Chemical Bonding I:
The Covalent Bond
1
Chemical Bonding I: The Covalent
Bond
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Lewis dot symbols (9.1)
The ionic bond (9.2)
Lattice energy of ionic compounds (9.3)
The covalent bond (9.4)
Electronegativity (9.5)
Writing Lewis structures (9.6)
Formal charge and Lewis structure (9.7)
The concept of resonance (9.8)
Exceptions to the octet rule (9.9)
Bond enthalpy (9.10)
Chemical Bonding I
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Valence electrons of atoms
Ionic bonding
◦ Lattice energy
◦ Born-Haber cycle
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Covalent bonding
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Lewis dot structures
Bond lengths
Electronegativity
Resonance
Formal charges
Exceptions to the octet rule
Bond Enthalpy
9.1 Lewis dot symbols
Main group elements
 What do the symbols show?
 What do the symbols not show?
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Figure 9.1, p. 288
9.2 The ionic bond
What is an ionic bond?
 How have we already discussed ionic
bonds?
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Definition
Formula
Nomenclature
Formula calculations
Redox (formation of ionic compounds)
Electron configurations
Ionization energy
Electron affinity
9.3 Lattice energy
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What is lattice energy?
What is the associated reaction for this (use
NaCl)?
◦ Can this be measured directly?
◦ Is this a positive or negative energy?
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How can we think about this in terms of
opposite charges?
◦ What is Coulomb’s law and what is the equation
for lattice energy?
◦ Is this a positive or negative values?
◦ How does this relate to lattice energy?
Born-Haber Cycle
9.3 Lattice energy
How is lattice energy affected by ionic
charges?
 How is lattice energy affected by the
distance between the ions?
 What is the Born-Haber cycle?
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◦ How does this relate to energy diagrams?
◦ How is this based on Hess’s Law?
Born-Haber Cycle
9.3 Lattice energy
Figure 9.2, p. 293
Born-Haber Cycle – NaCl
9.3 Lattice energy
What is the thermochemical equation
for the formation of NaCl?
 What is the energy diagram for this?
 How will these be integrated for the
Born-Haber Cycle?
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What does the length of the arrow mean?
What does the direction of the arrow
mean?
Born-Haber Cycle – NaCl
9.3 Lattice energy
What are the thermochemical equations for the:
Enthalpy of formation of NaCl(s)
Enthalpy of sublimation of Na
Bond dissociation energy of Cl2(g)
First ionization energy of Na
Electron affinity for Cl
What is the lattice energy (in kJ/mol) of sodium
chloride?
Born-Haber Cycle – NaCl
9.3 Lattice energy
9.3 Lattice energy
Table 9.1, p. 293
Born-Haber Cycle – NaCl
9.3 Lattice energy
Ek
Q Q
r
Lattice energy
increases as Q
increases and/or
as r decreases.
Q+ is the charge on the cation
Q- is the charge on the anion
r is the distance between the ions
k is a constant (that is positive)
Lattice Energy /
kJ·mol-1
MgF2
2957
MgO
3890
LiF
1017
LiCl
828
9.4 The covalent bond
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How is the representation
different for ionic versus
covalent bonds?
What are lone pairs of
electrons?
How does bond length vary?
Are multiple bonds longer or
shorter than single bonds?
Table 9.2, p. 295
9.5 Electronegativity
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What is the definition of
electronegativity?
◦ The ability of an atom to attract toward itself
the electrons in a chemical bond
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What is the trend for electronegativity?
9.5 Electronegativity
Figure 9.5, p. 296
9.5 Electronegativity
How does this relate to metallic
character?
 What is a polar covalent bond and how
is this modeled?
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9.5 Electronegativity
Polar covalent bond or polar bond is a covalent bond with
greater electron density around one of the two atoms
electron poor
region
H
electron rich
region
F
e- poor
H
+
e- rich
F
-
Figure 9.4, p. 296
9.5 Electronegativity
How does the
difference in
electronegativity
relate to covalent
versus ionic bonding?
 How does this relate
to percent ionic
character?
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Figure 9.7, p. 297
9.5 Electronegativity
Figure 9.5, p. 296
9.6 Writing Lewis structures
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This should be review (can use either a
previous method you have learned, the
method provided in the lecture activity or
the method given in the text, p. 299).
9.7 Formal charge
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What is formal charge?
◦ The electrical charge difference between the
valence electrons in an isolated atom and the
number of electrons assigned to that atom in
a Lewis structure
How does it differ from oxidation state?
 How does formal charge assist in
“placing” a charge on an atom for a
polyatomic ion?
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9.8 Resonance
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What is a resonance structure?
◦ One of two or more Lewis structures for a
single molecule that cannot be represented
accurately by only one Lewis structure
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What is the criteria for a structure to be
a resonance structure?
9.9 Exceptions to the octet rule
Must all atoms in molecules or ions obey
the octet rule?
 How can formal charge be useful in
determining expanded or reduced octets?
 Can molecules have an odd number of
electrons?
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9.10 Bond enthalpy
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What is bond enthalpy?
◦ The enthalpy change required to break a particular
bond in 1 mole of gaseous molecules
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What is the associated reaction?
Is bond enthalpy exothermic or endothermic?
Always?
How can reactions be considered using bond
enthalpies?
How do Lewis dot structures help in
determining enthalpy of reaction using bond
enthalpies?
9.10 Bond enthalpy
Table 9.3, p. 314
9.10 Bond enthalpy
Imagine reaction proceeding by
breaking all bonds in the reactants
and then using the gaseous atoms to
form all the bonds in the products.
H0 = total energy input – total energy released
H0 = BE(reactants) – BE(products)
9.10 Bond enthalpy
endothermic reaction
exothermic reaction
Figure 9.8, p. 315
Bond Enthalpy – Examples
2H2 (g) + O2 (g)
2H2O (g)
Margin figure, p. 315
9.10 Bond enthalpy
Table 9.3, p. 314
Bond Enthalpy – Examples
H2 (g) + Cl2 (g)
2HCl (g)
Margin figure, p. 315
9.10 Bond enthalpy
Table 9.3, p. 314
Chapter 9 – Practice
The lattice energy for magnesium oxide is 3890 kJ∙mol–1. Which chemical equation describes this?
A. Mg(s) + ½O2(g)  MgO(s)
B. MgO(s)  Mg(s) + ½O2(g)
2+
2–
C. Mg (g) + O (g)  MgO(s) D. MgO(s)  Mg2+(g) + O2–(g)
Which portion of the periodic table contains the elements with the highest electronegativity?
What is the correct Lewis dot structure of NO2?
A.
B.
C.
D.
Chapter 9 – Practice
Which thermochemical equation describes the bond enthalpy of hydrogen?
A 2H(g) H2(g) ∆Ho < 0
B 2H(g) H2(g) ∆Ho > 0
C H2(g) 2H(g) ∆Ho < 0
D H2(g) 2H(g) ∆Ho > 0
How many resonance structures are there for the formate ion, CO2H–?
A. 0
B. 1
C. 2
D. 3
Complete (balance) the chemical equation using structural formulas –
you may NOT use any stoichiometric coefficients.
What is the change in enthalpy for this reaction using bond enthalpies?
What is the thermochemical equation for the bond enthalpy of chlorine?
All bond enthalpies are EXOTHERMIC / ENDOTHERMIC.