ
Chem 101
H-F
1.82
1 82
H-Cl
1.08
H-Br
0.82
diff. in electronegativity
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Continuum of bond types
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Ionic and covalent bonding
represent the two extremes of the continuum
ionic
Properties:
Chem 101
covalent
Properties:
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28/09/2010
Reading assignment: 8.5-8.8
As you read ask yourself:
How can I use Lewis structures to account for bonding in covalent
m
molecules?
What are the differences between single, double and triple bonds in
terms of bond distance and strength?
When is it useful to assess the formal charges on atoms in a Lewis
structure?
How does a resonance hybrid structure differ from a regular Lewis
structure? How do resonance structures affect the predicted bond
distances?
What are the situations when the octet rule is disobeyed? How will
I recognize molecules that disobey the octet rule?
How can I make use of average bond enthalpies?
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Lewis structures
1. Find the total number of valence
electrons (account for any charges) =
TOTAL
Choose central atom
correctly
2. decide connection between atoms, draw
a line to represent 1 electron pair for
each connection, count the electrons
SHARED
3. calculate the remaining electrons =
TOTAL – SHARED, assign these to the
terminal atoms to make octet (or 2 for H
atom)
more than 1 central
atom (e.g. N2O4)? 
make a symmetric
arrangement
4. any electrons left? – put them on the
central atom
5. if central atom doesn’t have an octet,
make multiple bonds from nonbonded
electron pairs on terminal atoms
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examples:
electrons:
NH3
central atom?
 have?
 connections:
share?
 remaining?
 &  central atom octet?
ethylene, C2H2
electrons:
central atom?
 connections:
 have?
share?
 remaining?
 central atom?
 octet?
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Formal charges
often can make more than one Lewis structure
which one is
correct?
bookkeeping of electrons
calculate the charge on atom
assign to the atom
formal charge =
Evaluate Lewis structures
more stable if
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Example:
H
N
H
H
Chem 101
examples:
electrons:
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CO2
central atom?
 have?
 connections:
share?
 remaining?
 &  central atom octet?
Formal charge C:
Formal charge O:
Formal charge O:
Formal charge O:
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Resonance
sometimes experimental data show that a single Lewis structure is
not an accurate description of bonding
or we find that more than one valid Lewis structure is possible
example: ozone, O3
A
B
actual structure is
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Resonance hybrids
hybrid is
intermediate
between the two
parent
“parent”
structures
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Resonance has impact on bond lengths and strengths
compare NO+, NO2- and NO3NO+ :
NO2- :
NO3- :
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aromatic compounds show resonance
benzene
shorthand notation omits H atoms
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Exceptions to the octet rule
1. less than an octet
or?
2. odd number of electrons
3. more than an octet
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Exceptions to the octet rule…
can expand valence shell to make a Lewis structure with
lower formal charge
SO42-
experimental
info:
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Covalent bond strength
stability of molecule is related to strength of covalent bonds
energy change when a particular bond is broken in one mole of
gaseous substance is
HCl(g)
H(g) + Cl(g) H = 431 kJ
bond enthalpies are
the greater the ΔH,
depend on
For polyatomic molecules the bond enthalpies
H-OH
OH
H + OH H = 498.7 kJ
H+O
H = 428.0 kJ
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Estimate enthalpy change of a reaction
overall change is the difference
between bonds broken
and
bonds formed
example:
CH4 + Cl2
Chem 101
CH3Cl + HCl
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ΔH is negative (rxn. is
exothermic) when
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Bond lengths
also depend on nature of atom and type of bond
also calculated as averages
Chem 101
N
N
1.47
N
N
1.24
N
N
1.10
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